What are helper functions in C++?

"Helper functions" are described in Bjarne Stroustrups book, and I was just reading about them yesterday. According to Stroustrup good design of a class should keep the number of functions implementing a class to a minimum. You dont want to have 50 functions in a class, according to Stroustrup. Instead you use "helper functions" that use the class interface (call the member functions). They could perhaps (not sure about this) be defined in a shared namespace to give meaning to their "relationship". You can find the paragraph in the book in chapter 9 section 9.7.5

"helper function" is not a term that you would find in a standard, neither it has an exact definition... standard mentions "helper class" or "helper template" few times to refer to a class, which is not meant to be instantiated by end-users but it provides an useful functionality internally used within another class.

Helper functions are (what I believe the most people mean when they say it) usually functions that wrap some useful functionality that you're going to reuse, most likely over and over again. You can create helper functions meant to be used for many different kinds of purposes...

An example might be conversion function of any kind, for example function converting multi-byte encoded std::string to std::wstring:

std::wstring s2ws(const std::string& str)
{
    int size_needed = MultiByteToWideChar(CP_UTF8, 0, &str[0], (int)str.size(), NULL, 0);
    std::wstring wstrTo( size_needed, 0 );
    MultiByteToWideChar(CP_UTF8, 0, &str[0], (int)str.size(), &wstrTo[0], size_needed);
    return wstrTo;
}

An example could be the input validation function that you will be reusing in the entire main function. Let's say you have a program that asks for the user's age, since age is an integer > 0, you'll need to have a separate function that takes care of the "cin >> users_age;". If the input satisfies the condition statement then proceed, otherwise ask the user to re-enter their age.

This is just an example of "helper function". Correct me readers if I'm wrong. Thanks!

There is a great definition of a helper function from the CppCoreGuidline:

A helper function is a function (usually supplied by the writer of a class) that does not need direct access to the representation of the class, yet is seen as part of the useful interface to the class. Placing them in the same namespace as the class makes their relationship to the class obvious and allows them to be found by argument dependent lookup.

For more info you can check the paragraph with a clear example, from which the upper quote is taken.