map with class value segmentation fault - c++

Question

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.
get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
set(key, value) - Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before inserting a new item.
#include <iostream>
#include <map>
using namespace std;
struct node{
int val;
struct node* next;
struct node* prev;
};
class dlist{
public:
dlist(){}
dlist(int capacity){
cap=capacity;
}
void add(int value){
node* n=new node;
n->val=value;
if (size==0){
size++;
tail=n;
head=tail;
}
else {
if (size==cap){
node* buf=head;
head=head->next;
head->prev=NULL;
delete buf;
size--;
}
tail->next=n;
n->prev=tail;
tail=n;
size++;
}
}
int getVal(){
if (tail==NULL)
return -1;
return tail->val;
}
private:
int cap;
int size;
node* tail;
node* head;
};
class LRUCache{
public:
LRUCache(int capacity) {
cap=capacity;
}
int get(int key) {
if(cap!=0&&cache.find(key)!=cache.end())
return cache[key].getVal();
return -1;
}
void set(int key, int value) {
if (cap==0)
return;
if(cache.find(key)==cache.end()){
dlist d=dlist(cap);
cache.insert(make_pair(key,d));
}
cache[key].add(value);
}
private:
int cap;
map<int,dlist> cache;
};
int main()
{
LRUCache lru(3);
cout<<"asd";
lru.set(1,9);
lru.set(1,8);
lru.set(1,1);
lru.set(1,7);
lru.set(2,9);
cout<<lru.get(1)<<endl;
cout<<lru.get(2)<<endl;
cout<<lru.get(3)<<endl;
return 0;
}
so I used a map and a custom double linked list, it seems to working fine with if I add the cout line right after initializing LRU, but it will have seg fault if I don't, I and not very sure what should I do to manage the memory use of LRU(if this is the problem)
Also if there's any line that could be better written(aside from std namespace) please tell me, I would really appreciate that.

Answer 1

Your program exhibits undefined behavior since the member variables size, tail, and head of dlist are not initialized before being used.
Use
dlist() : dlist(0) {}
dlist(int capacity) : cap(capacity), size(0), tail(nullptr), head(nullptr) {}
That fixes the segmentation violation problem in my testing.
I recommend adding a constructor to node also:
struct node{
node(int v) : val(v), next(nullptr), prev(nullptr) {}
int val;
struct node* next;
struct node* prev;
};
and use
node* n=new node(value);
instead of
node* n=new node;
n->val=value;