How to automatically add handler to the global map?

I have Application singleton wich has method

void addHandler(const std::string& command, std::function<std::string (const std::string&)> handler)

I want to create a lot of cpp files with handlers like this

//create_user_handler.cpp
Application::getInstance()->addHandler("create_user", [](std::string name) {
   UserPtr user = User::create(name);
   return user->toJson();
});

How automatically call this from my cpp files?

I try to change from void addHandler to bool addHandler and than use

namespace {
  bool b = Application::getInatance()->addHandler......
}

but it didn't work for me