Strict aliasing rule

Question

I\'m reading notes about reinterpret_cast and it\'s aliasing rules ( http://en.cppreference.com/w/cpp/language/reinterpret_cast ).

I wrote that code:

Answer 1

Yeah, it's invalid, but not because you're converting a char* to an A*: it's because you are not obtaining a A* that actually points to an A* and, as you've identified, none of the type aliasing options fit.

You'd need something like this:

#include 
#include 

struct A
{
  int t;
};

char *buf = new char[sizeof(A)];

A* ptr = new (buf) A;
ptr->t = 1;

// Also valid, because points to an actual constructed A!
A *ptr2 = reinterpret_cast(buf);
std::cout << ptr2->t;

Now type aliasing doesn't come into it at all (though keep reading because there's more to do!).

• (live demo with -Wstrict-aliasing=2)

In reality, this is not enough. We must also consider alignment. Though the above code may appear to work, to be fully safe and whatnot you will need to placement-new into a properly-aligned region of storage, rather than just a casual block of chars.

The standard library (since C++11) gives us std::aligned_storage to do this:

using Storage = std::aligned_storage::type;
auto* buf = new Storage;

Or, if you don't need to dynamically allocate it, just:

Storage data;

Then, do your placement-new:

new (buf) A();
// or: new(&data) A();

And to use it:

auto ptr = reinterpret_cast(buf);
// or: auto ptr = reinterpret_cast(&data);

All in it looks like this:

#include 
#include 
#include 

struct A
{
  int t;
};

int main()
{
    using Storage = std::aligned_storage::type;

    auto* buf = new Storage;
    A* ptr = new(buf) A();

    ptr->t = 1;

    // Also valid, because points to an actual constructed A!
    A* ptr2 = reinterpret_cast(buf);
    std::cout << ptr2->t;
}

(live demo)

Even then, since C++17 this is somewhat more complicated; see the relevant cppreference pages for more information and pay attention to std::launder.

Of course, this whole thing appears contrived because you only want one A and therefore don't need array form; in fact, you'd just create a bog-standard A in the first place. But, assuming buf is actually larger in reality and you're creating an allocator or something similar, this makes some sense.