Const method that modifies *this without const_cast

Question

The following pattern has arisen in a program I'm writing. I hope it's not too contrived, but it manages to mutate a Foo object in the const method Foo::Questionable() const, without use of any const_cast or similar. Basically, Foo stores a reference to FooOwner and vice versa, and in Questionable(), Foo manages to modify itself in a const method by calling mutate_foo() on its owner. Questions follow the code.

#include "stdafx.h"
#include <iostream>
using namespace std;

class FooOwner;

class Foo {
    FooOwner& owner;
    int data;

public:
    Foo(FooOwner& owner_, int data_)
        : owner(owner_),
          data(data_)
    {
    }

    void SetData(int data_)
    {
        data = data_;
    }

    int Questionable() const;       // defined after FooOwner
};

class FooOwner {
    Foo* pFoo;

public:
    FooOwner()
        : pFoo(NULL)
    {}

    void own(Foo& foo)
    {
        pFoo = &foo;
    }

    void mutate_foo()
    {
        if (pFoo != NULL)
            pFoo->SetData(0);
    }
};

int Foo::Questionable() const
{
    owner.mutate_foo();     // point of interest
    return data;
}

int main()
{
    FooOwner foo_owner;
    Foo foo(foo_owner, 0);      // foo keeps reference to foo_owner
    foo_owner.own(foo);         // foo_owner keeps pointer to foo

    cout << foo.Questionable() << endl;  // correct?

    return 0;
}

Is this defined behavior? Should Foo::data be declared mutable? Or is this a sign I'm doing things fatally wrong? I'm trying to implement a kind of lazy-initialised 'data' which is only set when requested, and the following code compiles fine with no warnings, so I'm a little nervous I'm in UB land.

Edit: the const on Questionable() only makes immediate members const, and not the objects pointed to or referenced by the object. Does this make the code legal? I'm confused between the fact that in Questionable(), this has the type const Foo*, and further down the call stack, FooOwner legitimately has a non-const pointer it uses to modify Foo. Does this mean the Foo object can be modified or not?

Edit 2: perhaps an even simpler example:

class X {
    X* nonconst_this;   // Only turns in to X* const in a const method!
    int data;

public:
    X()
        : nonconst_this(this),
          data(0)
    {
    }

    int GetData() const
    {
        nonconst_this->data = 5;    // legal??
        return data;
    }
};

Answer 1

Without actually getting to whether it is/should/could be allowed, I would greatly advice against it. There are mechanisms in the language for what you want to achieve that don't require writing obscure constructs that will most probably confuse other developers.

Look into the mutable keyword. That keyword can be used to declare members that can be modified within const member methods as they do not affect the perceivable state of the class. Consider class that gets initialized with a set of parameters and performs a complex expensive calculation that may not be needed always:

class ComplexProcessor
{
public:
   void setInputs( int a, int b );
   int getValue() const;
private:
   int complexCalculation( int a, int b );
   int result;
};

A possible implementation is adding the result value as a member and calculating it for each set:

void ComplexProcessor::setInputs( int a, int b ) {
   result = complexCalculation( a, b );
}

But this means that the value is calculated in all sets, whether it is needed or not. If you think on the object as a black box, the interface just defines a method to set the parameters and a method to retrieve the calculated value. The instant when the calculation is performed does not really affect the perceived state of the object --as far as the value returned by the getter is correct. So we can modify the class to store the inputs (instead of the outputs) and calculate the result only when needed:

class ComplexProcessor2 {
public:
   void setInputs( int a, int b ) {
      a_ = a; b_ = b;
   }
   int getValue() const {
      return complexCalculation( a_, b_ );
   }
private:
   int complexCalculation( int a, int b );
   int a_,b_;
};

Semantically the second class and the first class are equivalent, but now we have avoided to perform the complex calculation if the value is not needed, so it is an advantage if the value is only requested in some cases. But at the same time it is a disadvantage if the value is requested many times for the same object: each time the complex calculation will be performed even if the inputs have not changed.

The solution is caching the result. For that we can the result to the class. When the result is requested, if we have already calculated it, we only need to retrieve it, while if we do not have the value we must calculate it. When the inputs change we invalidate the cache. This is when the mutable keyword comes in handy. It tells the compiler that the member is not part of the perceivable state and as such it can be modified within a constant method:

class ComplexProcessor3 {
public:
   ComplexProcessor3() : cached_(false) {}
   void setInputs( int a, int b ) {
      a_ = a; b_ = b;
      cached_ = false;
   }
   int getValue() const {
      if ( !cached_ ) {
         result_ = complexCalculation( a_, b_ );
         cached_ = true;
      }
      return result_;
   }
private:
   int complexCalculation( int a, int b );
   int a_,b_;
   // This are not part of the perceivable state:
   mutable int result_;
   mutable bool cached_;
};

The third implementation is semantically equivalent to the two previous versions, but avoid having to recalculate the value if the result is already known --and cached.

