何时调用复制构造函数或赋值构造函数？

Question

Triny the quiz from web and see this code. The result prints is 02

Which means default copy constructor is used for list initialization while assignment constructor is used for vector. Why?

#include <algorithm>
#include <iostream>
#include <list>
#include <vector>

class Int
{
public:
    Int(int i = 0) : m_i(i) { }

public:
    bool operator<(const Int& a) const { return this->m_i < a.m_i; }

    Int& operator=(const Int &a)
    {
        this->m_i = a.m_i;
        ++m_assignments;
        return *this;
    }

    static int get_assignments() { return m_assignments; }

private:
    int m_i;
    static int m_assignments;
};

int Int::m_assignments = 0;

int main()
{
    std::list<Int> l({ Int(3), Int(1) });
    l.sort();
    std::cout << (Int::get_assignments() > 0 ? 1 : 0);

    std::vector<Int> v({ Int(2), Int() });
    std::sort(v.begin(), v.end());
    std::cout << (Int::get_assignments() > 0 ? 2 : 0) << std::endl;

    return 0;
}

Answer 1

Which means default copy constructor is used for list initialization while assignment constructor is used for vector

If you remove the std::sort(v.begin(), v.end()); instruction the program prints 00 . The assignment operator is used just for sorting.

NOTE : the list can be sorted simply by modifying pointers (so l.sort() doesn't require operator= ).