Visual C++: Warning C4100

Warning 4100: unreferenced formal parameter might appear when C++ code is compiled at Warning Level 4 (/W4) with the Visual C++ compiler. For example, a function that generates C4100:

void Foo::OnEvent( const FooEvent& event )
{
    return;
}

There are a few ways to deal with this warning:

• If this warning made you aware that you had a redundant parameter, that is good! Remove the unused parameter from both the function declaration and the definition:

void Foo::OnEvent()
{
    return;
}

• If the function signature cannot be changed but you do not really use the parameter in the function body, then remove the parameter name, while retaining the parameter type:

void Foo::OnEvent( const FooEvent& )
{
    return;
}

• If you are playing around with the code in the function body and will need the parameter in the future, then comment the parameter temporarily:

void Foo::OnEvent( const FooEvent& /* event */ )
{
    // Some commented code that uses "event" parameter
    return;
}

• pragma warning directives can be used to disable the warning locally:

#pragma warning( push )
#pragma warning( disable: 4100 )
void Foo::OnEvent( const FooEvent& event )
{
    return;
}
#pragma warning( pop )

• If you are having many such functions generating warnings and want to do nothing to fix them, then use a pragma warning directive over the entire source file:

// Foo.cpp
#pragma warning( disable: 4100 )  // Disable for entire source file Foo.cpp

void Foo::OnEvent( const FooEvent& event )
{
    return;
}

A final note: Visual C++ 2010 does not seem to have this warning at all! It has been mysteriously removed without providing any reason.