c++ decltype how to use to simplify variable definition

Question

Say, I have this piece of code, in one of my classes, that defines

• A map of key and another map
• The second map is another key and a function handler
• The function hander is a signature that takes 2 params

Right now, the signature to define the variable looks incredible.

std::map<std::string, std::map<std::string,
    std::function<void(std::shared_ptr<HTTPRequest>,
        std::shared_ptr<HTTPResponse>)>>> routeFunctions_;

I recently came to know about decltype, but unable to use this correctly.

decltype(x) routeFunctions_;  // What should be there in the place of x ?

Answer 1

• Use typedef to this type, if you often declare variables of it.
• Use auto or decltype, if you want to return value of this type from a function.
• Use decltype, if you want to get type of structure/class member.

Look at this articles:

http://en.cppreference.com/w/cpp/language/auto http://en.cppreference.com/w/cpp/language/decltype

http://www.cprogramming.com/c++11/c++11-auto-decltype-return-value-after-function.html

Your choice in this case is a typedef:

typedef std::map<std::string, std::map<std::string,
std::function<void(std::shared_ptr<HTTPRequest>,
std::shared_ptr<HTTPResponse>)>>> RouteFunctionsContainer;

RouteFunctionsContainer routeFunctions_;