Double Negation in C++

Question

I just came onto a project with a pretty huge code base.

I\'m mostly dealing with C++ and a lot of the code they write uses double negation for their boolean logic.

Answer 1

This may be an example of the double-bang trick, see The Safe Bool Idiom for more details. Here I summarize the first page of the article.

In C++ there are a number of ways to provide Boolean tests for classes.

An obvious way is operator bool conversion operator.

// operator bool version
  class Testable {
    bool ok_;
  public:
    explicit Testable(bool b=true):ok_(b) {}

    operator bool() const { // use bool conversion operator
      return ok_;
    }
  };

We can test the class,

Testable test;
  if (test) 
    std::cout << "Yes, test is working!\n";
  else 
    std::cout << "No, test is not working!\n";

However, opereator bool is considered unsafe because it allows nonsensical operations such as test << 1; or int i=test.

Usingoperator! is safer because we avoid implicit conversion or overloading issues.

The implementation is trivial,

bool operator!() const { // use operator!
    return !ok_;
  }

The two idiomatic ways to test Testable object are

  Testable test;
  if (!!test) 
    std::cout << "Yes, test is working!\n";
  if (!test2) {
    std::cout << "No, test2 is not working!\n";

The first version if (!!test) is what some people call the double-bang trick.