CODEWITHSUNDEEP
c++templates
doctopus
doctopus

Reputation: 5657

How to template containers?

I have this function that I'd like to use for a container of strings, like vector<string> or set<string>. Obviously templating seems like the best solution here but I'm not sure how to template the container. I tried this but it doesn't work:

template<class T>
string pack_addrs(T<string>& caddrs) {
  string res = "";
  for (string addr : caddrs) {
    res += addr + ",";
  }
  return res.substr(0, res.size()-1);
}

Upvotes: 5

Views: 856

Answers (2)

jxh
jxh

Reputation: 70502

You can allow your template to use any class generically, but assert that it must behave like a container. In particular, you can assert that it has a value_type that matches std::string.

template<class C>
std::string pack_addrs(C& caddrs) {
  static_assert(std::is_same<typename C::value_type, std::string>::value);
  std::string res = "";
  for (std::string addr : caddrs) {
    res += addr + ",";
  }
  return res.substr(0, res.size()-1);
}

Or, you can use SFINAE to disable the function if the type of the argument does not match the desired trait.

template<class C,
         typename std::enable_if<
             std::is_same<typename C::value_type,
                          std::string>::value>::type* = nullptr>
std::string pack_addrs(C& caddrs) {
  std::string res = "";
  for (std::string addr : caddrs) {
    res += addr + ",";
  }
  return res.substr(0, res.size()-1);
}

Upvotes: 4

Remy Lebeau
Remy Lebeau

Reputation: 598448

When a template parameter is itself a templated type, you can use a template template parameter for it, eg:

template <template<class...> class Container>
string pack_addrs(const Container<string>& caddrs) {
  string res;
  for (const string &addr : caddrs) {
    res += addr + ",";
  }
  return res.substr(0, res.size()-1);
}

Live Demo

This code will allow you to pass in any templated container that accepts std::string as its 1st template parameter (even if it takes additional template parameters, like an allocator, etc), and that satisfies the requirements of a range-for loop.

Upvotes: 4

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