20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.3 Requirements on types and expressions [utility.requirements]

20.5.3.1 Template argument requirements [utility.arg.requirements]

The template definitions in the C++ standard library refer to various named requirements whose details are set out in Tables 2027.
In these tables, T is an object or reference type to be supplied by a C++ program instantiating a template; a, b, and c are values of type (possibly const) T; s and t are modifiable lvalues of type T; u denotes an identifier; rv is an rvalue of type T; and v is an lvalue of type (possibly const) T or an rvalue of type const T.
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 20EqualityComparable requirements
Expression
Return type
Requirement
a == b
convertible to bool
== 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 21LessThanComparable requirements
Expression
Return type
Requirement
a < b
convertible to bool
Table 22DefaultConstructible requirements
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 23MoveConstructible requirements
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
:
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 24CopyConstructible requirements (in addition to MoveConstructible)
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 25MoveAssignable requirements
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
:
 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 26CopyAssignable requirements (in addition to MoveAssignable)
Expression
Return type
Return value
Post-condition
t = v
T&
t
t is equivalent to v, the value of v is unchanged
Table 27Destructible requirements
Expression
Post-condition
u.~T()
All resources owned by u are reclaimed, no exception is propagated.