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consider the following code:

namespace fruit { struct apple{ ... }; } namespace language{ struct english{ ... }; }

I want to do something like:

std::unordered_map<std::string, ? > mymap = { {"paul", {"like", fruit::apple} }, {"jonas", {"like", language::english} } };

the key of my map is a string and the value should be such a pair <string, ? >.
What type should I give as value for my map? I think things like union or functor can be used to solve the problem, but how? thanks in advance.

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  • 1
    Looks like you want std::variant or std::any Commented Aug 3, 2018 at 13:19
  • 3
    What's the goal of the exercise? How do you envision using mymap, once constructed? Sounds like an XY problem Commented Aug 3, 2018 at 13:40
  • Naive solution would be std::tuple<std::string,std::unique_ptr<fruit::apple>,std::unique_ptr<language::english>> and make either nullptr Commented Aug 3, 2018 at 13:59
  • Use factory pattern and register each object (fruit::apple, language::english etc). Use functor in the unordered_map. You may need proxy pattern to wrap the object before hand so that you can apply inheritance. Commented Aug 3, 2018 at 14:16
  • @seccpur can you provide an example? thanks Commented Aug 3, 2018 at 14:19

4 Answers 4

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Here's a snippet ( without the factory pattern => improve the syntax ) with proxy pattern so that you need not modify your existing objects:

struct base { }; namespace fruit { struct apple {}; struct apple_proxy: base { apple_proxy() :apple_ptr(make_shared<fruit::apple>()) {} shared_ptr<apple> apple_ptr; }; } namespace language { struct english {}; struct english_proxy : base { english_proxy() :english_ptr(make_shared<language::english>()) {} shared_ptr<english> english_ptr; }; } int main() { std::unordered_map<std::string, shared_ptr<base>> map; shared_ptr<fruit::apple_proxy> a = make_shared<fruit::apple_proxy>(); map.emplace(make_pair("apple",a)); shared_ptr<language::english_proxy> e = make_shared<language::english_proxy>(); map.emplace(make_pair("english", e)); }
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1

This code compiles for me:

#include <unordered_map> #include <utility> #include <variant> namespace fruit { struct apple{}; } namespace language{ struct english{}; } int main(){ std::unordered_map<std::string,std::pair<std::string,std::variant<fruit::apple, language::english> > > mymap { {"paul", {"like", fruit::apple()} }, {"jonas", {"like", language::english()}} }; }

Note that you may want to compile with something like -std=gnu++17.

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std::variant<fruit::apple, language::english> -i'm limited to c++11 -There is like 30 type as fruit::apple, language::english, .., It will be inconvenient to write them all in the map declaration.. is there another way?
Then use boost::variant
Can u use polymorphism? You may want to make both fruit::apple and language::english inherit from same base class and instead of using std::variant you would use the base class.
@Antua i can't use polymorphism. In my real case scenario, both fruit::apple and language::english have been designed years ago.. and don't inherit from any class..
Then construct your own wrapper. No other solution comes to my mind.
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So I found a solution using std::type_index. 

typedef std::pair<std::string, std::type_index> myPair; std::unordered_map<std::string, myPair> myMap = { {"paul", {"likes", std::type_index(typeid(fruit::apple))} }, {"jonas", {"likes", std::type_index(typeid(language::english))} } };

thanks guys for your valuable suggestions.

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0

If you cannot use std::variant or boost::variant this solution could work:

struct data { std::string action; std::unque_ptr<fruit::apple> apple; std::unque_ptr<language::english> english; data( std::string act, fruit::apple ap ) : action( std::move( act ) ), apple( std::make_unqiue<fruit::apple>( std::move( ap ) ) {} data( std::string act, language::english en ) : action( std::move( act ) ), english( std::make_unqiue<language::english>( std::move( en ) ) {} };

but most probably your problem has better solution.

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