[utility.arg.requirements] (original) (raw)
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]
The template definitions in the C++ standard library refer to various named requirements whose details are set out in Tables 25–32.
In these tables, T is an object or reference type to be supplied by a C++ program instantiating a template;a,b, andc 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 25: Cpp17EqualityComparable requirements [tab:cpp17.equalitycomparable]
| == is an equivalence relation, that is, it has the following properties:If a == b and b == c, then a == c. |
|---|
Table 27: 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 28: 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 29: Cpp17CopyConstructible requirements (in addition to Cpp17MoveConstructible) [tab:cpp17.copyconstructible]
| the value of v is unchanged and is equivalent to u |
|---|
| the value of v is unchanged and is equivalent to T(v) |
Table 30: 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 31: 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 32: Cpp17Destructible requirements [tab:cpp17.destructible]
| All resources owned by u are reclaimed, no exception is propagated. |
|---|
| [Note 3: Array types and non-object types are not Cpp17Destructible. — _end note_] |