Template Struct SubscriptionCallbackTypeHelper

• Defined in File subscription_callback_type_helper.hpp

Struct Documentation

template<typename MessageT, typename CallbackT, typename Enable = void>
struct SubscriptionCallbackTypeHelper

Template metaprogramming helper used to resolve the callback argument into a std::function.

Sometimes the CallbackT is a std::function already, but it could also be a function pointer, lambda, bind, or some variant of those. In some cases, like a lambda where the arguments can be converted between one another, e.g. std::function<void (shared_ptr<…>)> and std::function<void (unique_ptr<…>)>, you need to make that not ambiguous by checking the arguments independently using function traits rather than rely on overloading the two std::function types.

This issue, with the lambda’s, can be demonstrated with this minimal program:

#include <functional>
#include <memory>

void f(std::function<void (std::shared_ptr<int>)>) {}
void f(std::function<void (std::unique_ptr<int>)>) {}

int main() {
  // Fails to compile with an "ambiguous call" error.
  f([](std::shared_ptr<int>){});

  // Works.
  std::function<void (std::shared_ptr<int>)> cb = [](std::shared_ptr<int>){};
  f(cb);
}

If this program ever starts working in a future version of C++, this class may become redundant.

This helper works by using SFINAE with rclcpp::function_traits::same_arguments<> to narrow down the exact std::function<> type for the given CallbackT.

Public Types

using callback_type = typename rclcpp::function_traits::as_std_function<CallbackT>::type