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This article introduces the knowledge of "how to use factory functions in C++". In the operation of actual cases, many people will encounter such a dilemma, so 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!

If you need "virtual behavior" during construction, use the factory function Reason (reason)

If the state of the base class object must depend on the derived part of the object, we need to use virtual functions (or equivalents) to minimize the time window in which the imperfectly constructed object is misused.

Note (Note)

The return type of the factory should usually return unique_ptr; by default. If some uses need to be shared, the caller can move the unique_ptr to shared_ptr. However, if the author of the factory knows that all the uses of the returned object are shared, he can also return shared_ptr so that make_shared can be used inside the function body to save a memory allocation.

Example, bad (negative example) class B {

Public:

B () {

/ *. /

F (); / / BAD: C.82: Don't call virtual functions in constructors and destructors

/ *. /

}

Virtual void f () = 0

}; Example (example) class B {

Protected:

Class Token {}

Public:

Explicit B (Token) {/ *... * /} / / create an imperfectly initialized object

Virtual void f () = 0

Template

Static shared_ptr create () / / interface for creating shared objects

{

Auto p = make_shared (typename T::Token {})

P-> post_initialize ()

Return p

}

Protected:

Virtual void post_initialize () / / called right after construction

{/ *... * / f (); / *... * /} / / GOOD: virtual dispatch is safe

}

Class D: public B {/ / some derived class

Protected:

Class Token {}

Public:

Explicit D (Token): B {B::Token {}}

Void f () override {/ *... * /}

Protected:

Template

Friend shared_ptr B::create ()

}

Shared_ptr p = D::create (); / / creating a D object

The constructor called by make_shared must be public. By requiring a protected token to ensure that the constructor cannot be called publicly, we avoid the outflow of partially constructed objects. Since a factory method create () is provided, the construction process (on free storage) can be easily implemented.

Note (Note)

The conventional factory method allocates object memory on free storage, not within a stack or closed object.

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