Re: Type-punning

[Silvius Rus <rus at google dot com>]

Herman Geza wrote:
> struct A {
> 	float x, y;
> };
>
> struct B {
> 	float x, y;
> };
>
> int main() {
> 	A a;
> 	B &b = reinterpret_cast<B&>(a);
> }
>
> I get a type-punned warning for this code. However, A & B is exactly the 
> same type.  Is the warning appropriate here?  
Unfortunately gcc 4.3 will likely warn on this as well.  To avoid it, we 
would need a "prefix" analysis to distinguish between different structs 
that are compatible up to a point.  As far as I know, GCC does not have 
this capability as of now.  If it exists, it still needs to be connected 
to -Wstrict-aliasing, which is generally not trivial, as it requires 
figuring out what field is referenced where.  I guess it could be 
implemented in a simpler way for the special case when the structs are 
identical structurally.

> Where can I find the 
> definition of "almost the same [type]"?
>   
C and C++ standards:
C Standard ISO/IEC 9899:1999, section 6.5, paragraph 7, and the C++ 
Standard ISO/IEC 14882:1998, section 3.10, paragraph 15.  But there are 
a couple other references that touch this.

> What is the correct way to do this:
>
> void setNaN(float &v) {
> 	reinterpret_cast<int&>(v) = 0x7f800001;
> }
>
> without a type-prunning warning?  I cannot use the union trick here 
> (memcpy works though, but it's not the most efficient solution, I 
> suppose).
>   
The correct and efficient solution is to use memcpy.  GCC should 
recognize the memcpy call and transform it into a move or load/store or 
such.

You may want to try memcpy, generate the assembly code, and see if you 
are happy with it.