Question

I am developing C++ on Netbeans 7.1 on Ubuntu 11.04. I was wondering if someone could tell me why Stack fs(5); is seen as an undefined reference to to `Stack::Stack(int)’. Why is it not seen as a class with float used for T?

#include <iostream>

using namespace std ;

template <class T>
class Stack
{
public:
    Stack(int = 10) ;
    ~Stack() { delete [] stackPtr ; }
    int method1(const T&); 
    int method2(T&) ;  
    int method3()const { return top == -1 ; }
    int method4() const { return top == size - 1 ; }
private:
    int attribute1 ;  
    int attribute2 ;
    T* stackPtr ;
} ;


using namespace std; 
int main()
{
    // This line gives the error message "undefined reference to `Stack<float>::Stack(int)'"
    Stack<float> fs(5);

    return 0;
}

Thanks,
Peter.

Answer 1

I don’t see the actual code for Stack<float>::Stack(int) anywhere. You’ve declared it, but there’s no definition. So it doesn’t actually exist (even though you’re saying it does; the compiler often just takes your word for it), and the linker won’t be able to find it, if it even gets that far.

You need to either define the function similarly to how you’ve defined ~Stack(), or have it elsewhere in the code the same way you’d have any other member function.