Casting from long double to unsigned long long appears broken in the MSVC C++ compiler

Question

Consider the following code:

#include <iostream>

using namespace std;

int main(int argc, char *argv[])
{
    long double test = 0xFFFFFFFFFFFFFFFF;
    cout << "1: " << test << endl;
    unsigned long long test2 = test;
    cout << "2: " << test2 << endl;
    cout << "3: " << (unsigned long long)test << endl;
    return 0;
}

Compiling this code with GCC g++ (7.5.0) and running produces the following output as expected:

1: 1.84467e+19
2: 18446744073709551615
3: 18446744073709551615

However compiling this with the Microsoft Visual C++ compiler (16.8.31019.35, both 64-bit and 32-bit) and running produces the following output:

1: 1.84467e+19
2: 9223372036854775808
3: 9223372036854775808

When casting a value to an unsigned long long, the MSVC compiler won't give a value lager than the max of a (signed) long long.

Am I doing something wrong? 

Am I running into a compiler limitation that I do not know about?

Does anyone know of a possible workaround to this problem?

Answer 1

Because a MSVC long double is really just a double (as pointed out by @drescherjm in the comments), it does not have enough precision to contain the exact value of 0xFFFFFFFFFFFFFFFF. When this value is stored in the long double it gets "rounded" to a value that is lager than 0xFFFFFFFFFFFFFFFF. This then causes undefined behaviour when converting to an unsigned long long.