如果 class 具有模板，则定义 class 的 const static 成员时出错

Question

I have the following code in a .h file:

class Test1 {
    struct A1 {
        int x;
    };
    static const A1 a1;
};

template <class T>
class Test2 {
    struct A2 {
        int x;
    };
    static const A2 a2;
};

and I define values for both a1 and a2 in a .cpp file:

const Test1::A1 Test1::a1 = { 5 };

template<class T>
const Test2<T>::A2 Test2<T>::a2 = { 5 };

The odd thing is that for Test1 everything works, but for Test2 I get the following error in the last line of the .cpp file:

Error C2061 syntax error: identifier 'A2'

The only difference between Test1 and Test2 is that Test2 has a template. Why is this a problem, and how can I fix it? Also, this works as well:

test.h

struct A3 {
    int x;
};
template <class T>
class Test3 {
    static const A3 a3;
};

test.cpp

template<class T>
const A3 Test3<T>::a3 = { 5 };

So the problem is with using a struct defined in a templated class, but I can't find out what exactly is the problem.

(I'm compiling with c++14.)

Answer 1

The fix is suggested by a compiler, just be more careful in reading the compiler messages.

<source>(17): warning C4346: 'A2': dependent name is not a type
<source>(17): note: prefix with 'typename' to indicate a type
<source>(17): error C2061: syntax error: identifier 'A2'
<source>(17): error C2143: syntax error: missing ';' before '{'
<source>(17): error C2447: '{': missing function header (old-style formal list?)

Or

<source>:17:7: error: missing 'typename' prior to dependent type name 'Test2<T>::A2'
const Test2<T>::A2 Test2<T>::a2 = { 5 };
      ^~~~~~~~~~~~
      typename 
1 error generated.

That means the correct code:

template<class T>
const typename Test2<T>::A2 Test2<T>::a2 = { 5 };

Read more: Why do we need typename here? .