C++ :: Dynamic Char Array For Class Constructor

I'm writing a class that has two constructors. However, I can't get them to work quite right. One constructor has a parameter of an int (with a default value of 0) and the other has a parameter of a C-style string.

First of all, are these function prototypes correct for the constructors?

MyInt(int n = 0);// first constructor, int param, default value 0
MyInt(const char * c);// second constructor, c-style string param

Both constructors work fine in some cases but don't work in all cases. Here are some potential calls to these functions that are supposed to work:

// These two work fine
MyInt x(12345),
y("9876543210123456789"),
// The array assignment doesn't work when the value is negative
// I'm not allowing negative numbers, but I want to create the object and assign the array to 0
r1(-1000),

[Code] .....

Here's the private data from the class (from the header file):

private:
int currentSize,
maxSize;
char* digits;// Pointer to an array of digits

// Increase the size of digits array by 5
void Grow ();