Using boost::numeric_cast<>

Question

When I want to convert between different integer types, it seems the best syntax is to use boost::numeric_cast<>():

int y = 99999;
short x         


        

Answer 1

If you're okay with C++17 but don't want your casting algorithm throwing exceptions internally, you can use std::clamp with a bit of wrapping to handle out-of-bounds values.

template 
constexpr TTo clamp_cast(const TFrom& src) noexcept
{
    using to_limits   = std::numeric_limits;
    using larger_type = std::conditional_t<(sizeof(TFrom) < sizeof(TTo)), TTo, TFrom>;

    if constexpr (std::is_same_v)
    {
        // don't bother if it is the same type
        return src;
    }
    else if constexpr (std::is_unsigned_v)
    {
        // if source is unsigned, we only need to worry about upper bound
        return TTo(std::min(larger_type(src), larger_type(to_limits::max())));
    }
    else if constexpr (std::is_unsigned_v)
    {
        // source is signed, but destination is not
        if (src < TFrom(0))
            return TTo(0);
        else
            return TTo(std::min(larger_type(src), larger_type(to_limits::max())));
    }
    else
    {
        // both are signed -- use regular clamping rules
        return TTo(std::clamp(larger_type(src),
                              larger_type(to_limits::min()),
                              larger_type(to_limits::max())
                             )
                  );
    }
}

Usage is basically what you'd expect:

static_assert(uint16_t(213)   == clamp_cast(213));
static_assert(uint16_t(65535) == clamp_cast(9872431));
static_assert(uint16_t(0)     == clamp_cast(-98721));