16 Library introduction [library]

16.3 Method of description [description]

16.3.2 Structure of each clause [structure] Detailed specifications [structure.specifications]

The detailed specifications each contain the following elements:
  • name and brief description
  • synopsis (class definition or function declaration, as appropriate)
  • restrictions on template arguments, if any
  • description of class invariants
  • description of function semantics
Descriptions of class member functions follow the order (as appropriate):139
  • constructor(s) and destructor
  • copying, moving & assignment functions
  • comparison operator functions
  • modifier functions
  • observer functions
  • operators and other non-member functions
Descriptions of function semantics contain the following elements (as appropriate):140
  • Constraints: the conditions for the function's participation in overload resolution ([over.match]).
    [Note 1: 
    Failure to meet such a condition results in the function's silent non-viability.
    — end note]
    [Example 1: 
    An implementation can express such a condition via a constraint-expression ([temp.constr.decl]).
    — end example]
  • Mandates: the conditions that, if not met, render the program ill-formed.
    [Example 2: 
    An implementation can express such a condition via the constant-expression in a static_assert-declaration ([dcl.pre]).
    If the diagnostic is to be emitted only after the function has been selected by overload resolution, an implementation can express such a condition via a constraint-expression ([temp.constr.decl]) and also define the function as deleted.
    — end example]
  • Preconditions: the conditions that the function assumes to hold whenever it is called; violation of any preconditions results in undefined behavior.
  • Effects: the actions performed by the function.
  • Synchronization: the synchronization operations ([intro.multithread]) applicable to the function.
  • Postconditions: the conditions (sometimes termed observable results) established by the function.
  • Result: for a typename-specifier, a description of the named type; for an expression, a description of the type of the expression; the expression is an lvalue if the type is an lvalue reference type, an xvalue if the type is an rvalue reference type, and a prvalue otherwise.
  • Returns: a description of the value(s) returned by the function.
  • Throws: any exceptions thrown by the function, and the conditions that would cause the exception.
  • Complexity: the time and/or space complexity of the function.
  • Remarks: additional semantic constraints on the function.
  • Error conditions: the error conditions for error codes reported by the function.
Whenever the Effects element specifies that the semantics of some function F are Equivalent to some code sequence, then the various elements are interpreted as follows.
If F's semantics specifies any Constraints or Mandates elements, then those requirements are logically imposed prior to the equivalent-to semantics.
Next, the semantics of the code sequence are determined by the Constraints, Mandates, Preconditions, Effects, Synchronization, Postconditions, Returns, Throws, Complexity, Remarks, and Error conditions specified for the function invocations contained in the code sequence.
The value returned from F is specified by F's Returns element, or if F has no Returns element, a non-void return from F is specified by the return statements ([stmt.return]) in the code sequence.
If F's semantics contains a Throws, Postconditions, or Complexity element, then that supersedes any occurrences of that element in the code sequence.
For non-reserved replacement and handler functions, [support] specifies two behaviors for the functions in question: their required and default behavior.
The default behavior describes a function definition provided by the implementation.
The required behavior describes the semantics of a function definition provided by either the implementation or a C++ program.
Where no distinction is explicitly made in the description, the behavior described is the required behavior.
If the formulation of a complexity requirement calls for a negative number of operations, the actual requirement is zero operations.141
Complexity requirements specified in the library clauses are upper bounds, and implementations that provide better complexity guarantees meet the requirements.
Error conditions specify conditions where a function may fail.
The conditions are listed, together with a suitable explanation, as the enum class errc constants ([syserr]).
To save space, items that do not apply to a class are omitted.
For example, if a class does not specify any comparison operator functions, there will be no “Comparison operator functions” subclause.
To save space, elements that do not apply to a function are omitted.
For example, if a function specifies no preconditions, there will be no Preconditions: element.
This simplifies the presentation of complexity requirements in some cases.