Re: Mapping float to ulong in CTFE

[Petar via Digitalmars-d-learn]

On Thursday, 12 December 2019 at 19:21:22 UTC, berni44 wrote:

Is it possible to get to the bits of a float in CTFE? I tried the following, but this doesn't work:

```
import std.stdio;

union FloatBits
{
    float floatValue;
    ulong ulongValue;
}

ulong test(float f)
{
    FloatBits fb;
    fb.floatValue = f;
    return fb.ulongValue;
}

void main()
{
    static assert(test(3.0) == 1077936128);
}
```

test.d(13): Error: reinterpretation through overlapped field ulongValue is not allowed in CTFE

test.d(18):        called from here: test(3.00000F)

test.d(18): while evaluating: static assert(test(3.00000F) == 1077936128LU)

You can use a C-style pointer reinterpret cast like this:

uint test(float f) { return *cast(uint*)&f; }

Make sure that source and destination types have the same size.

Or more generally:

IntegerOfSize!T bitRepresentation(T)(T f)
{
    return *cast(IntegerOfSize!T*)&f;
}

pragma (msg, bitRepresentation(3.0f)); // 1077936128u
pragma (msg, bitRepresentation(3.0));  // 4613937818241073152LU

void main()
{
    static assert(bitRepresentation(3.0f) == 1077936128);
}

template IntegerOfSize(T)
{
    static if (T.sizeof == 1)
        alias IntegerOfSize = ubyte;
    else static if (T.sizeof == 2)
        alias IntegerOfSize = ushort;
    else static if (T.sizeof == 4)
        alias IntegerOfSize = uint;
    else static if (T.sizeof == 8)
        alias IntegerOfSize = ulong;
    else
        static assert(0);
}