Working around the char array on stack align problem

Question

See: http://stackoverflow.com/questions/3874615/placement-new-issue

Simple question, would this solve the align problem?

union
{
    char real_array[sizeof(T)*size];
    T    fake_array[size];
};

Answer 1

I don't think so, if you look back at the link you posted it said 'OK I finally get it, it may start on a wrong address.' You still have no control over the address of the first member of the union.

Answer 2

Yes, that should solve the alignment problem. There's no need to make fake_array an array though. Just a single member of type T is enough.

This is actually a rather widely used trick for forcing specific alignment on some array.

As a pedantic side-note: anonymous unions only exist in C++, but not in C.

Answer 3

Yes, and even the simpler struct below could do the trick.

union
{
    char real_array[sizeof(T)*size];
    T    dummy;
};

I believe the citation below from ISO standard is guarantee enough that it works.

One special guarantee is made in order to simplify the use of unions: If a POD-union contains several POD-structs that share a common initial sequence, and if an object of this POD-union type contains one of the POD-structs, it is permitted to inspect the common initial sequence of any of POD-struct members;

However, as standard is worded you could wonder if there is not some loophole leading to Undefined Behavior if you use non POD classes... (but I would bet it will work with any compiler anyway).