How to have const members in stl container values in C++?

Question

I like to make my C++ member variables const if they should not be changed once the object is constructed, however, sometimes they need to be modified by STL. For example, if I have a vector of my class with const members and I try to swap two elements in the vector, STL tries to use the default generated operator=() and fails because of the const member variables.

I feel like the operator=() is like a constructor in that the whole object is being created and thus would like some way to allow operator=() while still having my const member variables.

Is there anyway to do this in C++03? If not, what about in C++11, perhaps in-place construction is for this?

class Foo {
  const int _id;

  static int _generate_unique_id();

public:
  Foo()
    : _id(_generate_unique_id()) {
  }
};

vector<Foo> foo_vector;

// Fill foo_vector with several entries:
//   [...]

// Try to swap the first and second elements of the vector:
swap(*foo_vector.begin(), *(foo_vector.begin() + 1));
// The above fails to compile due to const member variable _id
// prohibits us from using the default assignment operator.

Answer 1

A solution for storing not assignable objects in standard library containers is storing (smart) pointers to the objects. Not always ideal, but workable.

Answer 2

For example, if I have a vector of my class with const members and I try to swap two elements in the vector, STL tries to use the default generated operator=() and fails because of the const member variables.

Implement the "big three and a half" (default and copy constructor, assignment operator and swap), with the assignment operator explicitly skipping the reassignment if _id.

Answer 3

What you want is a thing like the Java immutable idiom. This is awesome with pointers (and thus, garbage collected languages) and less awesome with value-semantic languages like C++.

You have two solutions:

1 - Make your object immutable in the interface

The member is private (or it should be), so no one but the class itself (and its friends) can modify it. So all you need is to make sure no one does inside the class (which you control) and offer no way in the protected/public interface to leave others the power to do so.

TL;DR: Make your object non const. Don't modify it inside the class. Add a const getter. Remove the setter (if any).

2 - Use a std::unique_ptr<const Data>

Now we follow the Java idiom. The object is const, but the pointer can be reattributed, which is exactly what you want.

This is actually better than the const Data * member alternative because of its exception safety.

Bonus: Don't manually call the destructor to reconstruct again the object

There's an answer proposing that.

As mentionned first by sehe, don't do that.

Your point is to increase the quality of your code, which means your code will need to be exception safe, at one point or the other. And manually playing with your object lifetime will make it unusable in quality code.

Read Herb Sutter's article on the subject: http://www.gotw.ca/gotw/023.htm

Answer 4

const on members doesn't just prevent the programmer from modifying the value of the member during its lifetime; it also enables compiler optimisations by specifying that attempts to modify it are undefined behaviour (see const member and assignment operator. How to avoid the undefined behavior?).

One way to do what you want is to write a nonmodifiable container that gives semantic const while leaving you as the programmer the possibility of modifying the contained value:

template<typename T> class nonmodifiable {
   T t;
public:
   nonmodifiable(T t): t{std::move(t)} {}
   operator const T &() const { return t; }
   nonmodifiable &operator=(const nonmodifiable &) = delete;
};

You can now write:

class Foo {
  nonmodifiable<int> _id;
  // etc.
};

and because neither _id nor its contained value are const, use the destruct-placement new dance to reassign its value:

Foo &operator=(const Foo &foo) {
   if (this != &foo) {
      _id.~nonmodifiable<int>();
      new (&_id) nonmodifiable<int>(foo._id);
   }
   return this;
}