Question

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Where and why do I have to put the “template” and “typename” keywords?

I’m learning about template functions. I’m try to implement a static template function that clear a list of pointers. To do it I want to use templates. Its my code:

#include <cstdio>
#include <list>
using namespace std;

class util
{
public:
    template <class ARG>
    static void CleanPointers(list<ARG> &mylist) {
        list<ARG>::iterator it;
        for (it = mylist.begin(); it != mylist.end(); it++)
        {
            ARG obj = (ARG) *it;
            delete obj;
        }
        mylist.clear();
    };

    util();
    ~util();
};

int main()
{
    list<int*> mylist;
    mylist.push_back(new int(1));
    mylist.push_back(new int(2));
    util::CleanPointers<int*>(mylist);
    return 0;
}

I recived follow compile error message and I dont understand what is the point here. 🙂 Why need I put ; before it?

prog.cpp: In static member function ‘static void util::CleanPointers(std::list<ARG,         std::allocator<_Tp1> >&)’:
prog.cpp:10: error: expected `;' before ‘it’
prog.cpp:11: error: ‘it’ was not declared in this scope
prog.cpp: In static member function ‘static void util::CleanPointers(std::list<ARG,     std::allocator<_Tp1> >&) [with ARG = int*]’:
prog.cpp:28:   instantiated from here
prog.cpp:10: error: dependent-name ‘std::list::iterator’ is parsed as a non-type,   but instantiation yields a type
prog.cpp:10: note: say ‘typename std::list::iterator’ if a type is meant

Answer 1

needs typename, thus:

typename list<ARG>::iterator it;

This is not the only thing wrong with the code though. Why are you casting to int before deleting? That is a bad bad bug when it is not a collection of pointers to ints. You should delete through the proper pointer type if you are going to do it this way.

In addition you could put in a partial-specialisation since ARG has to be a pointer type.

C++ FAQ about templates. I would suggest reading all of that.

Also refer to the FAQ about why casts are evil, and it might also explain what happens with delete.