Overloading subscript operator in an user defined class C++

Question

Consider the following class:

class SocialPrefNode{

public:

// Constructors & Destructor
SocialPrefNode( );
SocialPrefNode( char self, int ind, int link, bool stack, std::vector<SocialPrefNode*> pref,
                std::vector<SocialPrefNode*> worse, std::vector<SocialPrefNode*> indiff );

SocialPrefNode( const SocialPrefNode& copy );

~SocialPrefNode( );

// Setters
void set_id( char self );

void set_index( int ind );
void set_lowlink( int link );

void set_onstack( bool stack );

void set_pref( std::vector<SocialPrefNode*> prefs );
void set_pref( SocialPrefNode& prefs );

void set_worse( std::vector<SocialPrefNode*> wrs );
void set_worse( SocialPrefNode& wrs );

void set_indiff( std::vector<SocialPrefNode*> indiff );
void set_indiff( SocialPrefNode& indiff );

// Getters
char get_id( ){ return id; }

int get_index( ){ return index; }
int get_lowlink( ){ return lowlink; }

bool get_onstack( ){ return onstack; }

std::vector<SocialPrefNode*> get_preferences( ){ return preferences; }
std::vector<SocialPrefNode*> get_worse( ){ return worsethan; }
std::vector<SocialPrefNode*> get_indiff( ){ return indifference; }

// Operators
SocialPrefNode& operator=( const SocialPrefNode& copy );

private:

char id{ };

int index{ };
int lowlink{ };

bool onstack{ };

std::vector<SocialPrefNode*> preferences{ };
std::vector<SocialPrefNode*> worsethan{ };
std::vector<SocialPrefNode*> indifference{ };
};

std::ostream& operator<<( std::ostream& os, SocialPrefNode& node );

Question: Is there a way to overload/override/redefine the subscript operator s.t. one can, for example, have access to a vector of choice, among the three options.

I.e., suppose: SocialPrefNode ordering{ }. I want to be able to use the subscript operator as in ordering[ i ] AND be able to choose one, among the three vectors in the class, for the subscript/index i to act up on.

Example: one wants to access the third element in the preferences vector of a SocialPrefNode ordering. Then, one does ordering[ 2 ] and, thus, have access to the desired element.

So you have three data: the instance ordering, the subsript i, and... where is the type of container to search in? — Ripi2
– Ripi2, Commented Nov 17, 2018 at 18:12
You could write an operator[] that takes 0-2 and returns a std::vector<SocialPrefNode*> (either preferences or worsethan or indifference). Of course that would require a multi dimensional subscript like x[0][2] for element 2 in vector 0. Or you could have a wrapper type for the index so that the wrapper comes with the information which vector to read. — Werner Henze
– Werner Henze, Commented Nov 17, 2018 at 18:19

Werner Henze · Accepted Answer · 2018-11-19 13:53:48Z

A possible solution is to use phantom types to wrap up the vector to use. I followed this blog post and came up with the following solution.

// Wrapper need once in the complete project:
template <typename T, typename Parameter>
class NamedType
{
public:
    explicit NamedType(T const& value) : value_(value) {}
    explicit NamedType(T&& value) : value_(std::move(value)) {}
    T& get() { return value_; }
    T const& get() const { return value_; }
private:
    T value_;
};

// Only showing the new parts in your class!
class SocialPrefNode {
public:
    // One phantom type for each array index
    using preferences_index = NamedType<std::ptrdiff_t, struct preferences_Parameter>;
    using worsethan_index = NamedType<std::ptrdiff_t, struct worsethan_Parameter>;
    using indifference_index = NamedType<std::ptrdiff_t, struct indifference_Parameter>;

    // One operator[] for each array index type
    SocialPrefNode* operator[](preferences_index i) { return preferences[i.get()]; }
    SocialPrefNode* operator[](worsethan_index i) { return worsethan[i.get()]; }
    SocialPrefNode* operator[](indifference_index i) { return indifference[i.get()]; }
}

// Usage is simple assuming a SocialPrefNode s.
s[0]; // This will not work!
s[SocialPrefNode::preferences_index{ 0 }];  // Accessing preferences
s[SocialPrefNode::worsethan_index{ 0 }];    // Accessing worsethan
s[SocialPrefNode::indifference_index{ 0 }]; // Accessing indifference

If you don't mind adding an external dependency, you can directly use the Github repo of the author of NamedType (where the class is a bit extended in comparison to here).

I really liked your comment above, this really seems like the best way to do what I want to. Thanks for the help!

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