C ++ STL-Map排序功能

Question

I'm trying to insert binary data into a B-Tree, implementing the STL interface. My typedefinition is the following:

typedef btree::btree_map<unsigned char*, int, less<unsigned char*>, allocator<unsigned char*>, node_size> Tree;

The problem is that less<unsigned char*> do not compare the elements as I expect.

Tree *bt = create_btree();

unsigned char *test = (unsigned char *) malloc(5);
unsigned char *test2 = (unsigned char *) malloc(5);

memset(test, 0, 5);
memset(test2, 0, 5);

cout << "Is equal: " << memcmp(test, test2, 5) << "\n";

memcpy(test + 0, "a\0", 2);
memcpy(test + 2, "HAM", 3);

memcpy(test2 + 0, "a\0", 2);
memcpy(test2 + 2, "HAM", 3);

bt->insert(make_pair(test, 1));
bt->insert(make_pair(test2, 2));

cout << "Is equal: " << memcmp(test, test2, 5) << "\n";


Tree::iterator iter;

for (iter = bt->begin(); iter != bt->end(); iter++) {

    cout << "Data: " << iter->first << " = " << iter->second << "\n";
}

When I run the code snippet above the function memcmp will return 0 which signals that both arrays are equal but the output of the programm is :

Is equal: 0
Is equal: 0
Data: a = 1
Data: a = 2

I would expect that the second insert would overwrite the first, and only one line with Data: a = 2 will be printed. I know that the key will be only printed till '\\0' occurs. So here is the key in hex representation: `61 00 48 41 4d'.

Correct me if I'm wrong but I think it has something to do with the less operator.

My question is: Is there a way to pass a function like memcmp to the map in order to compare the binary arrays correct.

Thank in advance

Answer 1

I think it has something to do with the less operator.

Correct. less<T*> function will compare pointers, not the content of array that might be pointed by them.

Is there a way to pass a function like memcmp to the map in order to compare the binary arrays correct.

Yes, see the third template parameter and the first constructor parameter. You can pass a functor of your own choice (assuming btree::btree_map has the same interface as the std::map ).

I would recommend using a std::vector or std::string as the key instead of a pointer. Not only do they work with less<> , but you also don't need to manage their memory.