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This article introduces the knowledge of why copy assignment is safe for self-assignment in C++. Many people will encounter this dilemma in the operation of actual cases. next, let the editor lead you to learn how to deal with these situations! I hope you can read it carefully and be able to achieve something!

C.62: secure copy assignment to self-assignment Reason (reason)

If xchangx changes the value of x, people will find it strange, and at the same time, very bad errors will occur. (usually contains leaks)

Example (sample)

The standard library container handles self-assignment in an elegant and efficient way:

Std::vector v = {3,1,4,1,5,9}

V = v

/ / the value of v is still {3,1,4,1,5,9} Note (attention)

The default assignment operation that arises from a member who correctly handles self-assignment will deal with the self-assignment problem.

Struct Bar {

Vector v

Map m

String s

}

Bar b

/ /...

B = b; / / correct and efficientNote (note)

You can prevent self-assignment by explicitly checking for self-assignment, but it is usually faster and more elegant not to use the above-mentioned checking methods (such as using swap).

Class Foo {

String s

Int i

Public:

Foo& operator= (const Foo& a)

/ /...

}

Foo& Foo::operator= (const Foo& a) / / OK, but there is a cost

{

If (this = & a) return * this

S = a.s

I = a.i

Return * this

}

This seems safe and efficient. But what if there is only one self-assignment in a million assignments? There are about a million extra checks (but since the results are always the same in nature, the computer's branch predictions are right every time). Consider the following code:

Foo& Foo::operator= (const Foo& a) / / simpler, and probably much better

{

S = a.s

I = a.i

Return * this

}

Std::string is safe for self-assignment, and so is int. All the costs come from (rarely) self-assignment.

Enforcement (implementation recommendations)

The (simple) assignment operator should not contain the following check: if (this = = & a) return * this

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