CODEWITHSUNDEEP
c++c++11
Giuseppe Pes
Giuseppe Pes

Reputation: 7912

Why can std::swap correctly swap custom objects?

I've written the following code as an example of a correct implementation of a swap function in c++.

#include <iostream>

class Complex
{
    int a;
    int b;
    friend std::ostream& operator<<(std::ostream&, const Complex&);
    friend void swap(Complex& rhs, Complex& lhs) noexcept;
public:
    Complex(): a(0), b(0) {};
    Complex(int a, int b): a(a), b(b) {};
};

void swap(Complex& rhs, Complex& lhs) noexcept
{
    using std::swap;
    swap(rhs.a, lhs.a);
    swap(rhs.b, lhs.b);
}

std::ostream& operator<<(std::ostream& os, const Complex& c)
{
    os << c.a << c.b;
    return os;
}

class Swapable
{
    int a;
    int b;
    Complex complex;
    friend std::ostream& operator<<(std::ostream&, const Swapable&);
    friend void swap(Swapable&, Swapable&) noexcept;
public:
    Swapable(): a(0), b(0) {};
    Swapable(int a, int b): a(a), b(b), complex(a+1, b+1) {};
};

void swap(Swapable& rhs, Swapable& lhs) noexcept
{
    std::cout << "Swapping" << std::endl;
    using std::swap;
    swap(rhs.a, lhs.a);
    swap(rhs.b, lhs.b);
    swap(rhs.complex, lhs.complex);
}

std::ostream& operator<<(std::ostream& os, const Swapable& s)
{
    os << s.a << s.b << s.complex;
    return os;
}

int main()
{
    Swapable s_1(1,1);
    Swapable s_2(2,2);

    std::cout << "Before swap" << std::endl;
    std::cout << s_1 << std::endl;
    std::cout << s_2 << std::endl;

    swap(s_1, s_2);

    std::cout << "After swap" << std::endl;
    std::cout << s_1 << std::endl;
    std::cout << s_2 << std::endl;

    std::swap(s_1, s_2); // It should fail. Shouldn't it?

    std::cout << "Second swap" << std::endl;
    std::cout << s_1 << std::endl;
    std::cout << s_2 << std::endl;
}

The output is :

Before swap
1122
2233
Swapping
After swap
2233
1122
Second swap
1122
2233

Every thing works as expected when I call the unqualified swap. However, I was expecting a compile error when I call the std::swap with my custom object. Why is the std::swap function able to swap correctly my custom object?

Upvotes: 2

Views: 1426

Answers (1)

songyuanyao
songyuanyao

Reputation: 172894

Class Swapable is MoveAssignable and MoveConstructible, then std::swap could work well with it.

Type requirements

This implies std::map could do the work with the move-assignment and move-construct operation provided by Swapable. Class Swapable meets the requirements, there're implicitly-declared move constructor and move assignment operator, (and implicitly-declared copy constructor and copy assignment operator) for it.

Upvotes: 3

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