Template for stl container

Question

Inside the declaration of the class Density, I built these member function:

class Density {
public:
    template <typename Container>
    void printStream (Container<Point>::iterator lo, Container<Point>::iterator hi);
......
};

In the cpp file:

template <typename Container>
void Density::printStream (Container<Point>::iterator lo, Container<Point>::iterator hi)
{
...
}

But get these errors when trying to compile:

src/../include/density.hpp:166:23: error: 'Container' is not a template
src/../include/density.hpp:166:50: error: expected unqualified-id before 'lo'
src/../include/density.hpp:166:50: error: expected ')' before 'lo'
src/../include/density.hpp:166:50: error: expected initializer before 'lo'
src/density.cpp: In member function 'void Density::startAlgorithm()':
src/density.cpp:291:43: error: 'printStream' was not declared in this scope
src/density.cpp: At global scope:
src/density.cpp:327:28: error: 'Container' is not a template
src/density.cpp:327:55: error: expected unqualified-id before 'lo'
src/density.cpp:327:55: error: expected ')' before 'lo'
src/density.cpp:327:55: error: expected initializer before 'lo'

What should I modify? And also, why, as I'd like to understand this issue.

score 4 · Accepted Answer · edited May 23 '17 at 12:18

4

NB. As commented, you might not be aware of the implications that using templates has on the visibility of template definitions in the header files. Let me point you to the c++-faq entry for that: Why can templates only be implemented in the header file?

Using a template template argument:

template <template <typename...> class Container>
void Density::printStream ()
{
    typename Container<Point>::iterator lo;
    typename Container<Point>::iterator hi;
}

What you are trying to do, seems to me to be impossible, since the iterator arguments are non-deducible context, so you'd end up explicitely specifying the container type anyways:

   density_instance.printStream<std::vector>(it1, it2);

Note, however, that's not really a problem, since you probably don't really care about the precise type of the container. The idiomatic way would be:

template <typename It>
    void printStream (It lo, It hi);

which you can freely call using

std::vector<int> v { 1,2,3 };
density_instance.printStream(begin(v), end(v));

But also with a non-class container, since the iterators are what matters:

const int a[] = { 1,2,3 };
density_instance.printStream(std::begin(a), std::end(b));

edited May 23 '17 at 12:18

Community

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answered Jun 09 '13 at 17:08

sehe

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Is it really standard conform to use `...` and only provide one argument (i.e. omit the allocator argument)? – celtschk Jun 09 '13 at 17:15
@juanchopanza Oh, good point. I'm going to "fix" that linking to the faq: **[A: Why can templates only be implemented in the header file?](http://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file/495056#495056)** – sehe Jun 09 '13 at 17:16
@celtschk I certainly hope so, since I don't think many people _always_ specify the allocator type. Now, I have a hunch you're probably on to the "things are not being deduced here" thing that I also addressed in the remainder of my answer :) – sehe Jun 09 '13 at 17:17
I think you forgot to remove `` from your last two examples. – catscradle Jun 09 '13 at 17:21
The point is, in C++03, it would have seemed just as natural to use `template – celtschk Jun 09 '13 at 17:26
@celtschk Well, as per the standard, `std::vector` is declared as a class template with two "formal" type template arguments. So, if you wanted it to match without variadics, you'd need `template – sehe Jun 09 '13 at 17:40
1

@celtschk You're right: that `template class` as a template template parameter matches `template class vector;` as an argument is a special rule. Generally speaking I recommend avoiding template template parameters altogether as they are unflexible (e.g. even `template class` won't accept `std::array`, as it involves a non-type template parameter) and the functionality they appear to provide can be reproduced in other ways (e.g. allocators have a mechanism to 'rebind' their template arguments, should they be class templates in the first place). – Luc Danton Jun 09 '13 at 17:42
@not-sehe: Thank you! I'll use the second option with ``. I don't know why you said it's overkill, but I understand it and find it simpler. And it's working. – Luis Jun 09 '13 at 18:24
@Luis I meant to say that what you were doing seemed like overkill, because it can be done much simpler :) I'll edit it to make it clearer. – sehe Jun 09 '13 at 18:28

Template for stl container

1 Answers1