Assignable (original) (raw)

SGI

Description

A type is Assignable if it is possible to copy objects of that type and to assign values to variables.

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 typepair<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 isEqualityComparable as well as Assignable, then a copy of xshould compare equal to x.