Question

EDIT: The main intent is to allow manipulating underlying data as part of an encapsulated struct as opposed to direct data manipulation.

Which of the following approaches is recommended when it comes to wrapping some data inside a struct:

1. Keep a pointer to the data within the struct:

   new s(buf), which stores buf in a local field (s->buf = buf)

2. reinterpret_cast-ing a memory address to a struct:

   reinterpret_cast<s*>(buf)

3. Use the new operator against the memory address where the data is located:

   new(buf) s;

Here is a sample program for these approaches:

#include <iostream>
using namespace std;

struct s {
    int* i;
    s(int* buf) : i(buf) {}
    int getValue() { return *i * 2; }
};

struct s2 {
    int i;
    int getValue() { return i * 2; }
};

int main() {
    int buf = 10;
    s a(&buf);
    cout << "value: " << a.getValue() << ", size: " << sizeof(a) << ", address: " << &a << ", buf-address: " << &buf << endl;

    s2* a2 = new(&buf) s2;
    cout << "value: " << a2->getValue() << ", size: " << sizeof(*a2) << ", address: " << a2 << ", buf-address: " << &buf << endl;

    s2* a3 = reinterpret_cast<s2*>(&buf);
    cout << "value: " << a3->getValue() << ", size: " << sizeof(*a3) << ", address: " << a3 << ", buf-address: " << &buf << endl;
}

And the output:

value: 20, size: 4, address: 0027F958, buf-address: 0027F964
value: 20, size: 4, address: 0027F964, buf-address: 0027F964
value: 20, size: 4, address: 0027F964, buf-address: 0027F964

Both size & time are important. Also, maintainability is important, e.g. someone might add by mistake a virtual function to s2 (which will mess up the data alignment).

Thanks!

Answer 1

Placement new will still call the constructor, wiping out anything that’s in the buffer already if such a constructor exists (or is created unknowingly in the future) so I don’t think that’s a safe option. reinterpret_cast is undefined behavior even though it may appear to work for you. Storing a local pointer seems to be the best option although you’ve only given a very tiny inkling of what you’re trying to do.

If you’re attempting serialization here, remember important issues like sizeof(int) and endianness.