Examples for move semantics (C++11)

move_semantics.cpp

#include <string>
#include <iostream>
#include <iomanip>
#include <utility>

struct A
{
	std::string s;
	A() : s("test") { }
	A(const A& o) : s(o.s) { std::cout << "move failed!\n"; }
	A(A&& o) noexcept : s(std::move(o.s)) { std::cout << "move success!\n"; }
};

A f(A a)
{
	return a;
}

struct B : A
{
	std::string s2;
	int n;
	// implicit move constructor B::(B&&)
	// calls A's move constructor
	// calls s2's move constructor
	// and makes a bitwise copy of n
};

struct C : B
{
	~C() { } // destructor prevents implicit move constructor C::(C&&)
};

struct D : B
{
	D() { }
	~D() { }          // destructor would prevent implicit move constructor D::(D&&)
	D(D&&) = default; // forces a move constructor anyway
};

int main()
{
	std::cout << "Trying to move A\n";
	A a1 = f(A()); // move-constructs from rvalue temporary
	A a2 = std::move(a1); // move-constructs from xvalue

	std::cout << "Trying to move B\n";
	B b1;
	std::cout << "Before move, b1.s = " << std::quoted(b1.s) << "\n";
	B b2 = std::move(b1); // calls implicit move constructor
	std::cout << "After move, b1.s = " << std::quoted(b1.s) << "\n";

	std::cout << "Trying to move C\n";
	C c1;
	C c2 = std::move(c1); // calls copy constructor

	std::cout << "Trying to move D\n";
	D d1;
	D d2 = std::move(d1);
}