inline int ProcessByValue(int i)
{
    // process i somehow
}

inline int ProcessByReference(const int& i)
{
    // process i somehow
}

int two = plus1( 1 );  //  compile error if plus1 is declared as "int plus1( int& )"

double y = sqrt( 1.1 * 2 );  // compile error if sqrt is declared as "double sqrt( const double& )"