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This article mainly explains "how to make mobile operations secure for self-assignment in C++", interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Next, let the editor take you to learn "how to make mobile operations secure for self-assignment in C++"!

C.65: make mobile operations secure to self-assigned Reason (reason)

If xchangx changes the value of x, people will be surprised and serious errors can occur. Although people generally do not use self-assignment directly in mobile operations, it still happens. But because std::swap is implemented to use mobile operations, serious and undetectable errors may occur if you accidentally call swap (aforce b) and an and b refer to the same object, if you don't handle the self-assignment properly.

Example (sample) class Foo {

String s

Int i

Public:

Foo& operator= (Foo&& a)

/ /...

}

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

{

If (this = & a) return * this; / / this line is redundant

S = std::move (a.s)

I = a.i

Return * this

}

The probability of violating self-assignment check is only 1/1000000; avoiding the discussion of self-assignment is more closely related to self-movement.

Note (Note)

There is no common way to avoid using if (this==&a) return * this; operations. Checking the method of moving the assignment will still yield the correct result (for example, the value of x does not change after xroomx).

Note (Note)

The ISO standard only guarantees a "legal but undefined" state for standard libraries. It doesn't seem to be a problem in more than 10 years of experience and product applications. If you encounter a counterexample, please contact the editor. This rule is more cautious and insists on being completely safe.

Example (sample)

Here is a way to move the pointer without checking (think of it as part of the implementation of the mobile assignment code):

/ / move from other.ptr to this- > ptr

T * temp = other.ptr

Other.ptr = nullptr

Delete ptr

Ptr = temp

Translator's Note

If other and this are the same object, other.ptr=nullptr will also leave this.ptr empty at the same time, causing the following delete ptr to not work. This ensures the safety of self-movement.

This is too difficult to understand.

Enforcement (sample)

(Moderate) In the case of self-assignment, a move assignment operator should not leave the object holding pointer members that have been deleted or set to nullptr.

(medium) in the case of self-assignment, the move operation operator should prevent the object pointed to by the pointer member of the object from being destroyed or left empty.

(Not enforceable) Look at the use of standard-library container types (incl. String) and consider them safe for ordinary (not life-critical) uses.

(unable to implement) find the use of standard library container types (including string) and consider them safe for general use (not declaring cycle sensitivity).

At this point, I believe you have a deeper understanding of "how to make mobile operations secure on self-assignment in C++". You might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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