No matching member function to call for 'push_back' error - c++

Question

While implementing LRU cache got this error.
Earlier I was implementing it via maps it works then but somehow even when doing it as vector it does not work.
#include <list>
class LRUCache {
list<pair<int,int>> lru;
int cap;
vector<list<pair<int, int>>::iterator> hash;
public:
LRUCache(int capacity) {
cap = capacity;
for(int i=0;i<=3000;i++)
hash.push_back(nullptr);
}
int get(int key) {
if(hash[(key)]!=nullptr)
{
int v = hash[key]->first;
lru.erase(hash[key]);
lru.push_front({v,key});
hash[key] = lru.begin();
return v;
}
else
return -1;
}
void put(int key, int value) {
if(hash[(key)]!=nullptr)
{
int v = value;
lru.erase(hash[key]);
lru.push_front({v,key});
hash[key] = lru.begin();
}
else if(lru.size()<cap)
{
lru.push_front({value,key});
hash[key] = lru.begin();
}
else
{
lru.push_front({value,key});
hash[key] = lru.begin();
auto it = lru.end();
it--;
hash[(it->second)] = nullptr;
lru.erase(it);
}
}
};
This way does not work either.
vector<list<pair<int, int>>::iterator> hash(3001,NULL);
Can we not create a vector of pointers?

Answer 1

Create an iterator variable, instead of nullptr value, as bellow:
list<pair<int, int>>::iterator emptyIt; // this iterator object refer to nothing
// Using `emptyIt` to initialize the hash
LRUCache(int capacity) {
cap = capacity;
for(int i=0;i<=3000;i++)
hash.push_back(emptyIt);
}
// Using emptyIt instead of nullptr
int get(int key) {
if(hash[(key)]!=emptyIt)
{
int v = hash[key]->first;
lru.erase(hash[key]);
lru.push_front({v,key});
hash[key] = lru.begin();
return v;
}
else
return -1;
}
void put(int key, int value) {
if(hash[(key)]!=emptyIt)
{
int v = value;
lru.erase(hash[key]);
lru.push_front({v,key});
hash[key] = lru.begin();
}
else if(lru.size()<cap)
{
lru.push_front({value,key});
hash[key] = lru.begin();
}
else
{
lru.push_front({value,key});
hash[key] = lru.begin();
auto it = lru.end();
it--;
hash[(it->second)] = emptyIt;
lru.erase(it);
}
}
};