How to mock a function with the signature `object ()`

Question

I want to mock a method with the declaration A::B X(void). The definition is something as follows.

class A {
    class B;
    virtual B X() = 0;
};

class A::B {
  public:
    auto_ptr<int> something;
};

My mock class, following this, is quite standard.

class mA : public A
{
  public:
    MOCK_METHOD0(X, A::B());
};

Compiled, however, this gives me this weirdo error, and I haven't been able to track it down. What is wrong with this?

In member function ‘virtual A::B mA::X()’:
...: error: no matching function for call to ‘A::B::B(A::B)’
...: note: candidates are: A::B::B()
...:                       A::B::B(A::B&)

---

Update I have found a failing code sample to demonstrate this.

#include <gmock/gmock.h>
#include <memory>
using std::auto_ptr;

class thing {
  public:
    class result;
    virtual result accessor () = 0;
};

class thing::result {
    auto_ptr<int> x;   // If this just "int", error goes away.
};

namespace mock {
    class thing : ::thing {
      public:
        MOCK_METHOD0 ( accessor, result() );
    };
}

Answer 1

It is hard to tell without the definitions of A and B. sounds like it is trying to construct a B from a temporary and failing because it can't bind the temporary to a non-const reference.

For example, your copy constructor might be defined as:

class A {
 public:
  class B {
   public:
    // This should be const, without good reason to make it otherwise.
    B(B&); 
  };
};

With the fix just making it a const reference.