value changes while reinterpret_cast

Question

while converting an integer to float by using reinterpret_cast the content of memory changes.

For example,

   float fDest = 0;
  __int32 nTempDest = -4808638;

  fDest = *reinterpret_cast<float*>(&nTempDest);

Hexadecimal representation for the variable value nTempest is '42 a0 b6 ff' but after reinterpret_cast the content of fDest is '42 a0 f6 ff'.

Can any one could give an answer why this third byte changed from b6 to f6.

Answer 1

In pure C++ this is in fact undefined behavior. Nevertheless there is an explanation for what you see.

I assume that the hexadecimal representations you give are from a bytewise view of memory. Obviously you are on a little endian architecture. So the 32-bit quantity we are starting from is 0xffb6a042, which is indeed the two's complement representation of -4808638.

As a IEC 60559 single-precision floating point number (also 32 bits) 0xffb6a042 is a negative, signaling NaN. NaNs in this representation have the form (in binary)

s1111111 1qxxxxxx xxxxxxxx xxxxxxxx

Here s is the sign, the x are arbitrary and q=1 for a quiet NaN and q=0 for a signaling NaN.

Now you are using the signaling Nan in that you assign it to fDest. This would raise a floating point invalid exception if floating point signaling is active. By default such exceptions are simply ignored and signaling NaN values are 'quieted' when propagated on.

So: In assigning to fDest, the NaN is propagated and the implementation converts it to a quiet NaN by setting bit 22. This is the change you observe.