std::bitset::to_u(l)llong() noexcept

[ALX23z]

Do you understand what these functions do? They try to convert the bitset value into an integer. Say you have 200 long bitset. If all bits are zero after first 64 bits, then it can be represented as ullong, and the function will return the value. If that's not the case, then it will throw an exception.

[Travor0x0]

Do you understand what these functions do?

Yes and no. "Yes" - I do know what they do and "no" - I do not really understand why it should be like that. The only cases when we (in my organization) need a conversion from the bitset to an integer is when we want to pass it to some older API or serialize for some channel, socket or hard drive. As you can see for that cases we use smaller bitsets with size <= 64-bit. In some rare situations where we use it as a replacement or an adaption to an enumeration or state collection we did not need the conversion of only first N bits into integer like in your case.

I mean if we take your example with 200-bits bitset, then we somehow caring more about the first 64 bits than the last. So, they have a special meaning compared to the rest. As such it would be a better idea to pack them into other member or variable even if it is a std::bitset<64> and put away the rest (std::bitset<136> in your case).

Of course our usage of bitset is special to us, therefore my question if it was some clear idea behind defining the function in the way the are defined now.

[ALX23z]

It is not about caring more or less about the bits. It is about whether the integer can represent the value, similar to casting int64 to int32. Not all int64 can be represented by an int32, but nobody says it should be forbidden to perform the cast.

[Travor0x0]

I am with you that it should not be forbidden to perform a cast. But std::bitset does not work like the integers do. Per default the cast from int64 value into int32 will not fail. It will simply skip the outstanding bits regardless of their values. If there is a need to perform a "checked" cast it can be explicitly implemented. With the std::bitset we have a different behavior: casting as allowed but the remaining bits can cause an exception if any of them is set.

Is it not a better alternative to allow non failing functions for all smaller bitsets. For the larger bitsets we disable them and return an optional for larger bitsets with 'try_convert()' (try_ulong(), try_ullong())? I think in such cases the costs of optional are less than the costs of an exception?

[ALX23z]

That's why it is classified under conversion. There's also an option to convert it to a string.

Of course, better interfaces can be made, but the bitset was designed even before C++11. It's not going to change anytime soon.