When to use reinterpret_cast?

Question

I am little confused with the applicability of reinterpret_cast vs static_cast. From what I have read the general rules are to use static cast when

Answer 1

First you have some data in a specific type like int here:

int x = 0x7fffffff://==nan in binary representation

Then you want to access the same variable as an other type like float: You can decide between

float y = reinterpret_cast(x);

//this could only be used in cpp, looks like a function with template-parameters

or

float y = *(float*)&(x);

//this could be used in c and cpp

BRIEF: it means that the same memory is used as a different type. So you could convert binary representations of floats as int type like above to floats. 0x80000000 is -0 for example (the mantissa and exponent are null but the sign, the msb, is one. This also works for doubles and long doubles.

OPTIMIZE: I think reinterpret_cast would be optimized in many compilers, while the c-casting is made by pointerarithmetic (the value must be copied to the memory, cause pointers couldn't point to cpu- registers).

NOTE: In both cases you should save the casted value in a variable before cast! This macro could help:

#define asvar(x) ({decltype(x) __tmp__ = (x); __tmp__; })