带有字符串模板参数的模板 class 的部分特化

Question

#include <iostream>

template<unsigned N>
struct FixedString 
{
    char buf[N + 1]{};
    constexpr FixedString(const char (&s)[N]) 
    {
        for (unsigned i = 0; i != N; ++i)
            buf[i] = s[i];
    }
};

template<int, FixedString name>
class Foo 
{
public:
    auto hello() const { return name.buf; }
};

template<FixedString name>
class Foo<6, name>
{
public:
    auto hello() const { return name.buf; }
};

int main() 
{
    Foo<6, "Hello!"> foo;
    foo.hello();
}

我正在尝试添加模板专业化 Foo<6, name> 并以此错误结束：

macros.h: At global scope:
macros.h:3:18: error: class template argument deduction failed:
 23 | class Foo<6, name>
      |                  ^
macros.h:3:18: error: no matching function for call to ‘FixedString(FixedString<...auto...>)’
macros.h:7:15: note: candidate: ‘template<unsigned int N> FixedString(const char (&)[N])-> FixedString<N>’
 7 |     constexpr FixedString(const char (&s)[N])
      |               ^~~~~~~~~~~
macros.h:7:15: note:   template argument deduction/substitution failed:
macros.h:13:18: note:   mismatched types ‘const char [N]’ and ‘FixedString<...auto...>’
 23 | class Foo<6, name>
      |                  ^
macros.h:4:8: note: candidate: ‘template<unsigned int N> FixedString(FixedString<N>)-> FixedString<N>’
 4 | struct FixedString
      |        ^~~~~~~~~~~
macros.h:4:8: note:   template argument deduction/substitution failed:
macros.h:13:18: note:   mismatched types ‘FixedString<N>’ and ‘FixedString<...auto...>’
 23 | class Foo<6, name>

使用字符串模板参数专门化模板类的正确方法是什么？ g++ (Ubuntu 9.4.0-1ubuntu1~16.04) 9.4.0

Answer 1

What is the proper way to specialise template classes with string template arguments?

给定的代码是 well-formed（在 C++20 中）但无法为 gcc 10.2 及更低版本编译。然而，它从 gcc 10.3 及更高版本编译得很好。 Demo

可能是因为并非所有 C++20 功能都在 gcc 10.2 及更低版本中完全实现。

带有字符串模板参数的模板 class 的部分特化

Partial specialiszation of a template class with string template argument

c++

template-specialization