Fix boost asio async_compose void error

The “deduced type void for err is incomplete” error in boost::asio::async_compose occurs when the completion handler signature doesn’t match what async_compose expects. This happens when the error parameter is incorrectly defined as void or has a default value that prevents proper type deduction. To fix this, ensure your async operation implementation exposes the correct completion handler signature without default values on error parameters and explicitly provide the handler signature when needed.

Contents

• Understanding the Error
• Why async_compose Deduces void for err
• Proper Implementation of async_compose
• Common Mistakes and Fixes
• Working Examples
• Best Practices for async_compose
• Advanced Usage Patterns
• Conclusion

Understanding the Error

The “deduced type void for err is incomplete” error is a compilation error that occurs specifically when using boost::asio::async_compose. This error indicates that the completion handler you’re passing to async_compose doesn’t match the signature that async_compose expects.

According to Stack Overflow, the error occurs when the completion handler’s first parameter is void, causing the type deduction to fail. The async_compose function needs to know the signature of the completion handler to properly wrap it in an intermediate handler.

When working with boost::asio, understanding the error handling mechanism is crucial. The official Boost documentation explains that async_compose simplifies the implementation of composed asynchronous operations by automatically wrapping a stateful function object with a conforming intermediate completion handler.

Why async_compose Deduces void for err

The root cause of this error lies in how C++ template argument deduction works in conjunction with async_compose. When you define your async operation implementation, if the error parameter has a default value, the compiler may deduce it as void instead of boost::system::error_code.

The technical analysis from think-async.com explains that when the lambda’s second parameter is declared with a default value like const boost::system::error_code& error = {}, the compiler might deduce the type of error as void in certain contexts.

This happens because:

1. Default arguments can interfere with template argument deduction
2. The parameter ordering or declaration affects how types are inferred
3. Incomplete type information propagates through the deduction chain

When implementing boost::asio async operations, you need to be precise about your function signatures to avoid these deduction issues.

Proper Implementation of async_compose

To correctly implement an async operation using boost::asio::async_compose, follow these key principles:

First, the function signature of your async operation implementation must match what async_compose expects. As noted in the official Boost documentation, the first argument to the function object should be a non-const reference to the enclosing intermediate completion handler.

The async_compose function signature is:

template<class CompletionToken, class Function>
auto async_compose(Function&& init, CompletionToken&& token, Executor&& ex);

Here’s the correct pattern for implementing an async operation with async_compose:

template <typename CompletionToken>
auto my_async_operation(CompletionToken&& token) {
  return boost::asio::async_compose<
    CompletionToken,
    void(boost::system::error_code, std::size_t)>(
      [](auto&& self, auto buffer) {
        // Your async operation implementation here
        self.complete(boost::system::error_code{}, 0);
      },
      token,
      boost::asio::get_associated_executor(token));
}

The key is to explicitly specify the completion handler signature when calling async_compose. This tells the compiler exactly what signature to expect, preventing the “deduced type void for err is incomplete” error.

Common Mistakes and Fixes

When implementing boost::asio async operations, several common patterns lead to the “deduced type void for err” error:

1. Default value on error parameter:
   Wrong:

   cpp

   template <typename Self>
   void operator()(Self& self, const boost::system::error_code& error = {}, std::size_t bytes_transferred = 0)
   

   Correct:

   cpp

   template <typename Self>
   void operator()(Self& self, const boost::system::error_code& error, std::size_t bytes_transferred)
   

2. Missing explicit signature specification:
   Wrong:

   cpp

   return boost::asio::async_compose(my_lambda, token, executor);
   

   Correct:

   cpp

   return boost::asio::async_compose<
     CompletionToken,
     void(boost::system::error_code, std::size_t)>(
       my_lambda, token, executor);
   

3. Incorrect parameter ordering:
   The error parameter must come before any other parameters in the handler signature.

As highlighted in Stack Overflow discussions, these mistakes prevent the compiler from correctly deducing the completion handler signature.

