Can't Assign 64 Bit Number to 64 Bit Min in C++

Question

I'm new to C++, so sorry if this sounds like a dumb question. I'm attempting to assign the 64 bit minimum (-9223372036854775808) to an int64_t (from cstdint), however I'm getting the current error message:

main.cpp:5:27: warning: integer literal is too large to be represented in a signed integer type, interpreting as unsigned [-Wimplicitly-unsigned-literal]
int64_t int_64_min = -9223372036854775808;

The code is as follows:

#include <iostream>
#include <cstdint>

int main() {
    int64_t int_64_min = -9223372036854775808;
    std::cout << int_64_min << std::endl;
    std::cout << INT64_MIN << std::endl;
    return 0;
}

Am I missing something obvious?

Answer 1

This is exactly why the INT_MIN family of macros are often defined like -INT_MAX - 1 on a 2's complement platform (virtually ubiquitous and compulsory from C++20).

There is no such thing as a negative literal in C++: just the unary negation of a positive number which produces a compile time evaluable constant expression.

9223372036854775808 is too big to fit into a 64 bit signed integral type, so the behavior of negating this and assigning to a signed 64 bit integral type is implementation defined.

Writing -9223372036854775807 - 1 is probably what your C++ standard library implementation does. If I were you, I'd use INT64_MIN directly or std::numeric_limits<std::int64_t>::min().

Answer 2

The problem is that -9223372036854775808 is not a literal. 9223372036854775808 is a literal, and -9223372036854775808 is an expression consisting of that literal with a unary - applied to it.

All unsuffixed decimal literals are of type int, long int, or long long int. Since 9223372036854775808 is 2⁶³, it's bigger than the maximum value of long long int (assuming long long int is 64 bits, which it almost certainly is).

The simplest solution is to use the INT64_MIN macro defined in <cstdint>.

Aside from that, the problem is that there are no literals for negative values. Another solution is to replace -9223372036854775808 by -9223372036854775807-1 -- or by (-9223372036854775807-1) to avoid ambiguity. (The <cstdint> header very likely does something similar).