This means you may get lots of errors about things like strcmp not being found. This is the most explicit way of saying what you mean. You will need to do this for each function or type you wish to use from the standard library. Never do this in a header file, as you will be forcing users of your header file to do the same. Here are some features that have been reported as bugs, but are not.
Nested classes can access private types of the containing class.
Defect report 45 clarifies that nested classes are members of the class they are nested in, and so are granted access to private members of that class. Classes in exception specifiers must be complete types. The first two are different, when virtual base classes are involved. Exceptions don't work in multithreaded applications. You cannot throw an exception in one thread and catch it in another.