In C++ if I do this:
__int64 var = LLONG_MIN;
__int64 var2 = -var;
cout << "var: "<< var << endl;
cout << "var2: "<< var2 << endl;
The output I get is:
var: -9223372036854775808
var2: -9223372036854775808
What is the part of the standard that covers this? I assume it's signed integer overflow. This was compiled using g++ (GCC) 4.7.2.
I have a subtract function and I'm writing an add function and I thought I could just do this:
add( someobj &obj, long long num ) { subtract( obj, -num ); }. I think that would work if it wasn't for LLONG_MIN.
It is indeed integer overflow, and an artefact of two's complement.
On your implementation, LLONG_MIN is -9223372036854775808 which is 0x8000000000000000 in hexadecimal (I will be using this hexadecimal notation because it's easier to see what happens to the bits).
When you compute -LLONG_MIN on a system that uses two's complement, under the hood you first make a bitwise-not (yielding 0x7FFFFFFFFFFFFFFF == LLONG_MAX) then add 1 which overflows the signed integer and gives back 0x8000000000000000 == LLONG_MIN.
Note that signed integer overflow is Undefined Behaviour, so there is no guarantee that it will behave consistently on every C++ implementation.