[SOLVED] template problem in c++

101b147 · 09-12-2010, 03:20 PM

hey guys. i'm having some problems in using templates in c++. here is the code i'm trying to compile. i'll send a shrinked version.

Code:

#include<list>
#include<utility>
#include<algorithm>
#include<string>
#include<vector>
using namespace std;

template<class K, class V> struct Hash{
	list<pair<K, V> > *hash;	
	int (*hashFunction)(K key);	
	int buckets;			

	Hash(int (*hashFunction)(K), int buckets){};

	~Hash(){};

	list<pair<K, V> >::iterator put(K key, V value, int pos_flag = -1){ return 0; };
};

int main(){
	return 0;
}

the problem is that i don't know how to specify the return type of the put() method.
when i change the return type to

Code:

list<pair<int, int> >::iterator

, it compiles normally.

thx in advance.

johnsfine · 09-12-2010, 06:12 PM

The problem is that the compiler can't know that
list<pair<K, V> >::iterator
is the name of a type. Even though the compiler has at that point seen the whole definition of list so you would expect it to know iterator is a name of a type inside list, the C++ standard has rules that prevent it from knowing.

In this situation, I strongly prefer to use several typedefs to break up the ugly declarations. So you could fix your problem just by adding the keyword typename to your declaration. But I personally think it is very ugly to use typename that way anywhere other than right after the keyword typedef

I haven't tested any of this, but I think it is right:

Code:

#include<list>
#include<utility>
#include<algorithm>
#include<string>
#include<vector>
using namespace std;

template<class K, class V> struct Hash{
        typedef list<pair<K, V> > my_list;
        typedef typename my_list::iterator iterator;
	my_list *hash;	
	int (*hashFunction)(K key);	
	int buckets;			

	Hash(int (*hashFunction)(K), int buckets){};

	~Hash(){};

	iterator put(K key, V value, int pos_flag = -1){ return 0; };
};

Actually I prefer the following, but that makes changes further from the scope of your question

Code:

#include<list>
#include<utility>
#include<algorithm>
#include<string>
#include<vector>

template<class K, class V> struct Hash{
        typedef std::pair<K, V> pair;
        typedef std::list<pair> list;
        typedef typename list::iterator iterator;
	list *hash;	
	int (*hashFunction)(K key);	
	int buckets;			

	Hash(int (*hashFunction)(K), int buckets){};

	~Hash(){};

	iterator put(K key, V value, int pos_flag = -1){ return 0; };
};

101b147 · 09-13-2010, 12:17 PM

thx, that solved my problem!! and thx for the extra explanation.