char array as storage for placement new

Question

Is the following legal C++ with well-defined behaviour?

class my_class { ... };

int main()
{
    char storage[sizeof(my_class)];
    new ((void *)storage) my_class();
}

Or is this problematic because of pointer casting/alignment considerations?

Answer 1

Yes, it's problematic. You simply have no guarantee that the memory is properly aligned.

While various tricks exist to get storage with proper alignment, you're best off using Boost's or C++0x's aligned_storage, which hide these tricks from you.

Then you just need:

// C++0x
typedef std::aligned_storage<sizeof(my_class),
                                alignof(my_class)>::type storage_type;

// Boost
typedef boost::aligned_storage<sizeof(my_class),
                        boost::alignment_of<my_class>::value>::type storage_type;

storage_type storage; // properly aligned
new (&storage) my_class(); // okay

Note that in C++0x, using attributes, you can just do this:

char storage [[align(my_class)]] [sizeof(my_class)];