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:

• (1.1)

  Template arguments

• (1.2)

  Derived classes

• (1.3)

  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.

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