Error passing object reference

[deathmetal]

#include <Data.h>
int main(int arg_count, char** argv)
{
    if(arg_count!=3){
        cerr<<"Files misisng\n";
        exit(1);
    }

    Data object;
    object.read_ref(argv[1]); //
    object.read_mmp(argv[2]); //


    std::map<std::string,Polymorphism> temp= object.ret_poly();
    object.test(temp);// this works completely fine.. 
    //object.test(object.ret_poly()); // this doesn't work

    return 0;

}

Data.h:

class Data
{
    public:
        /** Default constructor */
        Data();
        void read_ref(const char* ); 
        void read_mmp(const char*);
        void test(std::map<std::string,Polymorphism>&); //some issue if I use  object.test(object.ret_poly());
        void set_poly(std::map<std::string, Polymorphism>&);
        std::map<std::string, Polymorphism> ret_poly();

    protected:

    private:
    std::map<std::string, Polymorphism> polymorphisms;
    std::map<std::string, std::string> reference;

};

Data.cpp file

void Data::set_poly(std::map<std::string, Polymorphism>& t_poly){
    polymorphisms=t_poly;

}

std::map<std::string, Polymorphism> Data::ret_poly(){
    return(polymorphisms);
}

void Data::test(std::map<std::string,Polymorphism>& temp_poly){
}

//object.test(object.ret_poly());
This actually should work, because it is passing address of polymorphisms object.
I have tried changing prototype of test in Data.h, but failed.

Kindly help me understand, passing object address/pointers in C++.

[laserlight]

Does test need to change the map? If it doesn't then change its parameter to be a const reference. If it does change the map, then perhaps ret_poly should return a reference.

[deathmetal]

Hi laserlight,
No, map doesn't need to be changed, I shall change it to const.
Can you please help me understand why //object.test(object.ret_poly()); fails?

Does test need to change the map? If it doesn't then change its parameter to be a const reference. If it does change the map, then perhaps ret_poly should return a reference.

[laserlight]

ret_poly returns a copy of the map, so when you have test's parameter be a non-const reference, the non-const reference parameter, not being an rvalue reference, cannot bind to the temporary object returned by the function. If it was a const reference then there's no problem as a const reference can be bound to such temporaries.

Another thing to consider: does test need to have a parameter at all? Maybe it should be a const member function that operates on the member variable directly. If you prefer for test to have a parameter, and it does not act on any member directly, then perhaps it should be a non-member non-friend function.

[deathmetal]

Sorry didn't understand. Got lost in reference, bind.
I am noob in C++/C/pointer/reference world.

[Paul McKenzie]

Sorry didn't understand. Got lost in reference, bind.
I am noob in C++/C/pointer/reference world.

So why the overly complex code? You get the same error here:

int foo()
{
   return 10;
}

int main()
{
    int& x = foo();
}

That is basically what your code is doing, and that is trying to assign a temporary value to a non-const reference. This doesn't work because foo() returns a temporary int, and you cannot assign a temporary to a non-const reference. If you could then this code would make sense:

int foo()
{
   return 10;
}

int main()
{
    int& x = foo();
    x = 20;
}

The foo() returns a temporary, and that temporary disappears as soon as the first line finishes executing. So when you get to "x = 20", what is "x" referring to? Whatever it is, it is gone before the assignment of 10 is done. This is why you get the error.

Regards,

Paul McKenzie