C++ template instantiation pointer null error

Question

I want to write my own version of stack, this is what I have:

template<class myStackType> class myStackClass
{
    myStackType array[1000];
    int size;
public:
    myStackClass()
    {
        size = 0;
    }
    void pop()
    {
        size--;
    }
    void push(myStackType a)
    {
        array[size] = a;
        size++;
    }
    myStackType top()
    {
        return array[size-1];
    }
    bool empty()
    {
        return(size == 0);
    }
};

but when I try to actually use it

struct grade
{    
    int mid;
    int final;
    string name;
    grade(int mid1 = 0, int final1 = 0, string name1 = 0)
    {
        mid = mid1;
        final = final1;
        name = name1;
    }
};

myStackClass<grade> myStack;

I get a debug assertion failed: invalid null pointer

on the other hand, the std::stack works just fine in the same spot with the same data type

what am I doing wrong?

Thanks!

if I'm not wrong the stack grows as the user inserts more and more. why did you make it have maximum 1000 elements? — Davlog
– Davlog, Commented Oct 23, 2013 at 19:06

Daniel Frey · Accepted Answer · 2013-10-23 19:04:28Z

2

This is wrong:

string name1 = 0

It tries to construct a string from a const char* which is 0 - and this is not allowed. You probably meant:

string name1 = ""

answered Oct 23, 2013 at 19:04

Daniel Frey

57.2k13 gold badges129 silver badges182 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Dima · Accepted Answer · 2013-10-23 19:17:05Z

1

You are assigning 0 to a string in your constructor. That's the bug. The compiler is trying to interpret the 0 as a char *, i. e. a C-style string. But since it's a 0, it is interpreted as a NULL pointer.

You may also want to do some error checking to make sure your stack doesn't overflow, or that you don't try to pop off an empty stack.

edited Oct 23, 2013 at 19:17

answered Oct 23, 2013 at 19:04

Dima

39.6k14 gold badges80 silver badges116 bronze badges

2 Comments

user2813274 Over a year ago

That was it, thanks - although now I am confused why it worked with std::stack instead of mine.. oh well, thanks

Enrico Granata Over a year ago

That depends on how the stack class allocates its storage. If std::stack is using a T* it does not need to actually allocate any T in its constructor. Yours is using an array, so if I remember correctly, every element in the storage is going to be initialized, T() is going to be called, and, yes, then the string = nullptr is going to happen and crash ensues

Collectives™ on Stack Overflow

C++ template instantiation pointer null error

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related