'this' cannot be used in a constant expression error - c++

Question

The error is on line 76 int res[mSize]; the problem is on mSize. It seems like a simple fix but I can't figure it out. If someone can figure it out or point me in the right direction that would be greatly appreciated.
Also, the deconstructor ~MyContainer(), I am not sure if I am using it right or if there is a correct place to put it.
Here is my code:
#include <iostream>
using namespace std;
class MyContainer
{
private:
int* mHead; // head of the member array
int mSize; // size of the member array
public:
MyContainer();
MyContainer(int*, int);
//~MyContainer();
void Add(int);
void Delete(int);
int GetSize();
void DisplayAll();
int FindMissing();
~MyContainer() {}
};
MyContainer::MyContainer()
{
mHead = NULL;
mSize = 0;
}
MyContainer::MyContainer(int* a, int b)
{
mHead = a;
mSize = b;
}
void MyContainer::Add(int a)
{
*(mHead + mSize) = a;
mSize++;
}
void MyContainer::Delete(int a)
{
int index;
for (int i = 0; i < mSize; i++)
{
if (*(mHead + i) == a)
{
index = i;
break;
}
}
for (int i = index; i < mSize; i++)
{
*(mHead + i) = *(mHead + i + 1);
}
mSize--;
}
int MyContainer::GetSize()
{
return mSize;
}
void MyContainer::DisplayAll()
{
cout << "\n";
for (int i = 0; i < mSize; i++)
{
cout << *(mHead + i) << " ";
}
}
int MyContainer::FindMissing()
{
int res[mSize];
int temp;
int flag = 0;
for (int i = 1; i <= mSize; i++)
{
flag = 0;
for (int j = 0; j < mSize; j++)
{
if (*(mHead + j) == i)
{
flag = 1;
break;
}
}
if (flag == 0)
{
temp = i;
break;
}
}
return temp;
}
int main()
{
const int cSize = 5;
int lArray[cSize] = { 2, 3, 7, 6, 8 };
MyContainer lContainer(lArray, cSize);
lContainer.DisplayAll();
lContainer.Delete(7);
lContainer.DisplayAll();
cout << "Size now is: " << lContainer.GetSize() << endl; lContainer.Add(-1);
lContainer.Add(-10);
lContainer.Add(15);
lContainer.DisplayAll();
cout << "Size now is: " << lContainer.GetSize() << endl;
cout << "First missing positive is: " << lContainer.FindMissing() << endl;
system("PAUSE"); return 0;
}

Answer 1

int res[mSize];
The size of the array mSize must be known at compile time. You cannot use a variable here. An option may be to define a macro with an largish value that will not exceeded.
static const int kLargeSize =100;
int res[kLargeSize];
Edited in response to the comments - const and constexpr are a better option than a macro.
Or even better, you can use std::vector - https://en.cppreference.com/w/cpp/container/vector