CODEWITHSUNDEEP
c++objectstlunordered-map
JayB
JayB

Reputation: 397

Trying to place objects into unordered_map

I'm trying to create a unordered_map that has an integer as a key and a Transition object as the value.... Here's what I have:

Instantiation

unordered_map<int, Transition> transitions;

Transition Class Declaration:

class Transition{

    public:
            Transition(int n, char r, char m);
            ~Transition();
            int getNextState();
            char getReplacement();
            char getMovement();

    private:
            int nextState;
            char replacement;
            char movement;
};

Adding a transition to the map

// Create transition object
Transition t(r,b,x);
transitions[keyForMap] = t;

I'm getting this error:

/usr/include/c++/4.7/bits/hashtable_policy.h:445:24: error: no matching function for call to ‘Transition::Transition()’
/usr/include/c++/4.7/bits/hashtable_policy.h:445:24: note: candidates are:
In file included from ball_p1.cpp:6:0:
Transition.h:4:3: note: Transition::Transition(int, char, char)
Transition.h:4:3: note:   candidate expects 3 arguments, 0 provided
Transition.h:1:7: note: constexpr Transition::Transition(const Transition&)
Transition.h:1:7: note:   candidate expects 1 argument, 0 provided

Do I need to somehow specify the parameters that the constructor takes somewhere in the instantiation? What am I doing wrong? Thanks for any help ahead of time.

Upvotes: 1

Views: 8172

Answers (3)

JobHunter69
JobHunter69

Reputation: 2290

Your constructor got rid of the default constructor (which is needed).

Upvotes: 0

user4726402
user4726402

Reputation:

std::map::operator[] works as follows:
- If the specified key already exists, it returns a reference to the corresponding value.
- If the specified key doesn't exist, it inserts it, assigns it a default value, and return a reference to this recently created instance of the default value.

For example,
map<string, int> m; std::cout << m["new_key"]; // This prints the default value for type int, namely '0'.

If your value type does not support a default value (e.g. if it's a custom class that does not have a default constructor), then you can not use std::map::operator[]. You can, however, use std::map::insert to perform the same functionality. The latter will insert a pair directly into your map, without passing by the intermediate step of creating a default value.

Upvotes: 0

Aitch
Aitch

Reputation: 1697

I always use map.insert(std::make_pair(keyForMap, transition));.

Upvotes: 3

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