C ++错误C2440：“正在初始化”：无法从“类名”转换为“相同的类名”

Question

typedef unsigned int uint32_t;
struct Point {
    uint32_t x, y;
    Point() :x(0), y(0) {}
    Point(Point& p) {
        x = p.x;
        y = p.y;
    }
    Point operator=(Point& p) {
        x = p.x;
        y = p.y;
        return *this;
    }
};
struct Snake {
    Point pp;
    Point p() { return pp; }
};
int main() {
    Snake s;
    Point p1;
    Point p2 = p1; //ok
    Point p3 = s.pp; //ok
    Point wtf = s.p(); //error C2440??
}

I tried to compile the code above and get this weird error:

error C2440: 'initializing': cannot convert from 'Point' to 'Point'

note: Cannot copy construct struct 'Point' due to ambiguous copy constructors or no available copy constructor

I figured out 2 ways to make it works: changing

Point p() { return pp; }

into

Point& p() { return pp; }

or adding a move constructor to "Point":

Point(Point&& p) {
    x = p.x;
    y = p.y;
}

I decided to go with the second option. But I discover an even weirder problem:

I added some "debug code":

#include <iostream>
using std::cout;
typedef unsigned int uint32_t;
struct Point {
    uint32_t x, y;
    Point() :x(0), y(0) {
        cout << "\tPoint()\n";
    }
    Point(Point& p) {
        x = p.x;
        y = p.y;
        cout << "\tPoint(point& p)\n";
    }
    Point& operator=(Point& p) {
        x = p.x;
        y = p.y;
        cout << "\tPoint& operator=(Point& p)\n";
        return *this;
    }
    Point(Point&& p) {
        x = p.x;
        y = p.y;
        cout << "\tPoint(Point&& p)\n";
    }
};
struct Snake {
    Point pp;
    Point p() { return pp; }
};
int main() {
    cout << "Code 0:\n";
    Snake s;
    cout << "Code 1:\n";
    Point p1;
    cout << "Code 2:\n";
    Point p2 = p1; //ok
    cout << "Code 3:\n";
    Point p3 = s.pp; //ok
    cout << "Code 4:\n";
    Point wtf = s.p(); //error C2440??

    char c;
    std::cin >> c;
}

Which gives me:

Code 0:
    Point()
Code 1:
    Point()
Code 2:
    Point(point& p)
Code 3:
    Point(point& p)
Code 4:
    Point(point& p)

Which means the move constructor was not called at all.

Why does my first code not work? And why is move constructor not called even though it's the one fixing the problem?

(I use visual studio 2017)

Answer 1

A copy constructor and a copy assignment operator take their input parameter by const reference, but yours are both missing the const .

Also, assignment operators return *this by reference , but yours is returning by value .

Try this:

struct Point {
    uint32_t x, y;

    Point() : x(0), y(0) {}

    Point(const Point& p) : x(p.x), y(p.y) {}

    Point& operator=(const Point& p) {
        x = p.x;
        y = p.y;
        return *this;
    }
};

Note, however, that your struct is trivial, so you should let the compiler generate a default copy constructor and default copy assignment operator for you. Its implementations will suffice for your example:

struct Point {
    uint32_t x = 0;
    uint32_t y = 0;
};

Lastly, there is no need to manually define your own uint32_t type. C++11 has a standard uint32_t type defined in the <cstdint> header.

Answer 2

Your move constructor is not used because of copy elision, that is happening when you return from sp() And the reason you still need move constructor because before c++17 copy elision is optional so you have to have move constructor in case compiler decides not to use copy elision. And VS2017 is c++14 at the moment I think.