Delegates in C++

DelegateMQ is a C++ header-only library for invoking any callable (e.g., function, method, lambda):

• Synchronously
• Asynchronously
• Remotely across processes or processors

It unifies function calls across threads or systems via a simple delegate interface. DelegateMQ is thread-safe, unit-tested, and easy to port to any platform.

Key Concepts

• MakeDelegate – Creates a delegate instance bound to a callable (lambda, function, or method).
• MulticastDelegateSafe – A thread-safe container of delegates, allowing broadcast-style invocation.
• Thread – A cross-platform thread class capable of asynchronous delegate invocation.

Examples

Subscriber registers with Publisher to receive asynchronous callbacks.

class Publisher
{
public:
    // Thread-safe container to store registered callbacks
    dmq::MulticastDelegateSafe<void(const std::string& msg)> MsgCb;

    static Publisher& Instance() 
    {
        static Publisher instance;
        return instance;
    }

    void SetMsg(const std::string& msg) 
    {
        m_msg = msg;    // Store message
        MsgCb(m_msg);   // Invoke all callbacks
    }

private:
    Publisher() = default;
    std::string m_msg;
};

class Subscriber
{
public:
    Subscriber() : m_thread("SubscriberThread") 
    {
        m_thread.CreateThread();

        // Register for publisher callback on m_thread context
        Publisher::Instance().MsgCb += dmq::MakeDelegate(this, &Subscriber::HandleMsgCb, m_thread);
    }

private:
    // Handle publisher callback on m_thread
    void HandleMsgCb(const std::string& msg) { std::cout << msg << std::endl; }
    Thread m_thread;
};

Multiple callables targets stored and invoked asynchronously.

// Create an async delegate targeting lambda on thread1
auto lambda = [](int i) { std::cout << i; };
auto lambdaDelegate = dmq::MakeDelegate(std::function(lambda), thread1);

// Create an async delegate targeting Class::Func() on thread2
Class myClass;
auto memberDelegate = dmq::MakeDelegate(&myClass, &Class::Func, thread2);

// Create a thread-safe delegate container
dmq::MulticastDelegateSafe<void(int)> delegates;

// Insert delegates into the container 
delegates += lambdaDelegate;
delegates += memberDelegate;

// Invoke all callable targets asynchronously 
delegates(123);

Asynchronous public API reinvokes StoreAsync() call onto the internal m_thread context.

// Store data using asynchronous public API. Class is thread-safe.
class DataStore
{
public:
    DataStore() : m_thread("DataStoreThread")
    {
        m_thread.CreateThread();
    }

    // Store data asynchronously on m_thread context (non-blocking)
    void StoreAsync(const Data& data)
    {
        // If the caller thread is not the internal thread, reinvoke this function 
        // asynchronously on the internal thread to ensure thread-safety
        if (m_thread.GetThreadId() != Thread::GetCurrentThreadId()) {
            // Reinvoke StoreAsync(data) on m_thread context
            return dmq::MakeDelegate(this, &DataStore::StoreAsync, m_thread)(data);
        }
        m_data = data;  // Data stored on m_thread context
    }

private:
    Data m_data;        // Data storage
    Thread m_thread;    // Internal thread
};

Overview

In C++, a delegate function object encapsulates a callable entity, such as a function, method, or lambda, so it can be invoked later. A delegate is a type-safe wrapper around a callable function that allows it to be passed around, stored, or invoked at a later time, typically within different contexts or on different threads. Delegates are particularly useful for event-driven programming, callbacks, asynchronous APIs, or when you need to pass functions as arguments.

Synchronous and asynchronous delegates are available. Asynchronous variants handle both non-blocking and blocking modes with a timeout. The library supports anonymous invocation of any target callable regardless of the number of arguments or return value. All argument types are supported, including by value, pointers, pointers to pointers, and references. The delegate library takes care of the intricate details of function invocation across thread boundaries. Concrete porting examples below with easy porting to others.

• Operating System: Windows, Linux, FreeRTOS

Remote delegates enable function invocation across processes or processors by using customizable serialization and transport mechanisms. All argument data is marshaled to support remote callable endpoints with any function signature. Minimal effort is required to extend support to any new library. Concrete examples are provided using support libraries below.

• Serialization: MessagePack, RapidJSON, Cereal, Bitsery, MessageSerialize
• Transport: ZeroMQ, NNG, MQTT, UDP, data pipe, memory buffer

It is always safe to call the delegate. In its null state, a call will not perform any action and will return a default-constructed return value. A delegate behaves like a normal pointer type: it can be copied, compared for equality, called, and compared to nullptr. Const correctness is maintained; stored const objects can only be called by const member functions.