The mutable keyword is needed in other places, like in multithreaded applications the mutex in classes are often marked as mutable. Locking and unlocking a mutex are mutating operations for the mutex: its state is clearly changing. Now, a getter method in an object that is shared among different threads does not modify the perceived state but must acquire and release the lock if the operation has to be thread safe:

template <typename T>
class SharedValue {
public:
   void set( T v ) {
      scoped_lock lock(mutex_);
      value = v;
   }
   T get() const {
      scoped_lock lock(mutex_);
      return value;
   }
private:
   T value;
   mutable mutex mutex_;
};

The getter operation is semantically constant, even if it needs to modify the mutex to ensure single threaded access to the value member.

Answer 2

Consider the following:

int i = 3;

i is an object, and it has the type int. It is not cv-qualified (is not const or volatile, or both.)

Now we add:

const int& j = i;
const int* k = &i;

j is a reference which refers to i, and k is a pointer which points to i. (From now on, we simply combine "refer to" and "points to" to just "points to".)

At this point, we have two cv-qualified variables, j and k, that point to a non-cv-qualified object. This is mentioned in §7.1.​5.1/3:

A pointer or reference to a cv-qualified type need not actually point or refer to a cv-qualified object, but it is treated as if it does; a const-qualified access path cannot be used to modify an object even if the object referenced is a non-const object and can be modified through some other access path. [Note: cv-qualifiers are supported by the type system so that they cannot be subverted without casting (5.2.11). ]

What this means is that a compiler must respect that j and k are cv-qualified, even though they point to a non-cv-qualified object. (So j = 5 and *k = 5 are illegal, even though i = 5 is legal.)

We now consider removing the const from those:

const_cast<int&>(j) = 5;
*const_cast<int*>(k) = 5;

This is legal (§refer to 5.2.11), but is it undefined behavior? No. See §7.1.​5.1/4:

Except that any class member declared mutable (7.1.1) can be modified, any attempt to modify a const object during its lifetime (3.8) results in undefined behavior. ^{Emphasis mine.}

Remember that i is not const and that j and k both point to i. All we've done is tell the type system to remove the const-qualifier from the type so we can modify the pointed to object, and then modified i through those variables.

This is exactly the same as doing:

int& j = i; // removed const with const_cast...
int* k = &i; // ..trivially legal code

j = 5;
*k = 5;

And this is trivially legal. We now consider that i was this instead:

const int i = 3;

What of our code now?

const_cast<int&>(j) = 5;
*const_cast<int*>(k) = 5;

It now leads to undefined behavior, because i is a const-qualified object. We told the type system to remove const so we can modify the pointed to object, and then modified a const-qualified object. This is undefined, as quoted above.

Again, more apparent as:

int& j = i; // removed const with const_cast...
int* k = &i; // ...but this is not legal!

j = 5;
*k = 5;

Note that simply doing this:

const_cast<int&>(j);
*const_cast<int*>(k);

Is perfectly legal and defined, as no const-qualified objects are being modified; we're just messing with the type-system.

---

Now consider:

struct foo
{
    foo() :
    me(this), self(*this), i(3)
    {}

    void bar() const
    {
        me->i = 5;
        self.i = 5;
    }

    foo* me;
    foo& self;
    int i;
};

What does const on bar do to the members? It makes access to them go through something called a cv-qualified access path. (It does this by changing the type of this from T* const to cv T const*, where cv is the cv-qualifiers on the function.)

So what are the members types during the execution of bar? They are:

// const-pointer-to-non-const, where the pointer points cannot be changed
foo* const me;

// foo& const is ill-formed, cv-qualifiers do nothing to reference types
foo& self; 

// same as const int
int const i; 

Of course, the types are irrelevant, as the important thing is the const-qualification of the pointed to objects, not the pointers. (Had k above been const int* const, the latter const is irrelevant.) We now consider:

int main()
{
    foo f;
    f.bar(); // UB?
}

Within bar, both me and self point to a non-const foo, so just like with int i above we have well-defined behavior. Had we had:

const foo f;
f.bar(); // UB!

We would have had UB, just like with const int, because we would be modifying a const-qualified object.

In your question, you have no const-qualified objects, so you have no undefined behavior.