Working Examples

Here are complete working examples of how to properly implement async operations with boost::asio::async_compose:

Example 1: Simple Async Read Operation

template <typename CompletionToken, typename Buffer>
auto async_read_until_complete(boost::asio::ip::tcp::socket& socket, 
                               Buffer&& buffer, 
                               CompletionToken&& token) {
  return boost::asio::async_compose<
    CompletionToken,
    void(boost::system::error_code, std::size_t)>(
      [&](auto&& self, auto&& buf) {
        socket.async_read_some(
          boost::asio::buffer(buf),
          std::move(self));
      },
      token,
      boost::asio::get_associated_executor(token),
      std::forward<Buffer>(buffer));
}

Example 2: Custom Async Operation with State

class my_async_operation {
  boost::asio::ip::tcp::socket& socket_;
  std::vector<char> buffer_;
  std::size_t total_bytes_;
  
public:
  template <typename CompletionToken>
  auto operator()(CompletionToken&& token) {
    return boost::asio::async_compose<
      CompletionToken,
      void(boost::system::error_code, std::size_t)>(
        [this](auto&& self) {
          socket_.async_read_some(
            boost::asio::buffer(buffer_),
            std::move(self));
        },
        token,
        socket_.get_executor());
  }
};

// Usage:
my_async_operation op(socket, buffer);
boost::asio::async_compose<
  CompletionToken,
  void(boost::system::error_code, std::size_t)>(
    std::move(op), token, socket.get_executor());

The Boost example from beta.boost.org provides a more complete implementation pattern that demonstrates these concepts in action.

Best Practices for async_compose

When working with boost::asio and async_compose, follow these best practices:

1. Always specify the completion handler signature explicitly to avoid deduction issues:

   cpp

   return boost::asio::async_compose<
     CompletionToken,
     void(boost::system::error_code, std::size_t)>(
       my_lambda, token, executor);
   

2. Avoid default values on parameters that are part of the handler signature, especially the error parameter.

3. Use perfect forwarding for your function objects to avoid unnecessary copies:

   cpp

   return boost::asio::async_compose<...>(std::forward<Function>(f), token, ex);
   

4. Properly manage executor association using boost::asio::get_associated_executor.

5. Use the correct parameter ordering: error parameter first, then any additional parameters.

As noted in the tutorial from lastviking.eu, async_compose is the generic way to write an async operation that can be used with any continuation type, including callbacks, use_future, use_awaitable, and various coroutine types.

Advanced Usage Patterns

For more complex scenarios, consider these advanced patterns with boost::asio::async_compose:

Chaining Multiple Async Operations

template <typename CompletionToken>
auto chained_async_operation(CompletionToken&& token) {
  return boost::asio::async_compose<
    CompletionToken,
    void(boost::system::error_code)>(
      [](auto&& self, int step = 0) {
        if (step == 0) {
          // First async operation
          async_op1(std::move(self));
        } else if (step == 1) {
          // Second async operation
          async_op2(std::move(self));
        } else {
          // Complete
          self.complete(boost::system::error_code{});
        }
      },
      token,
      boost::asio::get_associated_executor(token));
}

Using Custom Completion Signatures

template <typename CompletionToken>
auto custom_async_operation(CompletionToken&& token) {
  // Custom signature with user-defined types
  return boost::asio::async_compose<
    CompletionToken,
    void(result_type)>(
      [](auto&& self) {
        // Implementation
        self.complete(result_type{});
      },
      token,
      boost::asio::get_associated_executor(token));
}

These patterns demonstrate the flexibility of boost::asio::async_compose while maintaining proper type safety and avoiding the “deduced type void for err” error.

Conclusion

The “deduced type void for err is incomplete” error when using boost::asio::async_compose stems from incorrect completion handler signatures that prevent proper template argument deduction. By understanding how async_compose works and following the implementation patterns outlined above, you can create robust async operations with boost::asio.

Key takeaways include:

• Always explicitly specify the completion handler signature when calling async_compose
• Avoid default values on parameters that are part of the handler signature
• Ensure the error parameter comes first in your handler signature
• Use proper executor association and perfect forwarding
• Study the official boost::asio examples to understand best practices

With these techniques, you can effectively implement complex async operations using boost::asio while avoiding common compilation errors related to type deduction.

Sources

• Stack Overflow: What’s going on with asio::async_compose?
• Boost.Asio official documentation: async_compose
• Boost.Asio example: composed_8.cpp
• The great mystery on how to make durable, asynchronous functions for asio
• Async Compose – How to Fix the “deduced type void for err is incomplete” Error