CODEWITHSUNDEEP
c++classtemplatespointersinstantiation
user2813274
user2813274

Reputation: 858

C++ template instantiation pointer null error

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!

Upvotes: 0

Views: 329

Answers (2)

Dima
Dima

Reputation: 39439

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.

Upvotes: 1

Daniel Frey
Daniel Frey

Reputation: 56921

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 = ""

Upvotes: 2

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