Howto cast from "fpm::fixed_a_b" to "fpm::fixed_c_d" ?

[Klummel69]

Is it possible to cast from e.g. fixed_8_24 to fixed_16_16 without an intermediate step via a float value?

With simple assignments "fixed2 = fixed1" the compiler reports an _>no known conversion for argument 1 from 'fpm::fixed_824' ... to 'const fpm::fixed<int, long long int, 16>&'

There is a function from_fixed_point(T value), but i dont understand it. I would have expected that the value is corrected by the difference of the FractionBits between new and old datatype.

[MikeLankamp]

Hi @Klummel69, this is not possible at the moment, but it makes sense to be able to do that.

However, the point of fpm is to be explicit about conversions that potentially throw away information so this functionality should be added as an explicit constructor so you can write:

fixed2 = fpm::fixed_16_16(fixed1);

[Klummel69]

Great, thanks for adding the feature. This increases the quality of the library.

I plan to store data types with different solutions.

• Angles: 8_24 format (-2 π … 2 π)
• Coordinates: 16_16 or 24_8 format.

This keeps angle calculations more accurate. Only after trigonometric calculations the resolution is reduced.

Example:

    fpm::fixed_8_24 w {1.047197551};
    fpm::fixed_16_16 x,y;
    //w = w1+w2+w3;
    x = 100 * fpm::fixed_16_16(fpm::sin(w));
    y = 100 * fpm::fixed_16_16(fpm::cos(w));

A possibility of conversion without explicit construction would be good, but it is bearable.

[MikeLankamp]

A possibility of conversion without explicit construction would be good.

I understand the desire to have this, but it's generally a bad idea. The C++ Core Guidelines also discourage this: https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es46-avoid-lossy-narrowing-truncating-arithmetic-conversions

Hence the decision to have these kind of conversions explicit.

[Klummel69]

You got me. Actually, I try to follow the guideline. I had used template code that was not compliant with the guideline. In the meantime I have corrected that. Thanks.