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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.

Refinement of

Associated types

Notation

`X`	A type that is a model of Assignable
`x`, `y`	Object of type `X`

Definitions

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; }

Complexity guarantees

Invariants

Models

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.