Suppose you want to write a template function that accepts any specialization of std::vector? What may work today may not work tomorrow.

On Writing Functions That Accept Any Specialization of a C++ Template Type

by Raymond Chen

From the article:

Suppose you want to write a template function that accepts any specialization of std::vector. Your first try would probably be something like this:

template<typename Value>
void accept_any_vector(std::vector<Value> v);

However, this does not actually accept any vector specialization. There is a second template argument for the allocator, which has a default value that nearly everyone uses. But if somebody has a vector with a custom allocator, then your function won’t match.