strict aliasing and alignment
- by cooky451
I need a safe way to alias between arbitrary POD types, conforming to ISO-C++11 explicitly considering 3.10/10 and 3.11 of n3242 or later.
There are a lot of questions about strict aliasing here, most of them regarding C and not C++. I found a "solution" for C which uses unions, probably using this section
union type that includes one of the aforementioned types among its
elements or nonstatic data members
From that I built this.
#include <iostream>
template <typename T, typename U>
T& access_as(U* p)
{
union dummy_union
{
U dummy;
T destination;
};
dummy_union* u = (dummy_union*)p;
return u->destination;
}
struct test
{
short s;
int i;
};
int main()
{
int buf[2];
static_assert(sizeof(buf) >= sizeof(double), "");
static_assert(sizeof(buf) >= sizeof(test), "");
access_as<double>(buf) = 42.1337;
std::cout << access_as<double>(buf) << '\n';
access_as<test>(buf).s = 42;
access_as<test>(buf).i = 1234;
std::cout << access_as<test>(buf).s << '\n';
std::cout << access_as<test>(buf).i << '\n';
}
My question is, just to be sure, is this program legal according to the standard?*
It doesn't give any warnings whatsoever and works fine when compiling with MinGW/GCC 4.6.2 using:
g++ -std=c++0x -Wall -Wextra -O3 -fstrict-aliasing -o alias.exe alias.cpp
* Edit: And if not, how could one modify this to be legal?
