callback.md

callback

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Header

#include <ext/callback>

Overview

Stores multiple callback functions and invokes all registered handlers with the supplied arguments. The tests cover add, remove, and argument-carrying callbacks.

Key APIs

• ext::callback<Args...> stores a list of handlers with the matching function signature.
• operator+= registers a handler and returns a cookie.
• operator-= removes a handler by the cookie returned from registration.
• operator()(args...) invokes the currently registered handlers in list order.

Behavior Notes

• Handlers are ordinary callable objects compatible with std::function for the callback signature.
• Removing a handler prevents later invocations from calling it.
• Use it when the publisher does not need the observer lifetime tracking provided by ext::observable.

Requirements

• GCC 8.3.0+
• Clang 10.0+
• Visual Studio 2008 SP1+ with Boost 1.69.0+
• Visual Studio 2017+

Examples

• Register a callbacks.

  #include <ext/callback>
  
  int sum = 1;
  ext::callback<int> int_callback;
  int_callback += [&sum](int val) { sum += val; };
  int_callback += [&sum](int val) { sum *= val; };
  int_callback(5);
  // sum == 10

• Register a callbacks.

  #include <ext/callback>
  
  int sum = 1;
  ext::callback<int> int_callback;
  int_callback += [&sum](int val) { sum *= val; };
  int_callback += [&sum](int val) { sum += val; };
  int_callback(5);
  // sum == 30

• Unregister a callback.

  #include <ext/callback>
  
  int sum = 0;
  ext::callback<int> int_callback;
  
  int_callback += [&sum](int val) { sum += val; };
  auto cookie = int_callback += [&sum](int val) { sum += val; };
  int_callback(1);
  // sum == 2
  
  sum = 0;
  int_callback -= cookie;
  int_callback(1);
  //sum == 1