How does one safely static_cast between unsigned int and int?

Question

I have an 8-character string representing a hexadecimal number and I need to convert it to an int. This conversion has to preserve the bit pattern for

Answer 1

While there are ways to do this using casts and conversions, most rely on undefined behavior that happen to have well-defined behaviors on some machines / with some compilers. Instead of relying on undefined behavior, copy the data:

int signed_val;
std::memcpy (signed_val, val, sizeof(int));
return signed_val;

Answer 2

Here's another solution that worked for me:

if (val <= INT_MAX) {
    return static_cast<int>(val);
}
else {
    int ret = static_cast<int>(val & ~INT_MIN);
    return ret | INT_MIN;
}

If I mask off the high bit, I avoid overflow when casting. I can then OR it back safely.

Answer 3

Quoting the C++03 standard, §4.7/3 (Integral Conversions):

If the destination type is signed, the value is unchanged if it can be represented in the destination type (and bit-field width); otherwise, the value is implementation-defined.

Because the result is implementation-defined, by definition it is impossible for there to be a truly portable solution.

Answer 4

You can negate an unsigned twos-complement number by taking the complement and adding one. So let's do that for negatives:

if (val < 0x80000000) // positive values need no conversion
  return val;
if (val == 0x80000000) // Complement-and-addition will overflow, so special case this
  return -0x80000000; // aka INT_MIN
else
  return -(int)(~val + 1);

This assumes that your ints are represented with 32-bit twos-complement representation (or have similar range). It does not rely on any undefined behavior related to signed integer overflow (note that the behavior of unsigned integer overflow is well-defined - although that should not happen here either!).

Note that if your ints are not 32-bit, things get more complex. You may need to use something like ~(~0U >> 1) instead of 0x80000000. Further, if your ints are no twos-complement, you may have overflow issues on certain values (for example, on a ones-complement machine, -0x80000000 cannot be represented in a 32-bit signed integer). However, non-twos-complement machines are very rare today, so this is unlikely to be a problem.

Answer 5

unsigned int u = ~0U;
int s = *reinterpret_cast<int*>(&u); // -1

Сontrariwise:

int s = -1;
unsigned int u = *reinterpret_cast<unsigned int*>(&s); // all ones

Answer 6

C++20 will have std::bit_cast that copies bits verbatim:

#include <bit>
#include <cassert>
#include <iostream>

int main()
{
    int i = -42;
    auto u = std::bit_cast<unsigned>(i);
    // Prints 4294967254 on two's compliment platforms where int is 32 bits
    std::cout << u << "\n";

    auto roundtripped = std::bit_cast<int>(u);
    assert(roundtripped == i);
    std::cout << roundtripped << "\n"; // Prints -42

    return 0;
}

cppreference shows an example of how one can implement their own bit_cast in terms of memcpy (under Notes).

While OpenVMS is not likely to gain C++20 support anytime soon, I hope this answer helps someone arriving at the same question via internet search.