cannot convert parameter 1 from 'std::_Vector_iterator<_Myvec>' to 'std::_Vector_iterator<_Myvec>'

Question

I have a question, its a bit hard to discribe it so be easy on me please.

I have two classes, A and B, class A have a private member- vector:

class A
{
private:
    struct complex
    {
       int x;
       vector< int > y;
    };

    vector< complex > m_resultVector; // <---- the private member

public:
    void getPointerToVector( vector< complex >::iterator it )
    {
        it = m_resultVector.begin();
    }
};

I need to get access (only read) from class B, to this m_resultVector; , I could write a get function but m_resultVector is very long vector and I don't want to copy the entire vector to a new one, I want to send its pointer. also the important part- I need class B cannot change the content of m_resultVector

class B
{
    struct complex
    {
        int x;
        vector< int > y;
    };

    void functionOf_B()
    {
        A class_A;
        vector< complex >::iterator p_resultVector;

        class_A.getPointerToVector(p_resultVector); // <------ compilation error here

        // some reading from the content of p_resultVector
    }
};

when I try to compile it, I get the error:

cannot convert parameter 1 from 'std::_Vector_iterator<_Myvec>' to 'std::_Vector_iterator<_Myvec>'

so basically, I have to questions-

1. why do I get this error? the complex struct is defined in both classes.
2. where and how do I need to declare on const iterator on class B, so it will be read only? I'm not sure ...

Answer 1

That is because A::complex and B::complex are different types (with same content, but that does not matter). So that vector<A::complex> and vector<B::complex> are different. Move definition of struct complex outside A and B .

Also there are more issues in your code. A::getPointerToVector does nothing, because it copies input vector iterator to temporary one, assigns a value to it and after return from that function, everything is lost. Using this approach, you would have to pass vector<complex>::iterator as reference (thus vector<complex>::iterator& ).

I would rather write a method in A like this

const vector<complex>& get_vector() const
{
    return m_resultVector;
}

I this way, you can doo this.

void foo()
{
    A class_A;
    // do something with A
    const vector<complex>& p_result = class_A.get_vector();

    // now you are holding whole vector and you can call methods
    // defined as const (which does not modify that vector)
    p_result.begin();
    p_result.at(0);
    p_result.end();
}

Answer 2

Zereges solution seems good. But I understand you do not want to return vector. I could not come up with any solution other than below.

In Class A:

void getPointerToVector(int position, int &var,vector<int>& o_Vec)
{

    vector<complex>::iterator itr;
    complex c;
    c = m_resultVector.at(position);
    var = c.x;
    o_Vec = c.y;

}

In class B :

 void functionOf_B()
{

    A class_A;
    int aVar;
    std::vector<int> vec;
    class_A.getPointerToVector(2, aVar, vec);


        // some reading from the content of p_resultVector

}

I am not sure how efficient and complex this is. I would suggest better to use Zereges solution