C ++定义静态成员的正确方法是什么

Question

I'm trying to use a static member in a normal member function. But the compiler report some errors. Pls take a look at this code

#include <memory>

template<typename T>
class MyClass {
private:
  static std::allocator<T> alloc;
  T *p;
public:
  void assign(T e) {
    p = alloc.allocate(1);
    alloc.construct(p, e);
  }
};

and this is how I use it:

#include 'myclass.h'

int main() {
  MyClass<int> cls;
  cls.assign(4);

};

And the compiler give this error:

/Users/liuziqi/CLionProjects/cpplearning/src/tt.h:17:9: warning: instantiation of variable 'MyClass<int>::alloc' required here, but no definition is available [-Wundefined-var-template]
    p = alloc.allocate(1);
        ^
/Users/liuziqi/CLionProjects/cpplearning/src/main.cpp:49:7: note: in instantiation of member function 'MyClass<int>::assign' requested here
  cls.assign(4);
      ^
/Users/liuziqi/CLionProjects/cpplearning/src/tt.h:13:28: note: forward declaration of template entity is here
  static std::allocator<T> alloc;
                           ^
/Users/liuziqi/CLionProjects/cpplearning/src/tt.h:17:9: note: add an explicit instantiation declaration to suppress this warning if 'MyClass<int>::alloc' is explicitly instantiated in another translation unit
    p = alloc.allocate(1);

I can not figure out which part is wrong.....I have defined that static member and any member function should be able to use it. Is this error relevant to template ? (I've just learned template and not sure If I use it correctly.)

Answer 1

I'm going to take a shot at describing what the warning is referring to in addition to answering the "how to fix it" question (which certainly has been answered many times before)...

Because MyClass is a template, the compiler expects all the code for the templated class to be available in the same file ( myclass.h ). Since you only declare but do not define alloc in myclass.h , the compiler assumes you made a mistake. It's not absolutely required to be there (hence the warning and not error ) and you could disable the warning if you define the variable elsewhere, but it's almost certainly just a mistake.

If you're using c++17, the easiest way to deal with this is to declare the static member as inline, so it will be defined right there:

  static inline std::allocator<T> alloc;

live: https://godbolt.org/g/cr1aUP

Prior to C++17, you would explicitly define the variable in myclass.h :

template<typename T>
std::allocator<T> MyClass<T>::alloc;

live: https://godbolt.org/g/2Znqst