How to work out types required for std::for_each lambda function - c++11

Question

I am confused about how to work out what type is required to use a lambda function in std::for_each. It seems I cannot use auto in this case as a parameter (Visual Studio 2013 complains anyway).
In the code below I would have thought I would use sections as the type for the for_each function, but in fact I have to use
const std::pair<std::string, std::unordered_map<std::string, std::string> >
And for the 'inner' type in this case what do I use?
My main question is how I work out what type to use?
#include <iostream>
#include <string>
#include <unordered_map>
#include <algorithm>
// key=value pairs within a section
typedef std::unordered_map<std::string, std::string> keyvalue;
// [section1] - top level
typedef std::unordered_map< std::string, std::unordered_map<std::string, std::string> > sections;
class config
{
public:
config() {
setup();
}
typedef sections::iterator iterator;
iterator begin() { return sections_.begin(); }
iterator end() { return sections_.end(); }
private:
sections sections_;
void setup() {
// obviously we wouldn't hard code like this in a real program
sections_["programming languages"].insert(std::make_pair("C", "imperative"));
sections_["programming languages"].insert(std::make_pair("C++", "OOP"));
sections_["programming languages"].insert(std::make_pair("Java", "OOP"));
sections_["programming languages"].insert(std::make_pair("Haskell", "functional"));
sections_["programming languages"].insert(std::make_pair("Prolog", "logic"));
}
};
int main() {
config cfg;
std::for_each(cfg.begin(), cfg.end(), [](const std::pair<std::string, std::unordered_map<std::string, std::string> > sec) {
std::cout << "section name: " << sec.first << std::endl;
// what is inner type - thought it would be keyvalue ???
//std::for_each(sec.second.begin(), sec.second.end(), [](const keyvalue& pr) {
//std::cout << "first: " << pr << std::endl;
//});
});
// I thought type would be sections ???
//std::for_each(cfg.begin(), cfg.end(), [](const sections& sec) {
// std::cout << "section name: " << sec.first << std::endl;
//});
}

Answer 1

You can also use decltype for this purpose like this:
config cfg;
std::for_each(cfg.begin(), cfg.end(), [](const decltype(*cfg.begin())& sec) {
std::cout << "section name: " << sec.first << std::endl;
std::for_each(sec.second.begin(), sec.second.end(), [](const decltype(*sec.second.begin())& pr) {
std::cout << "first: " << pr.first << std::endl;
});
});
I'm not 100% certain how reference types affect this, so removing it might be more appropriate like const std::remove_reference<decltype(*cfg.begin())>::type& sec
Although it should be noted that VS2013 ItelliSense has trouble with decltype and will mark it as an error even if it compiles fine.
STL containers always define a value_type so in that case you can use decltype(inst)::value_type and you don't need the remove_reference shenanigans. So I would advise you to also add a value_type typedef to your type like this:
class config
{
public:
typedef sections::value_type value_type;
Then you can do:
config cfg;
std::for_each(cfg.begin(), cfg.end(), [](const decltype(cfg)::value_type& sec) {
std::cout << "section name: " << sec.first << std::endl;
std::for_each(sec.second.begin(), sec.second.end(), [](const decltype(sec.second)::value_type& pr) {
std::cout << "first: " << pr.first << std::endl;
});
});