Question

I have some code that looks like:

template<unsigned int A, unsigned int B> int foo() {   int v = 1;   const int x = A - B;   if (x > 0) {     v = v << x;   }   bar(v); } 

gcc will complain about x being negative for certain instantiations of A, B; however, I do perform a check to make sure it is non-negative. What’s the best way around this? I know I can cast x to be unsigned int but that will cause warnings about x being larger than the width of v (since it is casting a negative number to be positive). I know there is a work-around that involves creating a new templatized shift function, but I’d like to avoid that if possible.

Answer 1

Since A and B are known at compile time, not only can you get rid of your warning, but you can also get rid of a runtime if, without any casts, like this:

#include <iostream> using namespace std;  template< unsigned int A, unsigned int B > struct my {     template< bool P >     static void shift_if( int & );      template<>     static void shift_if< false >( int & ) {}      template<>     static void shift_if< true >( int & v ) { v <<= A - B; }      static void op( int & v ) { shift_if< (A > B) >( v ); } };  template< unsigned int A, unsigned int B > int foo() {     int v = 1;     my< A, B >::op( v );     return v; }  int main() {     cout << foo< 1, 3 >() << endl;     cout << foo< 3, 1 >() << endl;     cout << foo< 300, 1 >() << endl;     cout << foo< 25, 31 >() << endl;     return 0; }