Can you invoke an instantiated object's class constructor explicity in C++?

Question

After creating a instance of a class, can we invoke the constructor explicitly? For example

class A{
    A(int a)
    {
    }
}

A instance;

instance.A(2);

Can we do this?

Answer 1

You can use placement new, which permits

new (&instance) A(2);

However, from your example you'd be calling a constructor on an object twice which is very bad practice. Instead I'd recommend you just do

A instance(2);

Placement new is usually only used when you need to pre-allocate the memory (e.g. in a custom memory manager) and construct the object later.