Inline float to uint "representation" not working?

Question

This is in C, but I tagged it C++ incase it's the same. This is being built with: Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 14.00.50727.220 for 80x86 if that makes any different

Why does this work?

(inVal is 0x80)

float flt = (float) inVal;
outVal = *((unsigned long*)&flt);

(results in outVal being 0x43000000 -- correct)

But this doesn't?

outVal = *((unsigned long*)&((float)inVal));

(results in outVal being 0x00000080 -- NOT CORRECT :( )

Before asking this question I googled around a bit and found this function in java that basically does what I want. If you're a bit confused about what I'm trying to do, this program might help explain it:

class hello
{
    public static void main(String[] args)
    {
        int inside = Float.floatToIntBits(128.0f);
        System.out.printf("0x%08X", inside);
    }
}

Answer 1

You're trying to take the address of a non-const temporary (the result of your (float) conversion) – this is illegal in C++ (and probably also in C). Hence, your code results in garbage.

In your first, working, code, you're not using a temporary so your code is working. Notice that from a standards point of view this is still ill-defined since the size and internal representation of the involved types isn't specified and may differ depending on platform and compiler. You're probably safe, though.

Answer 2

In C99, you may use compound literals to make this work inline:

unsigned long outVal = *((unsigned long *)&((float){ inVal }));

The literal (float){ inVal } will create a variable with automatic storage duration (ie stack-allocated) with a well-defined address.

Type punning may also be done using unions instead of pointer casts. Using compound literals and the non-standard __typeof__ operator, you can even do some macro magic:

#define wicked_cast(TYPE, VALUE) \
    (((union { __typeof__(VALUE) src; TYPE dest; }){ .src = VALUE }).dest)

unsigned long outVal = wicked_cast(unsigned long, (float)inVal);

GCC prefers unions over pointer casts in regard to optimization. This might not work at all with the MS compiler as its C99 support is rumored to be non-existant.

Answer 3

Assuming: inVal and outVal are parameters.

void func(int inVal,unsigned long* outVal)
{
    float flt = (float) inVal;

    *outVal = (unsigned long)flt; // convert flot to unsigned long.
                                  // Then assign to the variable by de-ref
                                  // the pointer.
}