641 lines
22 KiB
Markdown
641 lines
22 KiB
Markdown
---
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title: >-
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Unobvious C++
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sidebar: true
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notmine: false
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abstract: >-
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A collection of notes about the some of somewhat esoteric and unobvious aspects of C++
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...
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# Memory Safety
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Modern, mostly memory safe C++, is enforced by:\
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- Microsoft safety checker
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- Guidelines
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- language checker
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`$ clang-tidy test.cpp -checks=clang-analyzer-cplusplus*, cppcoreguidelines-*, modernize-*` will catch most of the issues that esr
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complains about, in practice usually all of them, though I suppose that as
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the project gets bigger, some will slip through.
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static_assert(__cplusplus >= 201703, "C version of out of date");
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Adds the std::span type, which makes pointer handling a whole lot simpler and safer.
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The size of the array pointed to is kept with the pointer for safe iteration and bounds checking during pointer maths. Also, translates std::array and old type C arrays to the same type, which makes life much simpler and safer. You get all the new good range stuff from both of them.
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Modern C++ as handles arrays as arrays where possible, but they quickly
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decay to pointers – which you avoid using spans. std::array is a C array
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whose size is known at compile time, and which is protected from decay to
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a pointer. std::vector is a dynamically resizable and insertable array
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protected from decay to a pointer – which can have significant overheads.
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std::make_unique, std::make_shared create pointers to memory managed
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objects. (But single objects, not an array, use spans for pointer
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arithmetic)
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```C++
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auto sp = std::make_shared<int>(42);
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std::weak_ptr<T> wp{sp};
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```
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# Array sizing and allocation
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```C++
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/* This code creates a bunch of "brown dog" strings on the heap to test automatic memory management. */
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char ca[]{ "red dog" }; //Automatic array sizing
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std::array<char,8> arr{"red dog"}; //Requires #include <array>
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/* No automatic array sizing, going to have to count your initializer list. */
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/* The pointer of the underlying array is referenced by &arr[0] but arr is not the underlying array, nor a pointer to it. */
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/* [0] invokes operator[], and operator[] is the member function that accesses the underlying array.*/
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/* The size of the underlying array is referenced by arr.size();*/
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/* size known at compile time, array can be returned from a function getting the benefits of stack allocation.*/
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// can be passed around like POD
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char *p = new char[10]{ "brown dog" }; //No automatic array
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// sizing for new
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std::unique_ptr<char[]>puc{ p }; // Now you do not have
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// to remember to delete p
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auto puc2 = std::move(puc); /* No copy constructor. Pass by reference, or pass a view, such as a span.*/
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std::unique_ptr<char> puc3{ new char[10]{ "brown dog" } };
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/* Array size unknown at compile or run time, needs a span, and you have to manually count the initialization list. */
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/* Compiler guards against overflow, but does not default to the correct size.*/
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/* You can just guess a way too small size, and the compiler in its error message will tell you what the size should be. */
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auto pu = std::make_unique<char[]>(10); // uninitialized,
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// needs procedural initialization.
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/* span can be trivially created from a compile time declared array, an std:array or from a run time std:: vector, but then these things already have the characteristics of a span, and they own their own storage. */
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/* You would use a span to point into an array, for example a large blob containing smaller blobs.*/
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// Placement New:
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char *buf = new char[1000]; //pre-allocated buffer
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char *p = buf;
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MyObject *pMyObject = new (p) MyObject();
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p += (sizeof(MyObject+7)/8)*8
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/* Problem is that you will have to explictly call the destructor on each object before freeing your buffer. */
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/* If your objects are POD plus code for operating on POD, you don’t have to worry about destructors.*/
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// A POD object cannot do run time polymorphism.
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/* The pointer referencing it has to be of the correct compile time type, and it has to explicitly have the default constructor when constructed with no arguments.*/
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/* If, however, you are building a tree in the pre-allocated buffer, no sweat. */
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/* You just destruct the root of the tree, and it recursively destructs all its children. */
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/* If you want an arbitrary graph, just make sure you have owning and non owning pointers, and the owning pointers form a tree. */
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/* Anything you can do with run time polymorphism, you can likely do with a type flag.*/
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static_assert ( std::is_pod<MyType>() , "MyType for some reason is not POD" );
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class MyClass
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{
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public:
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MyClass()=default; // Otherwise unlikely to be POD
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MyClass& operator=(const MyClass&) = default; // default assignment Not actually needed, but just a reminder.
