c++ - Copy constructor invoked 2 times, not 3 as expected? -

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here 1 program taken textbook featuring copy constructors:

#include <stdio.h> #include <conio.h> #include <iostream>  using namespace std;  class point {     private:         int x,y;     public:         point(int ox =0, int oy =0)         {             cout << " make object" << << endl;             cout << " using default constructor \n";             x = ox, y = oy;         }         point(const point &p)         {             cout << " make object" << << endl;             cout << " using copy constructor \n";             x = p.x, y = p.y;         }         void move(int dx, int dy);         void display(); };  point fct(point a);  int main() {     point a(5,2);     a.display();      point b = fct (a);     b.display();     getch();     return 0; }  void point::move(int dx, int dy) {     x += dx, y += dy; }  void point::display() {     cout << "coordinates :" << x << " " << y << "\n"; }  point fct(point a) {     point b=a;     //b.move(2,3);     return b; }

it should noted copy constructor of form : point (const point &p) instead of point (point &p) (the latter what's in textbook, couldn't compile had switch first one, still can't understand why :( )

the textbook said there 3 lines of "using copy constructor" , corresponding 3 calls copy constructor. think reason is, when call: b = fct(a)

  1. the function "fct" makes copy of (pass-by-value), therefor 1 call

  2. the line : point b = : again, copy constructor invoked

  3. the return value copied b (the variable in main, not 1 in fct) 3rd one.

however upon execution, there 2 calls. can give me explaination on this?

two copies occur because named return value optimization (nrvo) elides 1 of copies.

point fct(point a) {     point b=a;     return b; }  point b = fct (a);

the first copy 1 argument a in main parameter a in fct. occurs because point taken by-value.

the second copy a b inside func.

the copy elided 1 in returning b value. in case, b can directly allocated b @ call site, no copy takes place.


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