Small examples show copy elision in C++

#include <iostream>
using namespace std;

class Foo {
public:
    Foo() {cout<<"default constructor is called"<<endl;}
    Foo(const Foo& other) {cout<<"copy constructor is called"<<endl;}
    Foo(Foo&& other) {cout<<"move constructor is called"<<endl;}
};

Foo func()
{
    Foo f;
    return f;
}

int main()
{
    Foo a = func();
    return 0;
}

The compiler is clang 5.0.1:

# c++ --version
OpenBSD clang version 5.0.1 (tags/RELEASE_501/final) (based on LLVM 5.0.1)
Target: amd64-unknown-openbsd6.3
Thread model: posix
InstalledDir: /usr/bin

Build an execute it:

# c++ -std=c++11 test.cpp
# ./a.out
default constructor is called

You may expect Foo‘ copy constructor is called at least once:

Foo a = func();

However, the reality is that compiler may be clever enough to know the content in local variable f of func() will be finally copied to a, so it creates a first, and pass a into func(), like this:

Foo a;
func(&a);

Let’s modify the program:

#include <iostream>
using namespace std;

class Foo {
public:
    Foo() {cout<<"default constructor is called"<<endl;}
    Foo(const Foo& other) {cout<<"copy constructor is called"<<endl;}
    Foo(Foo&& other) {cout<<"move constructor is called"<<endl;}
};

Foo func(Foo f)
{
    return f;
}

int main()
{
    Foo a;
    Foo b = func(a);
    return 0;
}

This time, the temp variable f of func() is a parameter. Build and run it:

# c++ -std=c++11 test.cpp
# ./a.out
default constructor is called
copy constructor is called
move constructor is called

the temp variable fof func() is constructed by copy constructor:

Foo b = func(a);

In above statement, the func(a) returns a temporary variable, which is a rvalue, so the Foo‘s move constructor is used to construct b. If Foo doesn’t define move constructor:

Foo(Foo&& other) {cout<<"move constructor is called"<<endl;}

Then “Foo b = func(a);” will trigger copy constructor to initialize b.