级联

Question

我试图避免这种解决方案的一个问题：

static const int FOO_Test = 7; 

template < typename Derived > 
class Foo { 

    public: 
    static const int Type; 
}; 

const int Foo::Type = FOO_##Derived ; 

class Test : public Foo<Test> {}; 

正如你看到的，我试图让FOO_Test值，只存在如果有人从Foo派生了一个派生类（需要一些外部工具来编写一个头文件）。

那么宏连接不工作（毕竟还不确定），任何想法实现呢？

Answer 1

由于C++ 14，你可以使用一个变量模板来做到这一点。
它遵循一个最小的，工作示例：

class Test; 

template<typename> static constexpr int FOO; 
template<> constexpr int FOO<Test> = 7; 

template <typename Derived> 
struct Foo { 
    static const int Type; 
}; 

template<typename Derived> 
const int Foo<Derived>::Type = FOO<Derived> ; 

class Test : public Foo<Test> {}; 

int main() { 
    static_assert(Test::Type == 7, "!"); 
} 

它有助于将FOO值从Foo类分离。否则，你可以去完整的专业化，并抛弃这些变量。
作为示例：

class Test; 

template <typename Derived> 
struct Foo { 
    static const int Type; 
}; 

template<> 
const int Foo<Test>::Type = 7 ; 

class Test : public Foo<Test> {}; 

int main() { 
    static_assert(Test::Type == 7, "!"); 
} 

Answer 2

如果您可以使用C++ 14，请使用skypjack的方法。如果你不能，你可以写类似：

#define DEFINE_FOO_TYPE(Derived) \ 
    const int Foo<Derived>::Type = FOO_##Derived 

DEFINE_FOO_TYPE(Test); 

但我会尝试尽可能地避免它。

完整的例子：

// This probably wants to go in Foo.h 
template < typename Derived > 
class Foo { 

    public: 
    static const int Type; 
}; 
#define DEFINE_FOO_TYPE(Derived) \ 
    template <> const int Foo<Derived>::Type = FOO_##Derived 

// This probably wants to go in Test.h 
static const int FOO_Test = 7; 
class Test : public Foo<Test> {}; 

// This needs to go in Test.cpp (or some other central place) 
// Note that it must follow definition of Test. 
DEFINE_FOO_TYPE(Test); 

// an empty main so the whole example will compile and run. 
int main() 
{}