16 Library introduction [library]

16.4 Library-wide requirements [requirements]

16.4.4 Requirements on types and expressions [utility.requirements]

16.4.4.2 Template argument requirements [utility.arg.requirements]

1
#
The template definitions in the C++ standard library refer to various named requirements whose details are set out in Tables 28–35.
In these tables,
  • (1.1)
    T denotes an object or reference type to be supplied by a C++ program instantiating a template,
  • (1.2)
    a, b, and c denote values of type (possibly const) T,
  • (1.3)
    s and t denote modifiable lvalues of type T,
  • (1.4)
    u denotes an identifier,
  • (1.5)
    rv denotes an rvalue of type T, and
  • (1.6)
    v denotes an lvalue of type (possibly const) T or an rvalue of type const T.
2
#
In general, a default constructor is not required.
Certain container class member function signatures specify T() as a default argument.
T() shall be a well-defined expression ([dcl.init]) if one of those signatures is called using the default argument.
Table 28: Cpp17EqualityComparable requirements [tab:cpp17.equalitycomparable]
Expression
Return type
Requirement
a == b
decltype(a == b) models boolean-testable
== is an equivalence relation, that is, it has the following properties:
  • For all a, a == a.
  • If a == b, then b == a.
  • If a == b and b == c, then a == c.
Table 29: Cpp17LessThanComparable requirements [tab:cpp17.lessthancomparable]
Expression
Return type
Requirement
a < b
decltype(a < b) models boolean-testable
< is a strict weak ordering relation ([alg.sorting])
Table 30: Cpp17DefaultConstructible requirements [tab:cpp17.defaultconstructible]
Expression
Post-condition
T t;
object t is default-initialized
T u{};
object u is value-initialized or aggregate-initialized
T()
T{}
an object of type T is value-initialized or aggregate-initialized
Table 31: Cpp17MoveConstructible requirements [tab:cpp17.moveconstructible]
Expression
Post-condition
T u = rv;
u is equivalent to the value of rv before the construction
T(rv)
T(rv) is equivalent to the value of rv before the construction
rv's state is unspecified
[Note 1: 
rv must still meet the requirements of the library component that is using it.
The operations listed in those requirements must work as specified whether rv has been moved from or not.
— end note]
Table 32: Cpp17CopyConstructible requirements (in addition to Cpp17MoveConstructible) [tab:cpp17.copyconstructible]
Expression
Post-condition
T u = v;
the value of v is unchanged and is equivalent to u
T(v)
the value of v is unchanged and is equivalent to T(v)
Table 33: Cpp17MoveAssignable requirements [tab:cpp17.moveassignable]
Expression
Return type
Return value
Post-condition
t = rv
T&
t
If t and rv do not refer to the same object, t is equivalent to the value of rv before the assignment
rv's state is unspecified.
[Note 2: 
rv must still meet the requirements of the library component that is using it, whether or not t and rv refer to the same object.
The operations listed in those requirements must work as specified whether rv has been moved from or not.
— end note]
Table 34: Cpp17CopyAssignable requirements (in addition to Cpp17MoveAssignable) [tab:cpp17.copyassignable]
Expression
Return type
Return value
Post-condition
t = v
T&
t
t is equivalent to v, the value of v is unchanged
Table 35: Cpp17Destructible requirements [tab:cpp17.destructible]
Expression
Post-condition
u.~T()
All resources owned by u are reclaimed, no exception is propagated.
[Note 3: 
Array types and non-object types are not Cpp17Destructible.
— end note]