Convert Infix to Prefix Notation - c++

Question

I have a task: make a program (C++), which converts "infix" notation to "prefix" and uses own "stack and queue" realizations.
But I get: "Critical error detected c0000374" and "Free Heap block modified at ... after it was freed" at last string of void main() { /*...*/ system("pause"); } or at last string of void toPrefix();
Can somebody help me and point out my mistake(s), please?
Source.cpp:
#include "iostream"
#include "fstream"
#include "string"
#include "Stack.h"
#include "Queue.h"
void toPrefix(const std::string& first)
{
int length = first.length();
char test = NULL, operand = NULL;
char *ptr = &test, *op_ptr = &operand;
Queue<char> List;
std::string Output;
Stack<char> OpStack;
for (int i = length - 1; i >= 0; i--) List.push(first[i]); //
while (List.getsize() != 0)
{
List.pop(ptr);
if (test >= 48 && test <= 57) //is it number?
{
Output.insert(Output.begin(), test);
}
if (test == '*' || test == '/' || test == '-' || test == '+')
{
OpStack.push(test);
}
if (test == ')')
{
OpStack.push(test);
}
if (test == '(')
{
OpStack.pop(op_ptr);
while (operand != ')')
{
Output.insert(Output.begin(), operand);
OpStack.pop(op_ptr);
}
}
}
}
void main()
{
const std::string& first = "9-(2+2)";
toPrefix(first);
system("pause");
}
Queue.h:
#include<iostream>
template <typename T>
class Queue
{
private:
struct queue_element
{
T value;
queue_element *next;
};
queue_element *first;
queue_element *last;
int size;
public:
Queue()
{
first = new(queue_element);
last = first;
first->value = -1;
first->next = 0;
size = 0;
}
Queue(T x)
{
first = new(queue_element);
last = first;
first->value = x;
first->next = 0;
size = 1;
}
int getsize()
{
return size;
}
void push(T value)
{
if (size == 0)
{
size++;
last = first;
first->value = value;
first->next = 0;
}
else
{
size++;
queue_element *temp = new(queue_element);
temp->next = 0;
temp->value = value;
last->next = temp;
last = temp;
}
}
void pop(T* ret)
{
if (size == 0)
{
std::cout << "Queue is empty!" << std::endl;
return;
}
queue_element *temp = first;
*ret = first->value;
first = first->next;
size--;
}
void peek(T *ret)
{
if (size == 0)
{
std::cout << "Queue is empty!" << std::endl;
return;
}
*ret = first->value;
}
};
Stack.h
#include <iostream>
template <typename T>
class Stack
{
private:
struct stack_element
{
T value;
stack_element *next;
};
stack_element *first;
stack_element *last;
int size;
public:
Stack()
{
last = new(stack_element);
first = last;
last->value = -1;
last->next = first;
size = 0;
}
Stack(T x)
{
last = new(stack_element);
first = last;
last->value = x;
last->next = 0;
size = 1;
}
int getsize()
{
return size;
}
void push(T value)
{
if (size == 0)
{
size++;
last->value = value;
last->next = first;
}
else
{
size++;
stack_element *temp = new(stack_element);
temp->next = last;
temp->value = value;
last = temp;
}
}
void pop(T* ret)
{
if (size == 0)
{
std::cout << "Stack is empty!" << std::endl;
return;
}
stack_element *temp = last;
*ret = last->value;
last = last->next;
delete temp;
size--;
}
void peek(T *ret)
{
if (size == 0)
{
std::cout << "Stack is empty!" << std::endl;
return;
}
*ret = first->value;
}
};

Answer 1

Well... I think that the problem is in you Stack class.
The string that you pass to toPrefix() is "9-(2+2)"
So the operation in you Stack<char> OpStack, defined in toPrefix(), are (if I understand well)
construction with default (no arguments) constructor
push() in correspondence of -
pop() in correspondence od (
push() in correspondence of +
push() in correspondence of )
Well, let's see what's is happening in it
1) after the construction with default constructor
we have
1a) size == 0
1b) first, last, first->next and last->next that are pointing to the same allocated memory area
1c) first->value == last->value == (char)-1
2) after the first call to push() (with -)
we have
2a) size == 1
2b) first, last, first->next and last->next that are pointing to the same allocated memory area
2c) first->value == last->value == '-'
3) after the first call to pop()
we have
3a) size == 0
3b) first, last, first->next and last->next that are pointing to the same DELETED memory area
3c) first->value == last->value == '-'
4) calling for the second time push() (with +)
4a) size is incremented
4b) is written (last->value = value;) in a DELETED area
4c) is written again (last->next = first;) in a DELETED area
I suppose that this can explain your problem.
p.s.: the "use a debugger" suggestion from Rambo Ramon and Sam Varshavchik is a good suggestion (IMHO)
p.s.2: sorry for my bad English