c++ - Why is the copy constructor being used instead of the assignment operator -
this how myclass defined:
class myclass { double x, y; public: myclass (double = 0., double b = 0.) { x = a; y = b; cout << "using default constructor" << endl; } myclass (const myclass& p) { x = p.x; y = p.y; cout << "using copy constructor" << endl; } myclass operator =(const myclass& p) { x = p.x; y = p.y; cout << "using assignment operator" << endl; return *this; } }; and tested when each constructor or method called in main program:
int main() { cout << "myclass p" << endl; myclass p; cout << endl; cout << "myclass r(3.4)" << endl; myclass r(3.4); cout << endl; cout << "myclass s(r)" << endl; myclass s(r); cout << endl; cout << "myclass u = s" << endl; myclass u = s; cout << endl; cout << "s = p" << endl; s = p; cout << endl; } why copy constructor being used in fourth example, myclass u = s, instead of assignment operator?
edit
including output, asked:
myclass p using default constructor myclass r(3.4) using default constructor myclass s(r) using copy constructor myclass u = s using copy constructor s = p using assignment operator using copy constructor
because not actual assignment since declare u @ same time. thus, constructor called instead of assignment operator. , more efficient because if wasn't feature there have been redundancy of calling first default constructor , assignment operator. have evoked creation of unwanted copies , had deteriorate performance of c++ model significantly.
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