Visual Studio 2019拒绝"bool概念"，而gcc 8不会编译没有"bool"的概念。

Question

我认为c++概念是编写c++模板化代码的更好方法，具有更好的错误消息和更快的编译时间，所以我将Visual Studio升级到2019，仍在等待clang支持概念

然而，我用visualstudio2019的msvc和mingw-w64的g++8测试了一些简单的代码，我遇到了一些问题。

这就是测试：

#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 Studio2019CommunityVCToolsMSVC14.24.28314binHostX64x64cl.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中使用了不推荐使用或已删除的语法，而另一个编译器不支持它？

Answer 1

bool的语法不是C++20概念，而是以前的概念TS(技术规范(的语法。后者是对C++17的实验性扩展，被基于TS的C++20概念添加所取代，但对语法和语义进行了一些更改。

GCC 8只支持TS，而不支持C++20概念，后者需要GCC 10。

有关C++20概念添加的文档，请参阅cppreference.com页面，有关概念TS的文档，也请参阅此页面。有关编译器对概念(TS和C++20(的支持列表，请参阅此页。

Answer 2

正如胡桃木所说，concept bool是TS语法，而concept是C++20语法。

如果您真的需要支持这两个编译器，您可以使用功能测试宏来选择正确的语法(尽管从技术上讲，该表中没有列出Concepts TS(：

#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;

这将适用于所有支持概念的编译器。