Avoid warning 'Unreferenced Formal Parameter'

Question

I have a super class like this:

class Parent
{
public:
    virtual void Function(int param);
};

void Parent::Function(int param)
{
    std::cout << param << std::endl;
}

..and a sub-class like this:

class Child : public Parent
{
public:
    void Function(int param);
};

void Child::Function(int param)
{
    ;//Do nothing
}

When I compile the sub-class .cpp file, I get this error

warning C4100: 'param' : unreferenced formal parameter

As a practice, we used to treat warnings as errors. How to avoid the above warning?

Thanks.

Answer 1

In C++ you don't have to give a parameter that you aren't using a name so you can just do this:

void Child::Function(int)
{
    //Do nothing
}

You may wish to keep the parameter name in the declaration in the header file by way of documentation, though. The empty statement (;) is also unnecessary.

Answer 2

Another technique that you can use if you want to keep the parameter name is to cast to void:

void Child::Function(int param)
{
    (void)param;   //Do nothing
}

Answer 3

As @Charles Bailey mentioned, you can skip the parameter name.

However, in certain scenarios, you need the parameter name, since in debug builds you are calling an ASSERT() on it, but on retail builds it's a nop. For those scenarios there's a handy macros (at least in VC++ :-)) UNREFERENCED_PARAMETER(), which is defined like this:

#define UNREFERENCED_PARAMETER(x) x

Note that the simple cast @R Samuel Klatchko posted also works, but I personally find it more readable if the code is explicit that this is an unreferenced parameter vs. simple unexplained cast like that.