Containers, iterators, and algorithms that meet an interface convention
The string and iostream components use an explicit representation of operations required of template arguments. They use a class template char_traits to define these constraints.
Interface convention requirements are stated as generally as possible. Instead of stating “class X has to define a member function operator++()”, the interface requires “for any object x of class X, ++x is defined”. That is, whether the operator is a member is unspecified.
Requirements are stated in terms of well-defined expressions that define valid terms of the types that satisfy the requirements. For every set of well-defined expression requirements there is a table that specifies an initial set of the valid expressions and their semantics. Any generic algorithm (Clause [algorithms]) that uses the well-defined expression requirements is described in terms of the valid expressions for its template type parameters.
In some cases the semantic requirements are presented as C++ code. Such code is intended as a specification of equivalence of a construct to another construct, not necessarily as the way the construct must be implemented.156
Although in some cases the code given is unambiguously the optimum implementation.