По мотивам эпохального топика "С — зло... " http://www.rsdn.ru/forum/cpp.applied/4109801.1.aspx

Автор: LaptevVV
Дата: 10.01.11

struct SchoolInt
{
  int i;
  operator const int&() const { return i; }
  operator       int&()       { return i; }
  // other int operations like ++ etc
  
  struct SIA
  {
    SchoolInt& h;
    const int& x;
    mutable bool not_assigned_yet;
    
    template<class U> operator U() const
    {
      // Looks like you wrote if( a = b ) instead of if( a == b ).
      // If you really want to assign inside if-condition, do if( int(a = b) ).
      BOOST_STATIC_ASSERT(( ! boost::is_same< U, bool >::value ));
      not_assigned_yet=false;
      return h.i=x;
    }
    ~SIA() { if (not_assigned_yet) h.i=x; }
  };
  
  template<class RHS>
  SIA operator=(const RHS& x)
  {
    // Looks like you wrote int_a = bool_b.
    // SchoolInt is strict, please cast explicitly: int_a = (int)bool_b.
    BOOST_STATIC_ASSERT(( boost::is_same< RHS, int >::value ));
    SIA ret = {*this, x, true};
    return ret;
  }
};

Проверка:

int main()
{  
  SchoolInt i = { 5 };
  std::cout << i << std::endl;
  
  i = 14;                       // compiles
  std::cout << i << std::endl;
  
  double y = i = 13;            // compiles
  std::cout << i << " y=" << y << std::endl;
    
  //bool z = int(i = 12);       // compiles
  bool z = i = 12;              // doesn't compile
  std::cout << i << " z=" << z << std::endl;
    
  if ( int x = i = 11 )         // compiles
  std::cout << i << " x=" << x << std::endl;
  
  //if ( bool w = int(i = 10) ) // compiles
  if ( bool w = i = 10 )        // doesn't compile
  std::cout << i << " w=" << w << std::endl;
  
  //if ( int(i = 9) )           // compiles
  if ( i = 9 )                  // doesn't compile
    std::cout << i << std::endl;
  
  //if ( int(i = int(!0) ) )    // compiles
  if ( i =! 0 )                 // doesn't compile
    std::cout << i << std::endl;
}

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You will always get what you always got
  If you always do  what you always did