Question

classes in the stl, such as unique_ptr will occasionally show examples such as:

// unique_ptr constructor example
#include <iostream>
#include <memory>

int main () {
  std::default_delete<int> d;
  std::unique_ptr<int> u1;
  std::unique_ptr<int> u2 (nullptr);
  std::unique_ptr<int> u3 (new int);
  std::unique_ptr<int> u4 (new int, d);
  std::unique_ptr<int> u5 (new int, std::default_delete<int>());
  std::unique_ptr<int> u6 (std::move(u5));
  std::unique_ptr<void> u7 (std::move(u6));
  std::unique_ptr<int> u8 (std::auto_ptr<int>(new int));

  std::cout << "u1: " << (u1?"not null":"null") << '\n';
  std::cout << "u2: " << (u2?"not null":"null") << '\n';
  std::cout << "u3: " << (u3?"not null":"null") << '\n';
  std::cout << "u4: " << (u4?"not null":"null") << '\n';
  std::cout << "u5: " << (u5?"not null":"null") << '\n';
  std::cout << "u6: " << (u6?"not null":"null") << '\n';
  std::cout << "u7: " << (u7?"not null":"null") << '\n';
  std::cout << "u8: " << (u8?"not null":"null") << '\n';

 *emphasized text* return 0;
}

The line:

 std::cout << "u1: " << (u1?"not null":"null") << '\n';

shows the unique_ptr u1 being directly cast to false if it is tracking a null pointer.

I have seen this behaviour used in other custom classes. How is this managed and what operator decides whether a direct cast to bool such as this returns true or false?

Answer 1

It is implemented as a member conversion operator of the form explicit operator bool() const;. Whether it returns true or false is implemented in the logic of the class itself. For example, this class has an bool conversion operator that returns true if it’s data member has value 42, and false otherwise:

struct Foo
{
  explicit operator bool() const { return n==42; }
  int n;
};

#include <iostream>

int main()
{
  Foo f0{12};
  Foo f1{42};

  std::cout << (f0 ? "true\n" : "false\n"); 
  std::cout << (f1 ? "true\n" : "false\n"); 
}