On 4 September 2014 17:51, Andrew Haley wrote: > The "union trick" has always worked with GCC, and is now hallowed by > the standard. It's also easy to understand. It generates code as > efficient as all the other ways of doing it, AFAIAA. It's what we > have always recommended. Type punning with unions is allowed in C since C99 but not by any C++ standard (although it does work with GCC). Placement new might work with GCC in practice as long as the buffer is correctly aligned and the type being constructed does not have non-trivial initialization. However, my reading of the standard is that after the placement new, if the object's members are not initialized then they have indeterminate values (not the values that were at those memory addresses already, even though that's likely to be what happens in practice). Objects with indeterminate values can only be used in very limited ways and generally lead to undefined behaviour, and code with undefined behaviour does not usually mix well with aggressive optimizations. FWIW I prefer the memcpy approach, it usually generates the same code as type punning via a union, but is more portable and has guaranteed results that are well-defined (and if you're interested in maximum performance then you should not be using unoptimized code, so it shouldn't matter that the mempcy solution relies on optimizations).
