Is std::vector copying the objects with a push_back?

Yes, the push_back method of std::vector typically involves copying objects. When adding elements to std::vector, the push_back method copies the provided element to the container's internal storage. This process typically involves calling the copy constructor or move constructor of the element type (if applicable).

Example:

Suppose there is a simple class Widget:

cpp
class Widget {
public:
    Widget() {
        std::cout << "Default constructor" << std::endl;
    }
    
    Widget(const Widget&) {
        std::cout << "Copy constructor" << std::endl;
    }
    
    Widget(Widget&&) noexcept {
        std::cout << "Move constructor" << std::endl;
    }
    
    ~Widget() {}
};

When adding elements to std::vector<Widget>:

cpp
int main() {
    std::vector<Widget> v;
    Widget w;
    std::cout << "Adding to vector:" << std::endl;
    v.push_back(w);  // This will call the copy constructor of Widget
    v.push_back(std::move(w));  // This will call the move constructor of Widget
    return 0;
}

In the above code:

1. Creating an instance w of the Widget class will call the default constructor.
2. Using push_back(w) to add w to vector v will call the copy constructor.
3. Using push_back(std::move(w)) to add w to vector v will call the move constructor.

Optimization:

Since C++11, if the type supports move semantics, std::vector will prefer using the move constructor to reduce the cost of copying. Additionally, if you know you will be adding many elements, using the reserve method to pre-allocate sufficient memory can avoid reallocations and copying existing elements when adding new elements.

In summary, the push_back method of std::vector does involve copying objects, but the specific behavior (whether it is a copy or a move) depends on the element type and whether move semantics are enabled.