Книга: Standard Template Library Programmer
Assignable
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Assignable
Category: utilities
Component type: concept
Description
A type is Assignable if it is possible to copy objects of that type and to assign values to variables.
Notation
X A type that is a model of Assignable
x, y Object of type X
Valid expressions
| Name | Expression | Return type |
|---|---|---|
| Copy constructor | X(x) |
X |
| Copy constructor | X x(y); X x = y; |
|
| Assignment | x = y [1] |
X& |
| Swap | swap(x,y) |
void |
Expression semantics
| Name | Expression | Semantics | Postcondition |
|---|---|---|---|
| Copy constructor | X(x) |
X(x) is a copy of x [2] | |
| Copy constructor | X(x) |
X(x) is a copy of x [2] | |
| Copy constructor | X x(y); X x = y; |
x is a copy of y [2] | |
| Assignment | x = y [1] |
x is a copy of y [2] | |
| Swap | swap (x,y) |
Equivalent to{ X tmp = x; x = y; y = tmp; } |
Models
• int
Notes
[1] One implication of this requirement is that a const type is not Assignable. For example, const int is not Assignable: if x is declared to be of type const int, then x = 7 is illegal. Similarly, the type pair<const int, int> is not Assignable.
[2] The reason this says "x is a copy of y ", rather than "x == y ", is that operator== is not necessarily defined: equality is not a requirement of Assignable. If the type X is EqualityComparable as well as Assignable, then a copy of x should compare equal to x.
See also
DefaultConstructible
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