How can I improve this design that forces me to declare a member function const and declare variables mutable?

Question

For some reason I am iterating over elements of a class in an std::set and would like to slightly modify the keys, knowing that the order will be unchanged.

Answer 1

Another option is to const_cast to a reference type :

class Foo
{
public:
    void incrementB() const { ++ const_cast< int& >( b_ ); }
private:
    int b_;
};

But as sehe already said, you shouldn't modify set's elements.

Answer 2

One possibility might be to factor out the value part of Foo in a pimpl.

class Element
{
public:

    Element(int key, int value);

    Element( const Element& el );
    Element( Element&& el );

    ~Element();

    bool operator < (const Element& o) const;

    void incrementValue() const;
    int  getValue() const;

private:

    Element& operator=(const Element& );
    Element& operator=( Element&& el );

    struct Key
    {
        Key( const int key ) : m_KeyValue( key )
        {
        };

        const int m_KeyValue;
    };

    struct Value;

    const   Key                 m_Key;
    std::unique_ptr<Value>      m_Value;

};

struct Element::Value
{
    Value( int val ) : value(val)
    {

    }

    int value;
};

Element::Element(int key, int value) : 
    m_Key(key),
    m_Value( new Element::Value(value) )
{

}

Element::~Element()
{

}

Element::Element( const Element& el ) : 
    m_Key( el.m_Key ),
    m_Value( new Element::Value( *el.m_Value ) )
{

}

Element::Element( Element&& el ) : 
    m_Key(el.m_Key)
{
    m_Value = std::move(el.m_Value);
    el.m_Value.release();
}

bool Element::operator < (const Element& o) const 
{ 
    return m_Key.m_KeyValue < o.m_Key.m_KeyValue; 
}

void Element::incrementValue() const
{
    m_Value->value++;
}

int  
Element::getValue() const
{
    return m_Value->value;
}

void f()
{
    std::set<Element> s;

    s.insert(Element(1,2));
    s.insert(Element(2,3));

    std::for_each(s.begin(), s.end(), [](const Element& s) { s.incrementValue(); });

    std::for_each(s.begin(), s.end(), [](const Element& s) 
    { 
        std::cout << s.getValue() << std::endl; 

    }); 
}

int 
main()
{
    f();
    return 0;
}

EDIT: To be honest however you must decide if the extra level of indirection makes sense or you would be better off using a map.

Answer 3

You can't. Set elements are required to be const for container correctness:

It forces you to realize that the key part needs to be immutable, or the data structure invariants would be broken.

struct element 
{
     std::string key_part; // const in the set

     bool operator<(const element&o) const { return key_part<o.key_part; }

  private:
     mutable int m_cached; // non-key, *NOT* used in operator<
};

If you wanted to retain the possibility to 'express' const-ness in the non-key part, split it out into pairs and store them in a map:

std::map<std::string /*key_part*/, int /*m_cached*/> mapped;

or, more flexibly:

struct element 
{
     std::string key_part; // const in the set

     bool operator<(const element&o) const { return key_part<o.key_part; }

     struct value {
         int m_cached;
         int m_moredata; //...
     } /*not in the element itself*/;
};

std::map<element, element::value> mapped;