Class template, invalid null pointer(string)

Question

Created a template class that complies and runs fine with integers, when i attempt to run with strings I receive invalid null pointer error.

I only added in the methods where the heap is created, which is where the error was found.

//heap.h
#include <iostream>
#include <vector>
using namespace std;

template<class TYPE>
class Heap{
private:

    vector<TYPE> heap;
    int size;// number of elements in the heap
    bool maxheap = true;//default value
    TYPE bubble_up(TYPE item);
    TYPE bubble_down(TYPE item);

public:

    Heap();
    Heap(bool maxheap);
    Heap(vector<TYPE>, bool order);
    ~Heap();
    void build_heap();
    TYPE Insert(TYPE item);
    TYPE Delete(TYPE& item);
    const vector<TYPE> sort(bool order);
    const vector<TYPE> sort();// defualt sort if no variable given, max sort
    TYPE get_size();
    void print_heap();
    void clear_heap();
};

template<class TYPE>
Heap<TYPE>::Heap(){
    TYPE dummy = 0;
    heap.push_back(dummy);
    size = heap.size() - 1;
}

template<class TYPE>
Heap<TYPE>::Heap(bool order){
    maxheap = order; // true is max, false is min
    TYPE dummy = 0;
    heap.push_back(dummy);
    size = heap.size() - 1;

}

template<class TYPE>
Heap<TYPE>::Heap(vector<TYPE> x, bool order){
    maxheap = order;// true is max, false is min
    TYPE tempSize;
    TYPE dummy = 0;
    heap.push_back(dummy);
    size = heap.size() - 1;
    tempSize = x.size();
    for (TYPE y = 0; y < tempSize; y++){
        heap.push_back(x[y]);
    }
    size = heap.size() - 1;
    build_heap();
}

I have cut out the part where the issue was found in the next few lines of code.

//driver.cpp
#include <iostream>
#include <string>
#include "Heap.h"

using std::cout;
using std::endl;

typedef string TYPE;

int main(void) {

    Heap<std::string> *s_heap = new Heap<std::string>();  // string heap
    std::string s_item = "0";
}

"Debug assertion failed!" "Expression: Invalid null pointer"

TYPE dummy = 0; is no good if TYPE is a std::string - make it TYPE dummy (still questionable, but better) — user2249683
– user2249683, Commented Nov 17, 2014 at 20:06
@juanchopanza: Better the first one, who knows whether TYPE is moveconstructible / copyconstructible. (even though RVO applies) — Deduplicator
– Deduplicator, Commented Nov 17, 2014 at 20:11

James Curran · Accepted Answer · 2014-11-17 20:21:03Z

2

template<class TYPE>
Heap<TYPE>::Heap(){
    TYPE dummy = 0;
    heap.push_back(dummy);
    size = heap.size() - 1;
}

The first line would translate to:

 std:string dummy = 0;

I don't think you can set a string to zero.

try changing it to:

TYPE dummy = TYPE();

UPDATE: as juanchopanza points out, the modern syntax is:

TYPE dummy{};

(I'm still living in the past...)

edited Nov 17, 2014 at 20:21

answered Nov 17, 2014 at 20:06

James Curran

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3 Comments

juanchopanza Over a year ago

Correct. "setting" a string to zero invokes the const char* constructor, which assumes a pointer to a null-terminaded string.

surferfan Over a year ago

Another quick question, changing that in the code allowed it to create the heap as a string. Now I get several errors saying "No constructor could take the source type, or constructor overload resolution was ambiguous", do I need to make more constructor classes for the string cases? I thought that was the whole idea behind a template

James Curran Over a year ago

Where are you getting those errors? (post it as a new question)

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