Question

Excuse me for the elementary question: In C++, should all functions be inside a class or non-global namespace? In what sort of circumstances should one write a global function?

Answer 1

Defining a function outside a class might be useful for operator overloading (specifically operator<<) or perhaps a template function that allows many of your classes to be used such as:

template <class T>
void
display (T obj)
{
  obj.display ();
}

Of course, that says nothing about being inside or outside a namespace. Namespaces are useful to prevent name clashes (e.g. a function named “download” can be present in multiple libraries, so namespaces are useful in that you can use libraryX::download ("hello") and Nlib::download ("hello") in the same program whereas without a namespace the compiler would be unable to pick which function to use). However, there is nothing preventing you from making a function global by declaring the function outside of a namespace, and in fact this is quite common. If you’re creating a library, I’d recommend using a namespace to prevent another library from confusing the compiler.