“使用”类范围的类型别名：[何时] 方法中的用法可以先于类型别名吗？

Question

Yesterday, I was (pleasantly) surprised when I was able to compile code that had a method which used a using type alias even though the declaration of the alias was not until later in the class definition.

• Is this 'forward' usage of a type-alias valid? (I assume it is, since Clang 5 and GCC 4.9 both work this way.)
• What rules cover this behavior and the difference between the method body and method signature usages?

Case #1 - using declared after method, valid inside method body (only?)

#include <string>
#include <iostream>

struct X {
  std::string create() {     // fails to compile if Y used in signature
      return Y{"hello!"};    // compiles when Y here
  }

  using Y = std::string;     // declared at bottom
};

int main() 
{
    std::cout << X().create() << std::endl;
}

Case #2 - using declared above is [also] valid in signature

#include <string>
#include <iostream>

struct X {
  using Y = std::string;     // declared at top

  Y create() {               // can use Y here as well
      return Y{"hello!"};
  }
};

int main() 
{
    std::cout << X().create() << std::endl;
}

Answer 1

This has to do with the complete-class context . When you are in the body of a member function, the class is considered complete and can use anything defined in the class, no matter where in the class it is declared.

The return type of a member function is not part of the complete-class context so you can only use names that are known about at that point in the code.