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};
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```
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### alignment
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```C++
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// every object of type struct_float will be aligned to alignof(float) boundary
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// (usually 4)
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struct alignas(float) struct_float {
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// your definition here
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};
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// every object of type sse_t will be aligned to 256-byte boundary
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struct alignas(256) sse_t
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{
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float sse_data[4];
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};
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// the array "cacheline" will be aligned to 128-byte boundary
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alignas(128) char cacheline[128];
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```
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# Construction, assignment, and destruction
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six things: ([default
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constructor](https://en.cppreference.com/w/cpp/language/default_constructor),
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[copy
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constructor](https://en.cppreference.com/w/cpp/language/copy_constructor),
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[move
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constructor](https://en.cppreference.com/w/cpp/language/move_constructor),
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[copy
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assignment](https://en.cppreference.com/w/cpp/language/copy_assignment),
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[move
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assignment](https://en.cppreference.com/w/cpp/language/move_assignment)
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and [destructor](https://en.cppreference.com/w/cpp/language/destructor))
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are generated by default – except when they are not.
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So it is arguably a good idea to explicitly declare them as default or
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deleted.
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Copy constructors
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```C++
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A(const A& a)
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```
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Copy assignment
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```C++
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A& operator=(const A other)
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```
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Move constructors
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```C++
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class_name ( class_name && other)
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A(A&& o)
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D(D&&) = default;
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```
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Move assignment operator
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```C++
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V& operator=(V&& other)
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```
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Move constructors
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```C++
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class_name ( class_name && )
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```
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## delegating constructor
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```C++
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class Foo
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{
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public:
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Foo(char x, int y) {}
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Foo(int y) : Foo('a', y) {} // Foo(int) delegates to Foo(char, int)
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};
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```
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## rvalue references
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Move constructors and copy constructors primarily exist to tell the
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compiler how to handle temporary values, rvalues, that have references to possibly
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costly resources.
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`class_name&&` is rvalue reference, the canonical example being a reference to a compiler generated temporary.
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The primary purpose of rvalue references is to support move semantics in
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objects that reference resources, primarily unique_pointer.
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`std::move(t)` is equivalent to `static_cast<decltype(t)&&>(t)`, causing move
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semantics to be generated by the compiler.
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`t`, the compiler assumes, is converted by your move constructor or move assignment into a valid state where your destructor will not need to anything very costly.
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`std::forward(t)` causes move semantics to be invoked iff the thing referenced
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is an rvalue, typically a compiler generated temporary, *conditionally*
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forwarding the resources.
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where `std::forward` is defined as follows:
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template< class T > struct remove_reference {
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typedef T type;
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};
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template< class T > struct remove_reference<T&> {
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typedef T type;
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};
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template< class T > struct remove_reference<T&&> {
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typedef T type;
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};
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template<class S>
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S&& forward(typename std::remove_reference<S>::type& a) noexcept
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{
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return static_cast<S&&>(a);
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}
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`std::move(t)` and `std::forward(t)` don't actually perform any action
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in themselves, rather they cause the code referencing `t` to use the intended
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copy and intended assignment.
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## delegating constructors
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calling one constructor from another.
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```C++
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example::example(... arguments ...):
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example(...different arguments ...){
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};
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```
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## constructors and destructors
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If you declare the destructor deleted that prevents the compiler from
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generating its own, possibly disastrous, destructor, but then, of
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course, you have to define your own destructor with the exact same
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signature, which would ordinarily stop the compiler from doing that
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anyway.
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When you declare your own constructors, copiers, movers, and deleters,
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you should generally mark them noexcept.
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struct foo {
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foo() noexcept {}
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foo( const foo & ) noexcept { }
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foo( foo && ) noexcept { }
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~foo() {}
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};
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Destructors are noexcept by default. If a destructor throws an exception as
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a result of a destruction caused by an exception, the result is undefined,
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and usually very bad. This problem is resolved in complicated ad hoc
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ways that are unlikely to be satisfactory.
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If you need to define a copy constructor, probably also need to define
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an assignment operator.
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t2 = t1; /* calls assignment operator, same as "t2.operator=(t1);" */
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Test t3 = t1; /* calls copy constructor, same as "Test t3(t1);" */
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## casts
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You probably also want casts. The surprise thing about a cast operator
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is that its return type is not declared, nor permitted to be declared,
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DRY. Operator casts are the same thing as constructors, except declared
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in the source class instead of the destination class, hence most useful
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when you are converting to a generic C type, or to the type of an
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external library that you do not want to change.
