How to refer current class using decltype in C++11?

问题

When I declare a class static method, is it possible to refer current class using decltype (or in any other similar style)? For example,

class
AAA
{
    static AAA const make();
};

I am trying to make something like this.

class
AAA
{
    static decltype(*this) const make();  // Not working because there's no `this`.
};

The *this is used to describe what I want to do. I want to know some decltype() expression which can be resolved to AAA.

If it's possible how can I do that?

回答1:

In C++1y you could do this:

class
AAA
{
public:
    static auto make()
    {
        return AAA();
    }
};

int main()
{
    AAA aaa = AAA::make();
}

This is not legal in C++11 as you need to specify a return type for make(). In C++98/03/11 you can:

class
AAA
{
public:
    typedef AAA Self;

    static Self make()
    {
        return AAA();
    }
};

That is low-tech, but very readable.

<aside>

You should avoid returning const-qualified types by value. This inhibits efficient move semantics. If you want to avoid assigning to rvalues, then create an assignment operator qualified with &.

</aside>

回答2:

~~You could perhaps do something like this:~~

#include<iostream>

class AAA
{
   int _i;

   public:
   AAA(int i): _i(i) {}

   static auto make(int i) -> typename std::remove_reference<decltype(*this)>::type
   {
      return {i};
   }

   void print()
   {
      std::cout << _i << std::endl;
   }
};

int main()
{
    AAA aaa = AAA::make(1);
    aaa.print();
    return 0;
}

It compiles on GCC 4.7.2 at least :)

EDIT 26/11-13: The above code is not legal c++, even though it compiles with gcc, it does not with clang or icpc. My apologies.

回答3:

Just invented a way using member pointers, and it seems to work: https://godbolt.org/z/v-g5z0


    #define self_test(name) \
    void dummy__##name(void) {} \
    template  static T type__##name( void (T::*func)(void) ) { return T(); } \
    typedef decltype(type__##name(&dummy__##name)) self__##name; \
    static void test( void ) { self__##name::print(); }

    struct T1 {
      self_test(X);
      static void print() { printf( "this is T1\n" ); }
    };

    struct T2 {
      self_test(X);
      static void print() { printf( "this is T2\n" ); }
    };

    int main() {
      T1::test();
      T2::test();
    }

This is not perfect either, it requires -fpermissive to compile with gcc, but at least gcc/VS/Intel all compile it and it works. Also in fact gcc/mingw doesn't even require -fpermissive for this.

来源：https://stackoverflow.com/questions/20203640/how-to-refer-current-class-using-decltype-in-c11

标签

c++

c++11

static-methods

decltype