Optional function parameters: Use default arguments (NULL) or overload the function?

Question

I have a function that processes a given vector, but may also create such a vector itself if it is not given.

I see two design choices for such a case, where a func

Answer 1

Generally I agree with others' suggestion to use a two-function approach. However, if the vector created when the 1-parameter form is used is always the same, you could simplify things by instead making it static and using a default const& parameter instead:

// Either at global scope, or (better) inside a class
static vector default_vector = populate_default_vector();

void foo(int i, std::vector const& optional = default_vector) {
    ...
}