Conversion of big-endian long in C++?

Question

I need a C++ function that returns the value of four consecutive bytes interpreted as a bigendian long. A pointer to the first byte should be updated to point after the last. I have tried the following code:

inline int32_t bigendianlong(unsigned char * &p)  
{  
  return (((int32_t)*p++ << 8 | *p++) << 8 | *p++) << 8 | *p++;  
}  

For instance, if p points to 00 00 00 A0 I would expect the result to be 160, but it is 0. How come?

Answer 1

The issue is explained clearly by this warning (emitted by the compiler):

./endian.cpp:23:25: warning: multiple unsequenced modifications to 'p' [-Wunsequenced]
    return (((int32_t)*p++ << 8 | *p++) << 8 | *p++) << 8 | *p++;

Breaking down the logic in the function in order to explicitly specify sequence points...

inline int32_t bigendianlong(unsigned char * &p)
{
    int32_t result = *p++;
    result = (result << 8) + *p++;
    result = (result << 8) + *p++;
    result = (result << 8) + *p++;
    return result;
}

... will solve it