Question

My program makes a frequency map of characters (which I store in , surprise surprise, a Map), I am trying to copy each element from this map into a Priority Queue so that I can have a sorted copy of these values (I plan to make further use of the Q, that’s why am not sorting the map) , but whenever I try to copy these values , the program executes fine for the first two or three iterations and fails on the fourth citing an “Invalid heap” error.

I’m not sure how to proceed from here, so I am posting the code for the classes in question.

#include "srcFile.h"
#include <string>
#include <iostream>

srcFile::srcFile(std::string s_flName)
{
    // Storing the file name
    s_fileName= s_flName;
}

srcFile::srcFile()
{
    // Default constructor (never to be used)
}

srcFile::~srcFile(void)
{
}

void srcFile::dispOverallMap ()
{
    std::map<char,int>::iterator dispIterator;
dispIterator = map_charFreqDistribution.begin();
charElement *currentChar;

std::cout<<"\n Frequency distribution map \n";
while(dispIterator != map_charFreqDistribution.end())
{
    std::cout<< "Character : " << (int)dispIterator->first << " Frequency : "<< dispIterator->second<<'\n';
    currentChar = new charElement(dispIterator->first,dispIterator->second);

    Q_freqDistribution.push(*currentChar);
    dispIterator++;

    // delete currentChar;
}

while(!Q_freqDistribution.empty())
{
    std::cout<<'\n'<<"Queue Element : " << (int)Q_freqDistribution.top().ch_elementChar << " Frequency : " << Q_freqDistribution.top().i_frequency;
    Q_freqDistribution.pop();
}
}

map_charFreqDistribution has already been populated, if I remove the line
Q_freqDistribution.push(*currentChar);
Then I can verify that the Map is indeed there.

Also , both the Q and the use charElement as the template type , its nothing except the character and its frequency, along with 2 pointers to facilitate tree generation (unused upto this point)

Adding the definition of charElement on request

#pragma once
class charElement
{
public:
    // Holds the character for the element in question
char ch_elementChar;

// Holds the number of times the character appeared in the file
int i_frequency;

// Left pointer for tree
charElement* ptr_left;
// Right pointer for tree
charElement* ptr_right;

charElement(char,int);
charElement(void);
~charElement(void);
void operator=(charElement&);
};

class compareCharElt
{
public:
bool operator()(charElement &operand1,charElement &operand2)
{
    // If the frequency of op1 < op2 then return true
    if(operand1.i_frequency < operand2.i_frequency) return true;

    // If the frequency of op1 > op2 then return false
    if(operand1.i_frequency > operand2.i_frequency)return false;

    // If the frequency of op1 == op2 then return true (that is priority is indicated to be less even though frequencies are equal)
    if(operand1.i_frequency == operand2.i_frequency)return false;
}
};

Definition of Map and Queue

// The map which holds the frequency distribution of all characters in the file
    std::map<char,int> map_charFreqDistribution;
    void dispOverallMap();

// Create Q which holds character elements
std::priority_queue<charElement,std::vector<charElement>,compareCharElt>                 Q_freqDistribution;

P.S.This may be a noob question, but Is there an easier way to post blocks of code , putting 4 spaces in front of huge code chunks doesn’t seem all that efficient to me! Are pastebin links acceptable here ?

Answer 1

Your vector is reallocating and invalidating your pointers. You need to use a different data structure, or an index into the vector, instead of a raw pointer. When you insert elements into a vector, then pointers to the contents become invalid.

while(dispIterator != map_charFreqDistribution.end())
{
    std::cout<< "Character : " << (int)dispIterator->first << " Frequency : "<< dispIterator->second<<'\n';
    currentChar = new charElement(dispIterator->first,dispIterator->second);

    Q_freqDistribution.push(*currentChar);
    dispIterator++;

    delete currentChar;
}

Completely throws people off because it’s very traditional for people to have huge problems when using new and delete directly, but there’s actually no need for it whatsoever in this code, and everything is actually done by value.

You have two choices. Pick a structure (e.g. std::list) that does not invalidate pointers, or, allocate all charElements on the heap directly and use something like shared_ptr that cleans up for you.