Common Pitfalls

This Section highlights common mistakes or C++ subtleties that can show up when programming in CAF.

Defining Message Handlers

  • C++ evaluates comma-separated expressions from left-to-right, using only the last element as return type of the whole expression. This means that message handlers and behaviors must not be initialized like this:

    message_handler wrong = (
      [](int i) { /*...*/ },
      [](float f) { /*...*/ }
    );
    

    The correct way to initialize message handlers and behaviors is to either use the constructor or the member function assign:

    message_handler ok1{
      [](int i) { /*...*/ },
      [](float f) { /*...*/ }
    };
    
    message_handler ok2;
    // some place later
    ok2.assign(
      [](int i) { /*...*/ },
      [](float f) { /*...*/ }
    );
    

Event-Based API

  • The member function become does not block, i.e., always returns immediately. Thus, lambda expressions should always capture by value. Otherwise, all references on the stack will cause undefined behavior if the lambda expression is executed.

Requests

  • A handle returned by request represents exactly one response message. It is not possible to receive more than one response message.
  • The handle returned by request is bound to the calling actor. It is not possible to transfer a handle to a response to another actor.

Sharing

  • It is strongly recommended to not share states between actors. In particular, no actor shall ever access member variables or member functions of another actor. Accessing shared memory segments concurrently can cause undefined behavior that is incredibly hard to find and debug. However, sharing data between actors is fine, as long as the data is immutable and its lifetime is guaranteed to outlive all actors. The simplest way to meet the lifetime guarantee is by storing the data in smart pointers such as std::shared_ptr. Nevertheless, the recommended way of sharing informations is message passing. Sending the same message to multiple actors does not result in copying the data several times.