20 Library introduction [library]

Requirements describe constraints that shall be met by a C++ program that extends the standard library.

Such extensions are generally one of the following:

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.

Template argument requirements are sometimes referenced by name.

See [type.descriptions].

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