---

And just to add an appeal to authority, consider the const_cast trick by Scott Meyers, used to recycle a const-qualified function in a non-const function:

struct foo
{
    const int& bar() const
    {
        int* result = /* complicated process to get the resulting int */
        return *result; 
    }

    int& bar()
    {
        // we wouldn't like to copy-paste a complicated process, what can we do?
    }

};

He suggests:

int& bar(void)
{
    const foo& self = *this; // add const
    const int& result = self.bar(); // call const version
    return const_cast<int&>(result); // take off const
}

Or how it's usually written:

int& bar(void)
{
    return const_cast<int&>( // (3) remove const from result
            static_cast<const foo&>(*this) // (1) add const to this
            .bar() // (2) call const version
            ); 
}

Note this is, again, perfectly legal and well-defined. Specifically, because this function must be called on a non-const-qualified foo, we are perfectly safe in stripping the const-qualification from the return type of int& boo() const.

(Unless someone shoots themselves with a const_cast + call in the first place.)

---

To summarize:

struct foo
{
    foo(void) :
    i(),
    self(*this), me(this),
    self_2(*this), me_2(this)
    {}

    const int& bar() const
    {
        return i; // always well-formed, always defined
    }

    int& bar() const
    {
        // always well-formed, always well-defined
        return const_cast<int&>(
                static_cast<const foo&>(*this).
                bar()
                );
    }

    void baz() const
    {
        // always ill-formed, i is a const int in baz
        i = 5; 

        // always ill-formed, me is a foo* const in baz
        me = 0;

        // always ill-formed, me_2 is a const foo* const in baz
        me_2 = 0; 

        // always well-formed, defined if the foo pointed to is non-const
        self.i = 5;
        me->i = 5; 

        // always ill-formed, type points to a const (though the object it 
        // points to may or may not necessarily be const-qualified)
        self_2.i = 5; 
        me_2->i = 5; 

        // always well-formed, always defined, nothing being modified
        // (note: if the result/member was not an int and was a user-defined 
        // type, if it had its copy-constructor and/or operator= parameter 
        // as T& instead of const T&, like auto_ptr for example, this would 
        // be defined if the foo self_2/me_2 points to was non-const
        int r = const_cast<foo&>(self_2).i;
        r = const_cast<foo* const>(me_2)->i;

        // always well-formed, always defined, nothing being modified.
        // (same idea behind the non-const bar, only const qualifications
        // are being changed, not any objects.)
        const_cast<foo&>(self_2);
        const_cast<foo* const>(me_2);

        // always well-formed, defined if the foo pointed to is non-const
        // (note, equivalent to using self and me)
        const_cast<foo&>(self_2).i = 5;
        const_cast<foo* const>(me_2)->i = 5;

        // always well-formed, defined if the foo pointed to is non-const
        const_cast<foo&>(*this).i = 5;
        const_cast<foo* const>(this)->i = 5;
    }

    int i;

    foo& self;
    foo* me;
    const foo& self_2;
    const foo* me_2;
};

int main()
{
    int i = 0;
    {
        // always well-formed, always defined
        int& x = i;
        int* y = &i;
        const int& z = i;
        const int* w = &i;

        // always well-formed, always defined
        // (note, same as using x and y)
        const_cast<int&>(z) = 5;
        const_cast<int*>(w) = 5;
    }

    const int j = 0;
    {
        // never well-formed, strips cv-qualifications without a cast
        int& x = j;
        int* y = &j;

        // always well-formed, always defined
        const int& z = i;
        const int* w = &i;

        // always well-formed, never defined
        // (note, same as using x and y, but those were ill-formed)
        const_cast<int&>(z) = 5;
        const_cast<int*>(w) = 5;
    }

    foo x;
    x.bar(); // calls non-const, well-formed, always defined
    x.bar() = 5; // calls non-const, which calls const, removes const from
                 // result, and modifies which is defined because the object
                 // pointed to by the returned reference is non-const,
                 // because x is non-const.

    x.baz(); // well-formed, always defined

    const foo y;
    y.bar(); // calls const, well-formed, always defined
    const_cast<foo&>(y).bar(); // calls non-const, well-formed, 
                               // always defined (nothing being modified)
    const_cast<foo&>(y).bar() = 5; // calls non-const, which calls const,
                                   // removes const from result, and
                                   // modifies which is undefined because 
                                   // the object pointed to by the returned
                                   // reference is const, because y is const.

    y.baz(); // well-formed, always undefined
}

_{I refer to the ISO C++03 standard.}

Answer 3

IMO, you are not doing anything technically wrong. May-be it would be simpler to understand if the member was a pointer.

class X
{
    Y* m_ptr;
    void foo() const {
        m_ptr = NULL; //illegal
        *m_ptr = 42; //legal
    }
};

const makes the pointer const, not the pointee.

Consider the difference between:

const X* ptr;
X* const ptr;  //this is what happens in const member functions

As to references, since they can't be reseated anyway, the const keyword on the method has no effect whatsoever on reference members.

In your example, I don't see any const objects, so you are not doing anything bad, just exploiting a strange loophole in the way const correctness works in C++.