Undefined Behavior in C++

Type Punning

In a systems language like C++ you often want to interpret a value of type A as a value of type B where A and B are completely unrelated types. This is called type punning.

Take for example the ever popular Fast Inverse Square Root. The Wikipedia gives us the following code.

float Q_rsqrt( float number )
{
    long i;
    float x2, y;
    const float threehalfs = 1.5F;

    x2 = number * 0.5F;
    y  = number;
    i  = * ( long * ) &y;                       // evil floating point bit level hacking
    i  = 0x5f3759df - ( i >> 1 );               // what the fuck? 
    y  = * ( float * ) &i;
    y  = y * ( threehalfs - ( x2 * y * y ) );   // 1st iteration
//  y  = y * ( threehalfs - ( x2 * y * y ) );   // 2nd iteration, this can be removed

    return y;
}

We interpret a float value as integer in the “evil floating point bit level hacking” line. Similar type punning often happens when we want to interpret a stream of bytes as some structure. We try to simply cast the char* input stream to our structure and use the member elements to read values.

Another common way to do this is via a union.

union U {
    long i;
    float f;
};

U u;
u.f = number;
long number_as_int = u.i;

Unfortunately neither are valid C++.

Casting is invalid because of C++’s strict aliasing rules. Basically, you mustn’t cast a pointer to a different type and then dereference it (unless you cast to char*).

The union trick is also not valid, because only one member of a union can be “active”. When we set f it becomes active and i is thus inactive. Reading from an inactive member results in undefined behavior. At least that’s how I understand the standard. The union trick in valid in modern C99 (but not in C89).

Instead of this you should use memcpy and hope that your compiler knows how to optimize it.

memcpy(&i, &y, sizeof(long));