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This article mainly explains "how C++ keeps mobile source objects in an effective state". The explanation in this article is simple, clear and easy to learn and understand. let's study and learn "how C++ keeps mobile source objects in an effective state".

C.64: after the move operation is completed, the moving source object should remain in a valid state Reason (reason)

This is the semantics of the general assumption. When y=std::move (x) is executed, the value of y should become x, and x should be in a valid state.

Translator's Note

Removing the value of x is not the same thing as having an invalid state.

Example (sample) template

Class X {/ / OK: value semantics

Public:

X ()

X (X & a) noexcept; / / move X

Void modify (); / / change the value of X

/ /...

~ X () {delete [] p;}

Private:

T * p

Int sz

}

XRV (Xlux)

: p {a.p}, sz {a.sz} / / steal representation

{

A.P = nullptr; / / set to "empty"

A.sz = 0

}

Void use ()

{

X x {}

/ /...

X y = std::move (x)

X = X {}; / / OK

} / / OK: x can be destroyedNote (note)

Ideally, moving the source object should become the default value. This must be done unless there is a very good reason. However, not all types have default values, and some types have expensive code to build valid state. The standard requirement is that the object can be destroyed. In general, we can easily do better at little cost: the standard library assumes that values can be assigned to moving source objects. Ensure that the moved source object is in some (inevitably defined) valid state.

Note (Note)

Unless there is a particularly strong reason not to do so, it is important to make sure that after x=std::move (y) is executed, ybuttz can follow the usual semantics.

Enforcement (implementation recommendations)

(not executable) finds a situation in which a member in a move operation is assigned. If there is a default constructor, compare the assignment operation in the move operation with the assignment operation in the default constructor.

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