Question

#include <iostream>
#include <vector>
#include <cassert>

class a_class
{
public:

    int num_IN;

    a_class():num_IN(0){}
    a_class(a_class const & origin){/*Initialise();*/}   //if not called here, error occurs

    void Initialise(){num_IN =5;}
};

int main () 
{
    std::vector <a_class> the_vector;

    for(int q=0; q < 30; q++)
    {
        the_vector.push_back(a_class());
        the_vector[q].Initialise();             
        assert(5 == the_vector[q].num_IN);      //no problem here
    }

    for(int q=0; q < 30; q++)
        assert(the_vector[q].num_IN == 5);      //assertion fails
}

I don’t understand the difference between calling this from outside vs. inside the CC. I also don’t know why it should cause a problem in any case.

Answer 1

std::vector may reallocate the buffer it uses if it’s size outgrows it, in which case it has to copy the old elements over to the new buffer. If you don’t have a proper copy constructor that copies num_IN over, the old value is lost.

Fix this by providing a proper copy constructor:

a_class(a_class const & origin) : num_IN(origin.num_IN) {}

In the code posted the copy constructor isn’t even needed – if you don’t provide one the compiler will generate a suitable one here.