Adding an element <string, pointer> to boost unordered hashmap

Question

I am using boost unordered hashmap in C++ and I am unable to add elements to my hash map (my program has a segmentation fault). I am very new to C++ and the majority of my code (except the hash map handling) is C code. Could you please point out the problem.

// my simplified code

struct Record
{
        char *data;

};

typedef boost::unordered_map<std::string, std::vector<Record*> > MAP;
typedef std::pair<std::string,std::vector<Record*> > PAIR;

struct OuterRelation
{
        short num_keys;
        short join_key_ndx;
        MAP hash_table;
};

OuterRelation *outer_relation = (OuterRelation *) malloc (sizeof(OuterRelation)) ;

Record *new = (Record *) malloc(sizeof(Record));
new->data = "somestring";

outer_relation->hash_table[new->data].push_back(new);

The problem is in the last line.

RobH · Accepted Answer · 2014-03-05 14:44:23Z

3

STOP USING malloc. That's for C. Correct syntax would be:

OuterRelation *outer_relation = new OuterRelation;

Your use of malloc has allocated enough space for the OuterRelation struct alone. This might be enough if the struct contained only plain-old-data. However, the hash_table member is a class and using malloc has left it uninitialised.

new is (at its most basic) a combination of malloc and a call to the new'd object's constructor. The constructor for your struct will in turn call the constructors of its members, including the map. The map's constructor will initialise its data members.

You also need to stop using new as a variable name. That clashes with the new C++ keyword.

answered Mar 5, 2014 at 14:44

RobH

3,3481 gold badge24 silver badges27 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

NewToAndroid Over a year ago

If I would like to free OuterRelation at the end of my program, does C++ offer free(outer_relation) construct? Is it a common practice to do such frees or do I leave it to the garbage collector?

RobH Over a year ago

Call delete outer_relation;. delete calls the object's destructor then frees its memory. C++ has no garbage collection. (Well, technically garbage collection libraries exist, but I haven't seen code which uses one yet.)

Collectives™ on Stack Overflow

Adding an element <string, pointer> to boost unordered hashmap

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related