Solving leaky memory and syntax issues in a simple hash table - c++

Question

I'm implementing a basic hashtable. My logic for the table makes sense (at least to me), but I'm a bit rusty with my C++. My program returns a free memory error when I run it, but I can't seem to figure out where my problem is. I think is has to do with how I call the pointers in the various class functions.
#include <iostream>
#include <unordered_map>
#include <string>
#include <cmath>
#include <exception>
using namespace std;
int hashU(string in/*, int M*/){ //hThe hash function that utilizes a smal pseusorandom number
char *v = new char[in.size() + 1]; //generator to return an number between 0 and 50. (I arbitrarily chose 50 as the upper limit)
copy(in.begin(), in.end(), v); //First the input string is turned into a char* for use in the the function.
v[in.size()] = '\0';
int h, a = 31415, b = 27183;
for(h=0;*v!=0;v++,a=a*b%(49-1))
h = (a*h + *v)%50;
delete[] v; //Delete the char* to prevent leaky memory.
return (h<0) ? (h+50) : h; //Return number
}
struct hashNode{ //The node that will store the key and the values
string key;
float val;
struct hashNode *next;
};
struct hashLink{ //The linked list that will store additional keys and values should there be a collision.
public:
struct hashNode *start; //Start pointer
struct hashNode *tail; //Tail pointer
hashLink(){ //hashLink constructor
start=NULL;
tail=NULL;
}
void push(string key, float val); //Function to push values to stack. Used if there is a collision.
};
void hashLink::push(string key, float val){
struct hashNode *ptr;
ptr = new hashNode;
ptr->key = key;
ptr->val = val;
ptr->next = NULL;
if(start != NULL){
ptr->next = tail;
}
tail = ptr;
return;
}
struct hashTable{ //The "hash table." Creates an array of Linked Lists that are indexed by the values returned by the hash function.
public:
hashLink hash[50];
hashTable(){ //Constructor
}
void emplace(string in, float val); //Function to insert a new key and value into the table.
float fetch(string in); //Function to retrieve a stored key.
};
void hashTable::emplace(string in, float val){
int i = hashU(in); //Retrieve index of key from hash function.
hashNode *trav; //Create node traveler
trav = hash[i].start; //Set the traveler to the start of the desired linked list
while(trav!=hash[i].tail){ //Traverse the list searching to see if the input key already exists
if(trav->key.compare(in)==0){ //If the input key already exists, its associated value is updated, and the function returns.
trav->val = val;
return;
}
else //Travler moves to next node if the input key in not found.
trav = trav->next;
}
hash[i].push(in,val); //If the traveler does not see the input key, the request key must not exist and must be created by pushing the input key and associated value to the stack.
return;
}
float hashTable::fetch(string in){
int i = hashU(in); //Retrieve index of key
hashNode *trav; //Create node traveler and set it to the start of the appropriate list.
trav = hash[i].start;
while(trav!=hash[i].tail){ //Traverse the linked list searching for the requested key.
if(trav->key.compare(in)==0){ //If the the requested key is found, return the associated value.
return trav->val;
}
else
trav = trav->next; //If not found in the current node, move to the next.
}
return false; //If the requested key is not found, return false.
}
int main(){
hashTable vars; //initialize the hash table
float num = 5.23; //create test variable
vars.emplace("KILO",num);
cout<<vars.fetch("KILO")<<endl;
return 0;
}

Answer 1

The problem is that when you call delete[] v, you have advanced v such that it is pointing to the 0 at the end of the string, which is the wrong address to delete.
Also, you're wasting a lot of code unnecessarily copying the string out of where it is already available as a c-string.
unsigned int hashU(string in/*, int M*/) {
const char* v = in.c_str();
unsigned int h, a = 31415, b = 27183;
for(h=0;*v!=0;v++,a=a*b%(49-1))
h = (a*h + *v);
return h % 50;
}

Answer 2

for(h=0;*v!=0;v++,a=a*b%(49-1))
h = (a*h + *v)%50;
delete[] v; //Delete the char* to prevent leaky
You are incrementing v, then deleting an invalid memory location.