6 Library introduction [library]

Requirements describe constraints that shall be met by a C++ program that extends the Ranges 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 or satisfy a concept

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 concepts (Concepts TS [dcl.spec.concept]). Concepts are stated in terms of well-defined expressions that define valid terms of the types that satisfy the concept. For every set of well-defined expression requirements there is a named concept 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 formal type parameters.

Template argument requirements are sometimes referenced by name. See ISO/IEC 14882:2014 §[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.2

Required operations of any concept defined in this document need not be total functions; that is, some arguments to a required operation may result in the required semantics failing to be satisfied. [ Example: The required < operator of the StrictTotallyOrdered concept ([concepts.lib.compare.stricttotallyordered]) does not meet the semantic requirements of that concept when operating on NaNs. — end example ] This does not affect whether a type satisfies the concept.

A declaration may explicitly impose requirements through its associated constraints (Concepts TS [temp.constr.decl]). When the associated constraints refer to a concept (Concepts TS [dcl.spec.concept]), additional semantic requirements are imposed on the use of the declaration.