Copy elision is a compiler optimization that eliminates unnecessary copy or move operations when a temporary object is constructed directly into its destination. In C++17 and later, copy elision is guaranteed in certain contexts (like returning a temporary from a function), making it semantically part of the language, not just an optimization.

Copy elision improves performance automatically; rely on it but don't make code depend on side effects of copy constructors running or not running.

This example shows how the compiler elides copies when returning temporaries.

// compile: g++ -std=c++17 -o elision elision.cpp
// run: ./elision
// description: copy elision eliminates unnecessary copies of temporaries
 
#include <iostream>
#include <string>
 
class Widget {
public:
    Widget() { std::cout << "Default constructor\n"; }
    Widget(const Widget&) { std::cout << "Copy constructor\n"; }
    Widget(Widget&&) { std::cout << "Move constructor\n"; }
};
 
Widget makeWidget() {
    Widget w;
    return w;  // copy elision: w constructed directly in return value
}
 
int main() {
    Widget w = makeWidget();  // copy elision: temporary constructed directly into w
 
    return 0;
}