Typical errors of porting C++ code on the 64-bit platform

Bit shifting operations can cause a lot of troubles during the port from the 32-bit system on the 64-bit one if used inattentively. Let’s begin with an example of a function which defines the bit you’ve chosen as 1 in a variable of memsize type.

ptrdiff_t SetBitN(ptrdiff_t value, unsigned bitNum) {
  ptrdiff_t mask = 1 << bitNum;
  return value | mask;
}

The given code works only on the 32-bit architecture and allows to define bits with numbers from 0 to 31. After the program port on the 64-bit platform it becomes necessary to define bits from 0 to 63. What do you think which value will the following call of SetBitN(0, 32) function return? If you think that 0x100000000 the authors is glad because he hasn’t prepared this article in vain. You’ll get 0.

Pay attention that "1" has int type and during the shift on 32 positions an overflow will occur. To correct the code it is necessary to make the constant "1" of the same type as the variable mask.

ptrdiff_t mask = ptrdiff_t(1) << bitNum;
or
ptrdiff_t mask = CONST3264(1) << bitNum;