The template definitions in the C++ standard library
refer to various named requirements whose details are set out in
Tables [tab:equalitycomparable]–[tab:destructible].
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 ([dcl.fct.default]).
Table
20 —
EqualityComparable requirements
|
Expression | Return type | Requirement |
|
a == b |
convertible to bool |
== is an equivalence relation,
that is, it has the following properties:
|
Table
21 —
LessThanComparable requirements
|
Expression | Return type | Requirement |
|
a < b |
convertible to bool |
< is a strict weak ordering relation ([alg.sorting]) |
Table
22 —
DefaultConstructible 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
23 —
MoveConstructible 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
24 —
CopyConstructible 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
25 —
MoveAssignable 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
26 —
CopyAssignable 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
27 —
Destructible requirements
|
Expression | Post-condition |
|
u.~T() | All resources owned by u are reclaimed, no exception is propagated. |