class constructor and object instanciation/initialisation C++/GCC

Question

I have troubles with a C++ class constructor, using GCC.

The class "foo", hereunder, is supposed to emulate a processor register like AL, AH, AX, EAX,... and i need to have some basic arithmetic associated with this class. but i have a strange behaviour in the initialisation or a "foo" object.

I don't have have the same result for the 2 following cases:

foo w=0x12345678; //  case 1

foo w ;           //  case 2 init ( 2 steps)
    w=0x12345678;

For me, case 2 is working GCC calls foo() ( constructor 1) then the = operator. At the end, w.m_val is ok But for case 1 GCC calls directly foo( long *) ( constructor 2) and nothing else. Obviously that's not what i'm expecting.

If "foo" where char , int or long, the result would be the same for both cases.

I maybe misunderstood something about constructors or did something wrong. Could somebody help me ?

Thanks.

class foo
{
    public:
        foo(){ // constructor 1 
          m_val=0;
          m_ptr=NULL;
          };

        foo(long *p){ // constructor 2, should never be called!!!
          m_val=0;
          m_ptr=p;
        };
        friend foo  operator+( const foo &rhs, const unsigned int v );

        foo &operator+= (unsigned int v )
        {
            m_val+=v;
            return *this;
        }

        ~foo(){};

         foo &operator= ( const foo &rhs )
         {
            m_val=rhs.m_val;
            return *this;
         };
         foo &operator= ( const unsigned int v )
         {
            m_val=v;
            return *this;
         };

    private:
        unsigned int m_val;
        long *m_ptr;
};

the problem is not in the posted code (as I write this). post a complete, minimal example that exhibits the problem. — Cheers and hth. - Alf
I tried your code on a different compiler (Visual Studio 2010) and I got a compiler error on the line foo w=0x12345678; which is what I would expect. So either you have found a compiler bug (unlikely but not impossible) or you haven't posted your real code. — john

Mankarse Mankarse · Accepted Answer · 2012-11-17T09:38:26

You are misunderstanding something about constructors. Despite appearances, foo w=0x12345678; does not contain any calls to an assignment operator. It is copy-initialization, and (mostly) equivalent to:

foo w{foo{0x12345678}};

This should not compile, because foo does not have a constructor that takes an int parameter.

I say "mostly" because foo w=0x12345678; requires the existence of an implicit conversion from int to foo (or a subclass of foo), where foo w{foo{0x12345678}}; involves an explicit conversion, and will never construct a subclass of foo.

class constructor and object instanciation/initialisation C++/GCC

1 Answers