这应该是一个constexpr吗?

Should this be a constexpr or not?

考虑这个代码片段 (godbolt):

#include <cstdio>
#include <string>
#include <string_view>

struct Option
{
    std::string_view name;

    constexpr Option( std::string_view const n ) noexcept : name{n} {}
};

template< std::size_t N >
class TransformedOption : public Option
{
public:
    constexpr TransformedOption( std::string_view const nameStr ) :
        Option{ { nameStorage_, N - 1 } }
    {
        for ( auto i = 0U; i < N; ++i )
        {
            if ( nameStr[ i ] == '_' ) { nameStorage_[ i ] = '-'; }
            else                       { nameStorage_[ i ] = nameStr[ i ]; }
        }
    }
    private:
        char nameStorage_[ N ] = {};
};

template< std::size_t N >
constexpr TransformedOption< N > make( char const (&nameStr)[ N ] ) noexcept
{
    return TransformedOption< N  >{ nameStr };
}

int main()
{
    /*constexpr*/ auto t = make( "abcd_efgh_ijkl_mnop_peqst" );
    std::printf( "%s\n", t.name.data() );
    return 0;
}

基本上,我想通过将每个 _ 替换为 - 并确保最终二进制文件仅包含转换后的字符串(而不是原始字符串)来执行编译时字符串转换。

我试过 Clang 10.0.1、GCC 10.2 和 MSVC 19.24(见上文 godbolt link)。奇怪的东西如下:

嗯,根据 C++ 标准,哪个编译器是正确的?我应该向谁报告错误?给 GCC 和 Clang 的人还是给 Microsoft?

t 也声明为静态时,constexpr 声明适用于所有编译器。

constexpr static auto t = make( "abcd_efgh_ijkl_mnop_peqst" );

原因是string_view。它是引用正在初始化的对象的引用类型。所以不管怎样,你正在初始化一个 contexpr 指针。现在,constexpr 指针(未初始化为空指针)只能使用具有静态存储持续时间的对象的地址进行初始化。

[expr.const] (emphasis mine)

11 A constant expression is either a glvalue core constant expression that refers to an entity that is a permitted result of a constant expression (as defined below), or a prvalue core constant expression whose value satisfies the following constraints:

  • if the value is an object of class type, each non-static data member of reference type refers to an entity that is a permitted result of a constant expression,
  • if the value is of pointer type, it contains the address of an object with static storage duration, the address past the end of such an object ([expr.add]), the address of a non-immediate function, or a null pointer value,
  • if the value is of pointer-to-member-function type, it does not designate an immediate function, and
  • if the value is an object of class or array type, each subobject satisfies these constraints for the value.

An entity is a permitted result of a constant expression if it is an object with static storage duration that either is not a temporary object or is a temporary object whose value satisfies the above constraints, or if it is a non-immediate function.

当您将对象声明为自动存储持续时间时,string_view 中的指针未使用静态对象的地址进行初始化。因此 GCC 和 Clang 理所当然地抱怨。

自我引用使这变得有趣和棘手。