20 Library introduction [library]

20.4 Method of description (Informative) [description]

20.4.1 Structure of each clause [structure] Requirements [structure.requirements]

Requirements describe constraints that shall be met by a C++ program that extends the standard library.
Such extensions are generally one of the following:
  • Template arguments
  • Derived classes
  • 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 ([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.
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.157
Although in some cases the code given is unambiguously the optimum implementation.