C2593 'operator <<' is ambiguous

Cause

There is no operator << for __int64 type defined for the ostream class.

Resolution

Define your own version of operator <<. The following sample code section shows a simple solution for << operator that converts the __int64 variable to a char * type and passes it to the ostream << operator.

The following sample program demonstrates the problem and workaround:

//Sample.cpp
// Compiler Options : /GX

//#define WORKAROUND   //Uncomment this line to workaround

#include<iostream>
using namespace std;

#ifdef WORKAROUND
std::ostream& operator<<(std::ostream& os, __int64 i )
{
    char buf[20];
    sprintf(buf,"%I64d", i );
    os << buf;
    return os;
}

#endif

int main(){
__int64  i64;

cout << i64 ;

return 0;
}

Reference: "error C2593: 'operator <<' is ambiguous"[^]