It's not uncommon for a function to have some requirements regarding its parameters. Each requirement should be stated as a precondition. If a function guarantees that its result has some properties – for example, it is non-negative – the function should make that clear as well. Some developers resort to placing comments to inform others about this, but it doesn't really enforce the requirement in any way. Placing if statements is better, but hides the reason for the check. Currently, the C++ standard still doesn't offer a way to deal with this (contracts were first voted into the C++20 standard, just to be removed later on). Fortunately, libraries such as Microsoft's Guideline Support Library (GSL) provide their own checks.
Let's assume that, for whatever reason, we're writing our own queue implementation. The push member function could look like this:
template<typename T>
T& Queue...