Automatically pick a variable type big enough to hold a specified number

Question

Is there any way in C++ define a type that is big enough to hold at most a specific number, presumably using some clever template code. For example I want to be able to writ

Answer 1

I'm a bit late but...

#include 
#include 
#include 

template
struct getMinimalByteSize{
    using type= typename std::conditional< (bool)(data & (uint64_t)0xFFL << (test_bit*8)),
        typename std::tuple_element_t>,
        typename getMinimalByteSize::type>::type;};

template
struct getMinimalByteSize{using type = uint64_t;};

int main()
{
  static_assert(sizeof(getMinimalByteSize<0x0>::type)==8);
  static_assert(sizeof(getMinimalByteSize<0xFF>::type)==1);
  static_assert(sizeof(getMinimalByteSize<0xFFF>::type)==2);
  static_assert(sizeof(getMinimalByteSize<0xFFFFF>::type)==4);
  static_assert(sizeof(getMinimalByteSize<0xFFFFFFFFF>::type)==8);
}

The difference with all the other methods is on the testing. Instead of testing if the value is bigger than the biggest number possible given N bits, it goes byte for byte, testing if it is the last (most significant) non zero byte. If it is, then this is the minimal number of bits needed. Lastly we use a hand made list to fix the fact that there are not 24, 48, 56 bit integers defined in C++.

This is how this template metaprogram would look as a simple C function:

#include 

int tuple_element_t[]={8,16,32,32,64,64,64,64,64};

int getMinimalByteSize(uint64_t data, int8_t first_hi_byte = sizeof(data)-1){
    if (!data) return 0;
    /* Does the N bit of test is set? If so, we are done*/
    if (data &  (uint64_t)0xFFL << (first_hi_byte*8))
        return tuple_element_t[first_hi_byte];
    else/*Else, we tray with the next bit*/
        return getMinimalByteSize(data, first_hi_byte-1);}

Don't worry if you don't see it the first time, give yourself time . I've being working on AVRs for more than 10 years, in a platform where every byte counts. If you understand it in less than those 10 years, you already beat my.