map auto-based loop single element access C++

Question

I'm trying to understand the "theory" besides the auto loop over

std::map

elements in C++. I have a std::map with a std::string as KEY and a vector<std:string> as VALUE. I can access its elements with:

for ( auto &element : myMap ) {
    std::cout << element.first << ": " << '\t';
    for ( std::string subElement : element.second ) std::cout << subElement << ", ";
  }
}

as for the loop over the vector<string> elements I know that I could put "auto" instead of "std::string". But what would be the equivalent for the map in such case? I studied and searched around and I found in that post that each map element is accessed as a

map< K, V>::value_type

but how would I write it down? I tried:

for ( std::map<std::string, vector<std::string>> &pz : myMap ) {
    // print ...
}

and similars, but they just does not work.

songyuanyao · Accepted Answer · 2019-06-17 03:33:25Z

8

std::map::value_type is defined as the type of std::map's element, which is std::pair<const Key, T> (i.e. std::pair<const std::string, vector<std::string>> here). You should use it as

for ( std::map<std::string, vector<std::string>>::value_type &pz : myMap ) {
    //                                          ^^^^^^^^^^^^
    // print ...
}

edited Jun 17, 2019 at 3:33

answered Jun 17, 2019 at 3:11

songyuanyao

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7 Comments

cccnrc Over a year ago

it was that intuitive...so indicatively "::value_type" make C++ interpret it as a "single element" of which access the subelements?

songyuanyao Over a year ago

@cccnrc Yes, and the element of std::map is std::pair indeed.

user9254539 Over a year ago

@cccnrc std::map is a map. std::map::value_type is a member type of the std::map class, and as its name suggests, it is the type of a value in the map.

Daniel Langr Over a year ago

@cccnrc It's a bit confusing since a map contains key-value pairs, but value_type is not a type of stored values, but a type of whole entries (key-value pairs). The type of values is spelled as mapped_type.

songyuanyao Over a year ago

@UsingCpp For std::map<std::string, vector<std::string>>, typename is not required; all the template arguments have been determined.

|

khanh · Accepted Answer · 2019-06-17 03:18:57Z

0

songyuanyao has answered your question. One thing needed to be clarified here is that std::map is defined as

template<
    class Key,
    class T,
    class Compare = std::less<Key>,
    class Allocator = std::allocator<std::pair<const Key, T> >
> class map;

The class name should be std::map<key type, value type>.

answered Jun 17, 2019 at 3:18

khanh

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1 Comment

UserUsing Over a year ago

it is actually std::map<const key type, value type>

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