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```C++
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struct X {
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int y;
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operator int(){ return y; }
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operator const int&(){ return y; } /* C habits would lead you to
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incorrectly expect "return &y;", which is what is
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implied under the hood. */
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operator int*(){ return &y; } // Hood is opened.
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};
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```
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Mpir, the Visual Studio skew of GMP infinite precision library, has some
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useful and ingenious template code for converting C type functions of
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the form `SetAtoBplusC(void * a, void * b, void * c);` into C++
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expressions of the form `a = b+c*d;`. It has a bunch of intermediate
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types with no real existence, `__gmp_expr<>` and `__gmp_binary_expr<>`
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and methods with no real existence, which generate the appropriate
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calls, a templated function of potentially unlimited complexity, to
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convert such an expression into the relevant C type calls using
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pointers. See section mpir-3.0.0.pdf, section 17.5 “C++ Internals”.
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I don’t understand the Mpir code, but I think what is happening is that
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at run time, the binary expression operating on two base types creates a
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transient object on the stack containing pointers to the two base types,
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and the assignment operator and copy create operator then call the
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appropriate C code, and the operator for entities of indefinite
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complexity creates base type values on the stack and a binary expression
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operator pointing to them.
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Simpler, but introducing a redundant copy, to always generate
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intermediate values on the stack, since we have fixed length objects
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that do not need dynamic heap memory allocation, not that costly, and
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they are not that big, at worst thirty two bytes, so clever code is apt
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to cost in overheads of pointer management
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That just means we are putting 256 bits of intermediate data on the
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stack instead of 128, hardly a cost worth worrying about. And in the
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common bad case, (a+b)\*(c+d) clever coding would only save one stack
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allocation and redundant copy.
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# Introspection and sfinae
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Almost all weird and horrific sfinae code [has been rendered unnecessary by concepts](https://www.cppstories.com/2016/02/notes-on-c-sfinae/).
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```c++
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template <typename T> concept HasToString
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= requires(T v) {
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{v.toString()} -> std::convertible_to<std::string>;
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};
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```
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The `requires` clause is doing the sfinae behind your back to deliver a boolean.
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The concept name should be chosen to carry the meaning in an error message.
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And any time concepts cannot replace sfinae, sfinae can be done much better by `std::void_t`, which is syntactic sugar for "trigger substitution failure with the least possible distracting syntax"
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```c++
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// default template:
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template< class , class = void >
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struct has_toString : false_type { };
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// specialized as has_member< T , void > or sfinae
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template< class T>
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struct has_toString< T , void_t<decltype(&T::toString)> > :
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std::is_same< std::string, decltype(declval<T>().toString()) >
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{ };
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```
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# Template specialization
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```C++
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namespace N {
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template<class T> class Y { /*...*/ }; // primary template
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template<> class Y<double> ; // forward declare specialization for double
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}
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template<>
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class N::Y<double> { /*...*/ }; // OK: specialization in same namespace
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```
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is used when you have sophisticated template code, because you have to
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use recursion for looping as the Mpir system uses it to evaluate an
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arbitrarily complex recursive expression – but I think my rather crude
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implementation will not be nearly so clever.
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```C++
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extern template int fun(int);
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/*prevents redundant instantiation of fun in this compilation unit – and thus renders the code for fun unnecessary in this compilation unit.*/
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```
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# Abstract and virtual
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An abstract base class is a base class that contains a pure virtual
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function ` virtual void features() = 0;`.
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A class can have a virtual destructor, but not a virtual constructor.
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If a class contains virtual functions, then the default constructor has
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to initialize the pointer to the vtable. Otherwise, the default
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constructor for a POD class is empty, which implies that the default
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destructor is empty.
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The copy and swap copy assignment operator, a rather slow and elaborate
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method of guaranteeing that an exception will leave the system in a good
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state, is never generated by default, since it always relates to rather
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clever RAII.
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An interface class is a class that has no member variables, and where
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all of the functions are pure virtual! In other words, the class is
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purely a definition, and has no actual implementation. Interfaces are
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useful when you want to define the functionality that derived classes
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must implement, but leave the details of how the derived class
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implements that functionality entirely up to the derived class.