A delegate instance can be:

• Copied freely.
• Compared to same type delegates and nullptr.
• Reassigned.
• Called.

Typical use cases are:

• Synchronous and Asynchronous Callbacks
• Event-Driven Programming
• Inter-Process and Inter-Processor Communication
• Inter-Thread Publish/Subscribe (Observer) Pattern
• Thread-Safe Asynchronous API
• Asynchronous Method Invocation (AMI)
• Design Patterns (Active Object)
• std::async Thread Targeting

Originally published on CodeProject at Asynchronous Multicast Delegates in Modern C++ with a perfect 5.0 article feedback rating.

Modular Architecture

DelegateMQ uses an external thread, transport, and serializer, all of which are based on simple, well-defined interfaces.

DelegateMQ Layer Diagram

The library's flexible CMake build options allow for the inclusion of only the necessary features. Synchronous, asynchronous, and remote delegates can be used individually or in combination.

Getting Started

To build and run DelegateMQ, follow these simple steps. The library uses CMake to generate build files and supports Visual Studio, GCC, and Clang toolchains.

1. Clone the repository.
2. From the repository root, run the following CMake command:
   cmake -B build .
3. Build and run the project within the build directory.

See Example Projects to build other project examples.

Documentation

• See Design Details for implementation design documentation and more examples.
• See Doxygen Documentation for source code documentation.
• See Unit Test Code Coverage test results.

Motivation

Systems are composed of various design patterns or libraries to implement callbacks, asynchronous APIs, and inter-thread or inter-processor communications. These elements typically lack shared commonality. Callbacks are one-off implementations by individual developers, messages between threads rely on OS message queues, and communication libraries handle data transfer complexities. However, the underlying commonality lies in the need to move argument data to the target handler function, regardless of its location.

The DelegateMQ middleware effectively encapsulates all data movement and function invocation within a single library. Whether the target function is a static method, class method, or lambda—residing locally in a separate thread or remotely on a different processor—the library ensures the movement of argument data (marshalling when necessary) and invokes the target function. The low-level details of data movement and function invocation are neatly abstracted from the application layer.

Features

DelegateMQ at a glance.

Category	DelegateMQ
Purpose	DelegateMQ is a library used for invoking any callable synchronously, asynchronously, or remotely
Usages	Callbacks (synchronous and asynchronous), asynchronous API's, communication and data distribution, and more
Library	Allows customizing data sharing between threads, processes, or processors
Complexity	Lightweight and extensible through external library interfaces and full source code
Threading	No internal threads. External configurable thread interface portable to any OS (IThread).
Serialization	External configurable serialization data formats, such as MessagePack, RapidJSON, or custom encoding (ISerializer)
Transport	External configurable transport, such as ZeroMQ, TCP, UDP, serial, data pipe or any custom transport (ITransport)
Timeouts and Retries	Provided by a communication library (e.g. ZeroMQ REQ-REP (Request-Reply)), TCP/IP stack, or custom solution (ITransportMonitor)
Message Buffering	Provided by a communication library (e.g. ZeroMQ) or custom solution within transport
Dynamic Memory	Heap or DelegateMQ fixed-block allocator
Error Handling	Configurable for return error code, assert or exception
Embedded Friendly	Yes. Any OS such as Windows, Linux and FreeRTOS. An OS is not required (i.e. "superloop").
Operation System	Any. Custom IThread implementation may be required.
Language	C++17 or higher

Alternative Implementations

Alternative asynchronous implementations similar in concept to DelegateMQ, simpler, and less features.

• Asynchronous Callbacks in C++ - A C++ asynchronous callback framework.
• Asynchronous Callbacks in C - A C language asynchronous callback framework.

Other Projects Using DelegateMQ

Repositories utilizing the DelegateMQ library.

• C++ State Machine with Asynchronous Delegates - a framework combining a C++ state machine with the asynchronous delegate library.
• Asynchronous State Machine Design in C++ - an asynchronous C++ state machine implemented using an asynchronous delegate library.
• Integration Test Framework using Google Test and Delegates - a multi-threaded C++ software integration test framework using Google Test and DelegateMQ libraries.
• Asynchronous SQLite API using C++ Delegates - an asynchronous SQLite wrapper implemented using an asynchronous delegate library.