C++ 'overloading' the if() statement

Question

Is it possible to change the behavior of if() so that:

class Foo {
    int x;
};

Foo foo;
if(foo)

only proceeds if the value of <

Answer 1

You can define an operator to convert the object to bool

class Foo
{
  int x;
public:
  operator bool() const
  {
    return x > 0;
  }
};

But this can have unintended consequences because of implicit conversions to bool when you don't desire the conversion to take place. For instance

int x = 42 + Foo();

C++11 solves this problem by allowing you to declare the conversion operator as explicit, which then only allows implicit conversions in certain contexts, such as within an if statement.

explicit operator bool() const // allowed in C++11

Now

int x = 42 + Foo();  // error, no implicit conversion to bool
int x = 42 + static_cast(Foo()); // OK, explicit conversion is allowed