The Filesystem TS is nothing to do with C++1z support, it is a completely separate specification not part of the C++1z working draft. GCC’s implementation (in GCC 5.3 and later) is even available in C++11 mode. You just need to link with -lstdc++fs to use it. (The relevant library, libstdc++fs.a, is a static library, so … Read more
This is CWG active issue 2118: Defining a friend function in a template, then referencing that function later provides a means of capturing and retrieving metaprogramming state. This technique is arcane and should be made ill-formed. Notes from the May, 2015 meeting: CWG agreed that such techniques should be ill-formed, although the mechanism for prohibiting … Read more
The template <auto> feature (P0127R1) was accepted into C++ in the ISO C++ 2016 meeting in Oulu, Finland. An auto keyword in a template parameter can be used to indicate a non-type parameter the type of which is deduced at the point of instantiation. It helps to think of this as a more convenient way … Read more
You have to make the discarded statement dependent of the template parameters template <class…> constexpr std::false_type always_false{}; if constexpr(condition1){ … } else if constexpr (condition2) { …. } else if constexpr (condition3) { …. } else { static_assert(always_false<T>); } This is so because [temp.res]/8 – The program is ill-formed, no diagnostic required, if no valid … Read more
std::launder is aptly named, though only if you know what it’s for. It performs memory laundering. Consider the example in the paper: struct X { const int n; }; union U { X x; float f; }; … U u = {{ 1 }}; That statement performs aggregate initialization, initializing the first member of U … Read more
Don’t worry about that. According to the same information source: this library component should be retired to Annex D, along side , until a suitable replacement is standardized. So, you can still use it until a new standardized, more-secure version is done.
Template deduction guides are patterns associated with a template class that tell the compiler how to translate a set of constructor arguments (and their types) into template parameters for the class. The simplest example is that of std::vector and its constructor that takes an iterator pair. template<typename Iterator> void func(Iterator first, Iterator last) { vector … Read more
There is no X object, living or otherwise, so pretending that there is one results in undefined behavior. [intro.object]/1 spells out exhaustively when objects are created: An object is created by a definition ([basic.def]), by a new-expression ([expr.new]), when implicitly changing the active member of a union ([class.union]), or when a temporary object is created … Read more
Language features: Templates and Generic Code Template argument deduction for class templates Like how functions deduce template arguments, now constructors can deduce the template arguments of the class http://wg21.link/p0433r2 http://wg21.link/p0620r0 http://wg21.link/p0512r0 template <auto> Represents a value of any (non-type template argument) type. Non-type template arguments fixes template<template<class…>typename bob> struct foo {} ( Folding + … … Read more
There’s now a drop down (at least since VS 2017.3.5) where you can specifically select C++17. The available options are (under project > Properties > C/C++ > Language > C++ Language Standard) ISO C++14 Standard. msvc command line option: /std:c++14 ISO C++17 Standard. msvc command line option: /std:c++17 Visual Studio 2022 (MSVC C++20 and the … Read more