模板 class 中的非模板成员 function 的需要子句

Question

I have a simple template class A , I want to enable a function if some requirements are satisfied.

Solution 1 - requires clause

The first solution I tried is the following:

template <class T>
class A
{
public:
    void a(void) requires (same_as<T, int>)
    {
        std::cout << "a" << std::endl;
    };
};

This works pretty well, I can call A<int>().a() but not A<char>().b(); . Also IntelliSense correctly report correctly identifies usage errors in Visual Studio.

Then I tried to move the function definition outside the class and I got C2511 compiler error in Visual Studio while in GCC it works fine

template <class T>
class A
{
public:
    void a(void) requires (same_as<T, int>);
};

template <class T>
void A<T>::a(void)
requires (same_as<T, int>)
{
    std::cout << "a" << std::endl;
}

Do you think my code is incorrect or is it a visual studio compiler bug/incomplete feature?

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Solution 2 - static_assert

This solution works in some cases, but of course it would cause compile errors if you try an explicit template instantiation (for example template class A<char> ), also IntelliSense would not correctly identify improper usages

template <class T>
class A
{
public:

    void a(void);
};

template <class T>
void A<T>::a(void)
{
    static_assert(same_as<T, int>);
    std::cout << "a" << std::endl;
}

---

Soluton 3 - enable_if

(wrong)

template <class T>
class A
{
public:

    template <std::enable_if_t<same_as<T, int>, bool> = true>
    void a(void);
};

template <class T>
template <std::enable_if_t<same_as<T, int>, bool>>
void A<T>::a(void)
{
    std::cout << "a" << std::endl;
}

This solution has the same probelms as #2, moreover I would prefer not to add some non understandable at first sight templates.

---

Solution 4 - some weird compile time inheritance

template <class T>
class A_base { /*common stuff*/ };

template <class T>
class A : public A_base<T>
{
public:
    A_base<T>::A_base;

    /* non-int stuff*/
};

template <>
class A<int> : public A_base<int>
{
public:
    A_base<int>::A_base;

    void a(void) {};
};

This would work well, but it could get quite complex in some situations and very unpleasant for debugging when a bunch of levels are nested

---

Do you have any advice / better solution?

Thanks in advance

---

Edit:

• solution #3 does not work, I made a mistake;
• I am using msvc 16.7.0, Build Tools x64/x86 (v14.27)

Answer 1

MSVC C++ Features states that P0734R0 Concepts is available from VS 2019 16.3.

Could you possibly be using VS 2017 which does not have the same support for C++20 with MSVC?

Answer 2

I suggest you better not use C++20, because Visual Studio's support for C++20 is not yet complete. If you are not using VS2019, it is recommended that you could update to the latest version of VS2019. Then set Preview - Features from the Latest C++ Working Draft (std:c++latest) in Properties->General->C++ Language Standard .