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Interface classes are often named beginning with an I. Here’s a sample
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interface class:.
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class IErrorLog
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{
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public:
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virtual bool openLog(const char *filename) = 0;
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virtual bool closeLog() = 0;
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virtual bool writeError(const char *errorMessage) = 0;
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virtual ~IErrorLog() {} // make a virtual destructor in case we delete an IErrorLog pointer, so the proper derived destructor is called
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// Notice that the virtual destructor is declared to be trivial, but not declared =0;
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};
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[Override
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specifier](https://en.cppreference.com/w/cpp/language/override)
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struct A
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{
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virtual void foo();
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void bar();
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};
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struct B : A
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{
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void foo() const override; // Error: B::foo does not override A::foo
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// (signature mismatch)
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void foo() override; // OK: B::foo overrides A::foo
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void bar() override; // Error: A::bar is not virtual
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};
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Similarly [Final
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specifier](https://en.cppreference.com/w/cpp/language/final)
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[To obtain aligned
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storage](http://www.cplusplus.com/reference/type_traits/aligned_storage/)for
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use with placement new
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void* p = aligned_alloc(sizeof(NotMyClass));
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MyClass* pmc = new (p) MyClass; //Placement new.
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// ...
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pmc->~MyClass(); //Explicit call to destructor.
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aligned_free(p);.
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# spans
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A span is a non owning pointer and count.
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It would perhaps be more useful if we also hand owning pointers with counts
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# The Curiously Recurring Template Pattern
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[CRTP](https://www.fluentcpp.com/2017/05/16/what-the-crtp-brings-to-code/),
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makes the relationship between the templated base class or classes and
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the derived class cyclic, so that the derived class tends to function as
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real base class. Useful for mixin classes.
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template <typename T> class Mixin1{
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public:
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// ...
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void doSomething() //using the other mixin classes and the derived class T
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{
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T& derived = static_cast<T&>(*this);
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// use derived...
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}
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private:
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mixin1(){}; // prevents the class from being used outside the mix)
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friend T;
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};
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template <typename T> class Mixin2{
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{
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public:
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// ...
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void doSomethingElse()
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{
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T& derived = static_cast<T&>(*this);
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// use derived...
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}
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private:
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Mixin2(){};
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friend T;
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};
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class composite: public mixin1<composite>, public mixin2<composite>{
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composite( int x, char * y): mixin1(x), mixin2(y[0]) { ...}
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composite():composite(7,"a" ){ ...}
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}
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# Aggregate initialization
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A class of aggregate type has no constructors – the aggregate
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constructor is implied default.
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A class can be explicitly defined to take aggregate initialization
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Class T{
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T(std::initializer_list<const unsigned char> in){
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for (auto i{in.begin); i<in.end(); i++){
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do stuff with i
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}
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}
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but that does not make it of aggregate type.
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Aggregate type has *no* constructors
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except default and deleted constructors
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# functional programming
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A lambda is a nameless value of a nameless class that is a
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functor, which is to say, has `operator()` defined.
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That each lambda is a unique and nameless class allows the compiler
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to optimise it away, so that it becomes inline code.
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So doing the kind of stuff with a lambda you would ordinarily need
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a class name to do is likely to break stuff or lead to weirdness.
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It looks like you are defining a real class, but your compiler does
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not want to generate a real class except as a last resort.
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But, of course you can get the class with `decltype`
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and assign that nameless value to an `auto` variable,
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and all that, but because the design for lambdas was
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to allow them to be efficiently converted to straightforward
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code this may well result in strange complications and mysterious
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syntax and semantic errors.
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To tell the compiler to actually use the lambda as a lambda, and
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not do all this behind the scenes cleverness, you use `std::function`
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which is a regular templated class that defines `operator ()`, and if
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it is defined in terms of lambda, stashes that lambda on the heap
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after the style of `std::string`
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However for very small lambdas (not capturing any variables, or capturing
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on one variably by value, this can get optimised away), and under
|
||
the hood it does things C style.
|
||
|
||
However, depending on the compiler, `std::function` may do hidden heap
|
||
allocation.
|
||
|
||
But if you are doing all that, might as well explicitly define a
|
||
named functor class.
