Modern C++ Programming Cookbook

There's more...

These non-member functions are mainly intended for template code where the container is not known and can be a standard container, a C-like array, or a custom type. Using the non-member version of these functions enables us to write simpler and less code that works with all these types of containers.

However, the use of these functions is not and should not be limited to generic code. Though it is rather a matter of personal preference, it can be a good habit to be consistent and use them everywhere in your code. All these methods have lightweight implementations that will most likely be inlined by the compiler, which means that there will be no overhead at all over using the corresponding member functions.