Class overhead in C++

Question

I have a class e.g.:

class Vehicle {
  public:
    Vehicle(int handle);

    // Methods that use the handle e.g.:
    Color getColor() {
        return VEHICLE::GET_COLOR(handle);
    }

  protected:
    int handle;
};

I don't know if that example makes sense to you, but I built several wrapper classes around those handles to get a more OOP style of coding.

So my question now is, how much overhead is there when I pass a Vehicle object over just passing the vehicle handle to other methods?

Answer 1

If you split your class in a header and source file and use it in other compilation units there will be a sligth overhead due to the call to the constructor.

To solve this, the definition of your constructor has to be placed within your header file, so that the compiler can inline it.

You can do this with either changing your class declaration:

class Vehicle {
public:
    Vehicle(int handle)
     : handle(handle)
    {
    }
...

or by putting the definition inside your headerfile and decorating it with the inline keyword.

class Vehicle {
public:
    Vehicle(int handle);
...
}

inline Vehicle::Vehicle(int handle)
 : handle(handle)
{
}

Notice that there is no gurantee that your function will be inlined, but probably every major compiler out there will be able to do it.

Also notice, that additional work in your constructor, e.g handle - 1, will also very likely result in a overhead.

If your class is polymorphic or bigger, there might be additional overhead.