|
||
|
||
## lambda on the heap
|
||
|
||
To construct a lambda in the heap:
|
||
|
||
```c++
|
||
auto p = new auto([a,b,c](){})
|
||
auto q = new auto([](int x) { return x * x; });
|
||
int result = (*q)(5); // Calls the lambda with the argument 5
|
||
delete p;
|
||
delete q;
|
||
```
|
||
|
||
But if you have pointers to lambdas, std::function is more useful than lambda, because then you can declare their pointer type.
|
||
|
||
|
||
If you want to declare a pointer type that can point to any lambda with the same signature, or indeed any instance of
|
||
a class that supports operator(), you can use std::function:
|
||
|
||
```C++
|
||
#include <functional>
|
||
|
||
// Declare a pointer to a lambda that takes an int and returns an int
|
||
std::function<int(int)>* lambdaPtr = new std::function<int(int)>([](int x) { return x * x; });
|
||
|
||
// Use the lambda
|
||
int result = (*lambdaPtr)(5);
|
||
|
||
// Clean up
|
||
delete lambdaPtr;
|
||
```
|
||
|
||
This way, lambdaPtr is a pointer to a std::function that can store any callable object, including lambdas, that take an int and return an int.
|
||
|
||
similarly placement `new`, and `unique_ptr`.
|
||
|
||
Trouble is that an std::function object is a fixed sized object, like an `std::string`, typically sixteen bytes. which like an `std::string` points to a dynamically allocated object on the heap.
|
||
|
||
# auto and decltype(variable)
|
||
|
||
In good c++, a tremendous amount of code behavior is specified by type
|
||
information, often rather complex type information, and the more one’s
|
||
code description is in types, the better.
|
||
|
||
But specifying types everywhere violates the dry principle, hence,
|
||
wherever possible, use auto and decltype(variable) to avoid redundant
|
||
and repeated type information. Wherever you can use an auto or a
|
||
decltype for a type, use it.
|
||
|
||
In good event oriented code, events are not triggered procedurally, but
|
||
by type information or data structures, and they are not handled
|
||
procedurally, as by defining a lambda, but by defining a derived type
|
||
in the sense that you use the virtual method table as the despatch table
|
||
for handling different events.
|
||
|
||
# Variable length Data Structures
|
||
|
||
C++ just does not handle them well, except you embed an `std::vector` in them,
|
||
which can result in messy reallocations.
|
||
|
||
One way is to drop back into old style C, and tell C++ not to fuck
|
||
around.
|
||
|
||
struct Packet
|
||
{
|
||
unsigned int bytelength;
|
||
unsigned int data[];
|
||
|
||
private:
|
||
// Will cause compiler error if you misuse this struct
|
||
void Packet(const Packet&);
|
||
void operator=(const Packet&);
|
||
};
|
||
Packet* CreatePacket(unsigned int length)
|
||
{
|
||
Packet *output = (Packet*) malloc((length+1)*sizeof(Packet));
|
||
output->bytelength = length;
|
||
return output;
|
||
}
|
||
|
||
# for_each
|
||
|
||
template<class InputIterator, class Function>
|
||
Function for_each(InputIterator first, InputIterator last, Function fn){
|
||
while (first!=last) {
|
||
fn (*first);
|
||
++first;
|
||
}
|
||
return move(fn);
|
||
}
|
||
|
||
# Range-based for loop
|
||
|
||
for(auto x: temporary_with_begin_and_end_members{ code;}
|
||
for(auto& x: temporary_with_begin_and_end_members{ code;}
|
||
for(auto&& x: temporary_with_begin_and_end_members{ code;}
|
||
for (T thing = foo(); auto& x : thing.items()) { code; }
|
||
|
||
The types of the begin_expr and the end_expr do not have to be the same,
|
||
and in fact the type of the end_expr does not have to be an iterator: it
|
||
just needs to be able to be compared for inequality with one. This makes
|
||
it possible to delimit a range by a predicate (e.g. “the iterator
|
||
points at a null character”).
|
||
|
||
If range_expression is an expression of a class type C that has both a
|
||
member named begin and a member named end (regardless of the type or
|
||
accessibility of such member), then begin_expr is \_\_range.begin() and
|
||
end_expr is \_\_range.end();
|
||
|
||
for (T thing = foo(); auto x : thing.items()) { code; }
|
||
|
||
Produces code equivalent to:
|
||
|
||
T thing = foo();
|
||
auto bar = thing.items();
|
||
auto enditer = bar.end;
|
||
for (auto iter = bar.begin(); iter != enditer; ++iter) {
|
||
x = *iter;
|
||
code;
|
||
}
|