CRTP编译错误 [英] CRTP compiling error

问题描述

以下将使用GCC 5.2进行编译，但不能与Visual Studio 2015一起编译。

The following will compile with GCC 5.2 but not with Visual Studio 2015.

template <typename Derived>
struct CRTP {
    static constexpr int num = Derived::value + 1;
};

struct A : CRTP<A> {
    static constexpr int value = 5;
};

它抱怨 A 成员名为 value 。
如何修复代码，使其编译在两个编译器？

It complains that A does not have a member named value. How to fix the code so that it compiles on both compilers? Or is it illegal altogether?

推荐答案

尝试使用constexpr函数。

由于模板成员函数只在首次使用时初始化，请键入 A 将由该点完全定义。

Try making it a constexpr function instead. The way you have it setup now attempts to access an incomplete type.
Since a templated member function will only be initialized upon first being used, type A will be fully defined by that point.

#include <iostream>

template <typename Derived>
struct CRTP {
    static constexpr int num() { return  Derived::value + 1; }
};

struct A : CRTP<A> {
    static constexpr int value = 5;
};

int main()
{
    std::cout << A::num();
    return 0;
}

查看现场这里

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