初始化指向结构C的const指针

Question

I am a beginner in C++, and got confused about the initialization of const pointers declared in headers. To give but one example, I have in my header a structure and a class like:

/* header file.h */
typedef struct A{ ... } A;

class myClass{
    protected:
        A *const myPtrA;
}

And would like to instantiate the content of myPtrA, for instance in a constructor, knowing that A is a quite complicated structure composed of substructures, and needs to be dynamically instantiated:

/* source file.cpp */
#include file.h

myClass::myClass() {
   A *tmpA = new A;
   *myPtrA = *tmpA;
}

Is this the good way to initialize my const pointer myPtrA? And to the extent each new call needs a dedicated delete call, can I delete my pointer tmpA immediately after the line *myPtrA = *a; without risk of losing the content pointed by myPtrA?

Thank you in advance for your time (and pardon my English ;) )

qroh

Answer 1

 *myPtrA = *tmpA; 

Is this the good way to initialize my const pointer myPtrA?

No. You haven't initialized myPtrA at all. It was default-initialized and therefore has indeterminate value. Dereferencing the pointer with indeterminate value ( *myPtrA ) has undefined behaviour.

can I delete my pointer tmpA immediately after the line *myPtrA = *a; without risk of losing the content pointed by myPtrA?

Yes, it is safe. The object pointed by myPtrA is copy (by assignment) of the one pointed by tmpA . However, it is completely pointless to allocate a dynamic object, copy it and then detroy it in the first place, when you could simply create/modify the copy directly.

Here is an example of how to correctly initialize the member pointer:

class myClass{
    protected:
        A *const myPtrA = new A;
};

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PS. While it is useful to learn how to do this, you should hardly ever manage memory manually in actual programs. Instead use RAII containers such as std::unique_ptr .