C When member copy constructors are called-CodePudding

Given the code

#include <iostream>
#include <vector>


class Entity {
public:
    Entity(const int x, const int y) :
        x(x), y(y)
    {
        this->entities.push_back(*this);
    }

    int x, y;
    std::vector<Entity> entities;
};


int main() {

    auto p = Entity(10, 20);

    std::cout << p.entities.size() << '\n';
    std::cout << p.x << ' ' << p.y << "\n\n";

    std::cout << p.entities[0].entities.size() << '\n';
    std::cout << p.entities[0].x << ' ' << p.entities[0].y << '\n';
    
    return 0;
}

When entities.push_back is called, the Entity should be copied to the vector. When copied all members should have their copy constuctors called in the following order: (x, y, entities). The problem is in the entities constructor, the "std::vector" constructor should call the "Entity" constructor that will call the "std::vector" constructor again, resulting in a infinite loop.

But according to the output:

The "std::vector" copy constructor is not called.

Where is my error? I'm forgetting some implementation detail?

CodePudding user response：

the "std::vector" constructor should call the "Entity" constructor that will call the "std::vector" constructor again, resulting in a infinite loop

Nope.

The std::vector<Entity> copy constructor will call the Entity copy constructor.

The Entity copy constructor copies the member vector as it currently exists (empty). It does not run the code in the two-element Entity constructor that push_back something to the vector.

CodePudding user response：

You're copying yourself in a constructor, which is generally a bad practice since the object is not yet fully constructed. In this case, the vector is still empty when you call push_back.