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

Question

我认为c++概念是编写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升级到2019年，并且仍然在等待clang支持概念。

然而，我用visual 2019和g++ 8的msvc测试了一些简单的代码，这些代码来自明明威64，我遇到了一些麻烦。

这是一个考验：

#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 Studio\2019\Community\VC\Tools\MSVC\14.24.28314\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，我在一个编译器中使用不推荐的或删除的语法，而另一个编译器不支持它？

Answer 1

bool的语法不是C++20概念，而是以前概念TS (技术规范)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，但对语法和语义进行了一些更改。

GCC 8只支持TS，不支持C++20的概念，你需要GCC 10为后者。

有关C++20概念添加的文档，请参阅C++20，概念TS的这一个。还请参阅此页以获得编译器对概念(TS和C++20)的支持列表。

Answer 2

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

如果确实需要同时支持这两种编译器，则可以使用特征测试宏来选择正确的语法(尽管从技术上讲，该表中没有列出概念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。