rclcpp::detail::SubscriptionCallbackTypeHelper< MessageT, CallbackT, Enable > Struct Template Reference

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

#include <rclcpp/detail/subscription_callback_type_helper.hpp>

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

Detailed Description

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

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.

Definition at line 65 of file subscription_callback_type_helper.hpp.

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The documentation for this struct was generated from the following file:

• rclcpp/include/rclcpp/detail/subscription_callback_type_helper.hpp