Failing to use stl containers in templated functions/classes

Question

When I try and compile the following code...

#include <vector>

template <class T> void DoNothing()
{
    std::vector<T>::iterator it;
}

int main(int argc, char**argv)
{
    return 0;
}

g++ says:

test.cpp:5: error: expected `;' before ‘it’

And I don't understand why this is a problem. If I replace it with std::vector<int>::iterator, say, it works fine as expected.

As you can see i'm not instantiating the function, so g++ must have a problem with the template definition itself, but I can't see how its invalid.

Thanks for any advice about whats going on.

NB I'm actually trying to write a templated class and having issues with a map rather than a vector, but this is the simplest test case for my problem.

Answer 1

Does typename std::vector<T>::iterator it; work?

EDIT: change template to typename ... got my keywords mixed up.

Answer 2

You need to use the typename keyword because the std::vector<T>::iterator type is dependent on the template parameter:

template <class T> void DoNothing()
{
    typename std::vector<T>::iterator it;
}

It can actually be confusing when you need to use typename and when you don't need it (or are even not permitted to use it). This article has a decent overview:

• http://pages.cs.wisc.edu/~driscoll/typename.html

Answer 3

I agree it is confusing. Without the typename keyword, the name would be considered a static member. The book C++ Templates by Vandevoorde and Josuttis explains this in detail.