Stackoverflow

#include <iostream>

constexpr int _aSize = 20;
int pArray[_aSize];
int stackPointer = 0;

void pushOp(int input)
{
    pArray[stackPointer] = input;

    stackPointer = stackPointer + 1;

    //if (stackPointer > _aSize)
    //{
    //  std::cout << "Stack Overflow!";         //Exception handle for stack overflow
    //}


}



void popOp()
{
    stackPointer = stackPointer - 1;
    pArray[stackPointer] = NULL;

    if (stackPointer < 0) stackPointer = 0;     //Exception handle for stackPointer notNULL
}

void switchIndexForPush()
{
    for (int count = _aSize - 2; count >= stackPointer; --count)
    {
        pArray[count + 1] = pArray[count];
    }
}

void switchIndexForPop()
{
    for (stackPointer; stackPointer < _aSize-1; ++stackPointer)
    {
        pArray[stackPointer] = pArray[stackPointer + 1];
    }
}

void searchStackForPush()
{
    for (int count = stackPointer; pArray[count] != 0; ++count)
    {
        stackPointer = count +1;
    }
}

void searchStackForPop()
{
    for (int count = stackPointer; pArray[count] == 0; --count)
    {
        stackPointer = count;
    }
}

void pushDirect(int input)
{
    std::cout << "Give position(stack) to push: ";
    std::cin >> stackPointer;
    switchIndexForPush();
    pArray[stackPointer] = input;

    searchStackForPush();
}

void popDirect()
{
    std::cout << "Give position(stack) to pop: ";
    std::cin >> stackPointer;
    switchIndexForPop();

    searchStackForPop();
}

void fillArray(int _pArray[])
{
    for (int count = 0; count < _aSize; ++count)
    {
        _pArray[count] = NULL;
    }
}

void readArray(int _pArray[])
{
    for (int count = 0; count < _aSize; ++count)
    {
        std::cout << "[ " << _pArray[count] << " ]";
    }
    std::cout << "   Stackpointer: " << stackPointer;
}


int main()
{
    int In;
    int In2;
    fillArray(pArray);
    while (true) {
        std::cout << "Choose one function: (1) for PUSH | (2) for direkt PUSH | (3) for POP | (4) for direkt POP ";
        std::cin >> In;
        switch (In)
        {
        case 1:
            std::cout << "Give one Number for Push on Stack: ";
            std::cin >> In2;
            pushOp(In2);
            break;

        case 2:
            std::cout << "Give one Number for Push on Stack: ";
            std::cin >> In2;
            pushDirect(In2);
            break;

        case 3:
            popOp();
            break;

        case 4:
            popDirect();
            break;
        }
        readArray(pArray);
        std::cout << std::endl;
    }

}