This is a special case of the copy elision called return value optimization (the link explains precisely your case).
constructor and copy constructor behaviour in c++ [duplicate]
-
11-06-2023 - |
문제
I have the following code:
#include <iostream>
using namespace std;
class X
{
public:
int g;
X() { cout << "constr" << endl; }
X(const X& ref1) { cout << "copy constr" << endl; }
};
X f()
{
X ee;
ee.g = 1;
return ee;
}
int main()
{
X ff = f();
return 0;
}
Running the code I see that the constructor was called only once and the copy constructor was never called. Don't you expect two constructor and one copy constructor calls here? Thanks!
해결책
다른 팁
Copy Elision is an optimization implemented by many compilers to prevent extra, unnecessary, copies. Makes the return-by-value or pass-by-value possible in practice.
Take a look at the example in the following answer: https://stackoverflow.com/a/12953129/1938163
struct C {
C() {}
C(const C&) { std::cout << "A copy was made.\n"; }
};
C f() {
return C();
}
int main() {
std::cout << "Hello World!\n";
C obj = f();
}
(http://en.wikipedia.org/wiki/Return_value_optimization#Summary)
Perhaps incredible to believe the first time, depending on the compiler & settings, the following outputs are all valid:
Hello World!
A copy was made.
A copy was made.
Hello World!
A copy was made.
Hello World!
In your case, this is a special copy elision optimization called RVO - Return Value Optimization where an object returned by value from a method has its copy elided.