c++ - invalid new-expression of abstract class type error -

homework
i'm getting type of error merge sort function. think error has merge() method, thought took care of using in mergesort(). appreciated.

sort.h

#ifndef sort_h_ #define sort_h_  class sort { protected:    unsigned long num_cmps; // number of comparisons performed in sort function public:    virtual void sort(int a[], int size)= 0;    // main entry point    bool testifsorted(int a[], int size);        // returns false if not sorted                                             // true otherwise    unsigned long getnumcmps() { return num_cmps; }        // returns # of comparisons    void resetnumcmps() { num_cmps = 0; } };  class mergesort:public sort {       // mergesort class public:   void sort(int a[], int size, int low, int high);      // main entry point };  #endif 

sort.cpp

 sort* s;    switch(op.getalg())        {       case 's':          s=new selectionsort();          break;       case 'i':          s=new insertionsort();          break;       case 'b':          s=new bubblesort();          break;       case 'h':          s=new heapsort();          break;       case 'r':          s=new radixsort();          radixsortq = true;          break;       case 'm':          s=new mergesort(); --> error          break;       } 

merge sort.cpp

#include <iostream> #include "sort.h" using namespace std; void mergesort::sort(int a[], int size, int low, int high)          // main entry point {     if (low < high){         int middle = (low + high) / 2;         mergesort(a, size, low, middle);         mergesort(a, size, middle+1, high);         merge(a, size, low, middle, high);     } }  void merge::sort(int a[], int size, int low, int middle, int high){     int temp[size];     for(int = low; <= high; i++){         temp[i] = a[i];     }     int = low;     int j = middle + 1;     int k = low;      while(i <= middle && j <= high){         if(temp[i] <= temp[j]){             a[k] = temp[i];             ++i;         }         else {             a[k] = temp[j];             ++j;         }         ++k;     }     while(i <= middle){         a[k] = temp[i];         ++k;         ++i;     } } 

you aren't overriding sort function, mergesort still abstract class:

sort::sort(int a[], int size)= 0; mergesort::sort(int a[], int size, int low, int high);      // main entry point 

these sorts have different signatures, different functions.

to solve this, either need change mergesort::sort method not take in low , high argument, or add low , high argument other sort methods.