Copy constructor C++ -

sorry struggling grasp copy constructors,i know why copy constructor invoked when call object in function value "op.return_value(op)<

class operation{    public:        int add(int x,int y){            *total=x+y;        return (*total);        }         operation(){        cout<<"this constructor"<<endl;        total=new int;        }        operation(const operation &op){        cout<<"this copy of start"<<endl;        total=new int;        *total=*op.total;        }         ~operation(){        cout<<"this end";        delete total;        }          int *total;         int return_value(operation op){         return *total;        }  };  class child_operation:operation{     public:         int sub(int x,int y){            *total=x-y;        return(*total);        }  };    int main() {    operation op;    child_operation op1;     cout<<op.add(5,6)<<endl<<op1.sub(6,5)<<endl;    cout<<op.return_value(op)<<endl; } 

basically in ways copy constructor invoked?

it pretty straightforward, , explicitly stated in standard (12.8.1):

a class object can copied in 2 ways, initialization (including function argument passing , function value return) , assignment. conceptually, these 2 operations implemented copy constructor , copy assignment operator.

so, if initializing new object, either explicitly, or implicitly (e.g. passing object function by copy, case, or returning object function), copy constructor called:

myclass = b;//initialization, copy constructor called //or void foo(myclass a){}//a passed value, copy made, , copy constructor called //or myclass foo() {     myclass result;     return result;//a copy of result returned, , copy constructor called } 

on contrary, if making assignment, operator= called instead, e.g.

myclass x; myclass y; y = x;//assignmanet