flatmouse flatmouse - 28 days ago 19
C++ Question

Avoid narrowing conversion for bitset template

How can the following be written in a portable way to avoid narrowing conversions?

#include <bitset>
#include <iostream>
#include <climits>

template <typename T>
auto int_to_bitset(T x)
{
//return std::bitset<sizeof(T)*CHAR_BIT>{x}; // does not work, narrowing conversion to unsigned type
//return std::bitset<sizeof(T)*CHAR_BIT>{static_cast<unsigned int>(x)}; // might not have the same size as T
//return std::bitset<sizeof(T)*CHAR_BIT>{static_cast<unsigned T>(x)}; // What I would like to do, but does not work. I've never seen so many errors.
return std::bitset<sizeof(T)*CHAR_BIT>(x); // works, but selects unsigned long long for the constructor's parameter on my system. Can this conversion be relied on?
}

int main()
{
std::cout << int_to_bitset<short>( 1 ) << '\n';
std::cout << int_to_bitset<short>(-1 ) << '\n';
std::cout << int_to_bitset ( 1 ) << '\n';
std::cout << int_to_bitset (-1 ) << '\n';
std::cout << int_to_bitset ( 1L ) << '\n';
std::cout << int_to_bitset (-1L ) << '\n';
std::cout << int_to_bitset ( 1LL) << '\n';
std::cout << int_to_bitset (-1LL) << '\n';
}


Produces:

0000000000000001
1111111111111111
00000000000000000000000000000001
11111111111111111111111111111111
00000000000000000000000000000001
11111111111111111111111111111111
0000000000000000000000000000000000000000000000000000000000000001
1111111111111111111111111111111111111111111111111111111111111111

Answer

You can use std::make_unsigned:

template <typename T>
auto int_to_bitset(T x)
{
    return std::bitset<sizeof(T)*CHAR_BIT>{static_cast<std::make_unsigned_t<T>>(x)};
}

live example

Comments