Visual Studio 2019 拒绝“bool 概念”，而 gcc 8 不编译没有“bool”的概念

Question

我认为 c++ concepts 是编写 c++ 模板代码的更好方法，具有更好的错误消息和更快的编译时间，因此我将 Visual Studio 升级到 2019 并仍在等待 clang 支持 concepts

但是我用 visual studio 2019 的 msvc 和 mingw-w64 的 g++ 8 测试了一些简单的代码，我遇到了一些麻烦。

这是测试：

#include <iostream>

using namespace std;

// this compiles under g++ 8 but not visual studio 2019
template <class T>
bool concept CharT = std::is_same_v<T, char> || std::is_same_v<T, wchar_t> ||
    std::is_same_v<T, char16_t> || std::is_same_v<T, char32_t>;

// this compile under visual studio 2019 but not g++
//template <class T>
//concept CharT = std::is_same_v<T, char> || std::is_same_v<T, wchar_t> ||
//  std::is_same_v<T, char16_t> || std::is_same_v<T, char32_t>; 

template <CharT char_type>
void PrintChar(char_type ch)
{
    wcout << ch << endl;
}

int main()
{
    PrintChar('c');
    PrintChar(L'h');
    PrintChar('a');
    PrintChar('r');
}

Visual Studio 调用：

cl /std:c++latest concepts.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.24.28316 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

/std:c++latest is provided as a preview of language features from the latest C++
working draft, and we're eager to hear about bugs and suggestions for improvements.
However, note that these features are provided as-is without support, and subject
to changes or removal as the working draft evolves. See
https://go.microsoft.com/fwlink/?linkid=2045807 for details.

concepts.cpp
concepts.cpp(7): error C2988: unrecognizable template declaration/definition
concepts.cpp(7): error C2059: syntax error: 'concept'
concepts.cpp(7): fatal error C1903: unable to recover from previous error(s); stopping compilation
Internal Compiler Error in C:\Program Files (x86)\Microsoft Visual Studio19\Community\VC\Tools\MSVC.24.28314\bin\HostX64\x64\cl.exe.  You will be prompted to send an error report to Microsoft later.

g++ 调用

g++ -std=c++2a -fconcepts concepts.cpp -o c.exe
concepts.cpp:12:9: error: 'CharT' does not name a type; did you mean 'char'?
 concept CharT = std::is_same_v<T, char> || std::is_same_v<T, wchar_t> ||
         ^~~~~
         char
concepts.cpp:15:11: error: 'CharT' has not been declared
 template <CharT char_type>
           ^~~~~
concepts.cpp:16:16: error: variable or field 'PrintChar' declared void
 void PrintChar(char_type ch)
                ^~~~~~~~~
concepts.cpp:16:16: error: 'char_type' was not declared in this scope
concepts.cpp:16:16: note: suggested alternative: 'wchar_t'
 void PrintChar(char_type ch)
                ^~~~~~~~~
                wchar_t
concepts.cpp: In function 'int main()':
concepts.cpp:23:2: error: 'PrintChar' was not declared in this scope
  PrintChar('c');
  ^~~~~~~~~

应该支持这两种语法吗？或者我在一个编译器上使用已弃用或删除的语法，而另一个编译器不支持它？

Answer 1

带bool的语法不是C++20的概念，而是之前的Concepts TS(Technical Specification)的语法。后者是对 C++17 的实验性扩展，取代了基于 TS 的 C++20 概念添加，但对语法和语义做了一些更改。

GCC 8 仅支持 TS，不支持 C++20 概念，后者需要 GCC 10。

有关概念（TS 和 C++20）的编译器支持列表，请参阅 this cppreference.com page for documentation of the C++20 concepts additions and this one for the Concepts TS. See also this page。

Answer 2

正如所说，concept bool是TS语法，而concept是C++20语法。

如果你真的需要支持两个编译器，你可以使用feature test macro到select正确的语法（尽管从技术上讲Concepts TS' t 列在那 table):

#if __cpp_concepts >= 201707
  // working paper, C++20 concepts
  #define CONCEPT concept
#else
  // TS
  #define CONCEPT concept bool
#endif

template <typename T>
CONCEPT C = true;

这适用于所有支持概念的编译器。

Visual Studio 2019 拒绝“bool 概念”，而 gcc 8 不编译没有“bool”的概念

Visual Studio 2019 rejects `bool concept` while gcc 8 doesn't compile concepts without `bool`

c++

g++

visual-c++

c++-concepts

c++20