Overload a method or use default values? c++

Question

I'm still relatively new to C++ and I can't seem to figure out the difference in the following two ways of coding a function that may take one parameter or maybe two or three or more. Anyway, here's my point

function overload:

int aClass::doSomething(int required)
{
    //DO SOMETHING
}

int aClass::doSomething(int required, int optional)
{
    //DO SOMETHING
}

how is this different to, default value:

int aClass::doSomething(int required, int optional = 0)
{
    //DO SOMETHING
}

I know in different circumstances one may be more suitable than another but what kinds of things should I be aware of when choosing between each of these options?

Answer 1

There are several technical reasons to prefer overloading to default arguments, they are well laid out in Google's C++ Style Guide in the Default Arguments section:

Function pointers are confusing in the presence of default arguments, since the function signature often doesn't match the call signature. Adding a default argument to an existing function changes its type, which can cause problems with code taking its address. Adding function overloads avoids these problems.

and:

default parameters may result in bulkier code since they are replicated at every call-site -- as opposed to overloaded functions, where "the default" appears only in the function definition.

On the positive side it says:

Often you have a function that uses default values, but occasionally you want to override the defaults. Default parameters allow an easy way to do this without having to define many functions for the rare exceptions.

So your choice will depend on how relevant the negative issues are for your application.

Answer 2

First off, you're talking about overloading, not overriding. Overriding is done for virtual functions in a derived class. Overloading refers to the same function name with a different signature.

The difference is logical - in the first case (2 versions), the two functions can behave completely different, whereas the second case will have more or less the same logic. It's really up to you.