Component of the Week #28: bdlf_noop

Summary:

• A functor class that accepts any type and number of parameters and does nothing, useful for doing nothing even when an interface requires providing a callback.

The bdlf::NoOp class provides a functor whose function-call operator accepts any number of arguments of any type, does nothing (leaving all arguments unmodified), and returns void. This component is particularly useful when working with callback-based interfaces that require a callable object, but you don’t need the callback to perform any action. It is more concise than using an empty lambda expression because it avoids the need to define arguments that match the callback signature.

The component also provides bdlf::noOp, a constant variable of type bdlf::NoOp for convenient usage.

Basic Usage with Callback Interfaces

Many asynchronous systems and APIs require callback functions, even when you don’t need the callback to do anything. bdlf::NoOp provides a clean solution for these situations:

#include <bdlf_noop.h>
#include <bsl_functional.h>
#include <bsl_iostream.h>
#include <bsl_string.h>

using namespace BloombergLP;

/// Hypothetical async system interface
struct AsyncSystemUtil {
    /// A callback that accepts an integer status code
    typedef bsl::function<void(int)> StatusCallback;

    /// A callback that accepts a message and a status code
    typedef bsl::function<void(const bsl::string&, int)> MessageAndStatusCallback;

    static void sendPing(const StatusCallback& callback)
    {
        // Simulate async operation
        bsl::cout << "Sending ping..." << bsl::endl;
        // Later, callback would be invoked with status
        callback(0);  // 0 = success
    }

    static void fetchServerInfo(const MessageAndStatusCallback& callback)
    {
        // Simulate async operation that returns server information
        bsl::cout << "Fetching server info..." << bsl::endl;
        // Later, callback would be invoked with server details
        callback("Server: Grady, Release: Caretaker-2", 0);
    }
};

int main()
{
    // Use NoOp when we don't care about the callback result
    AsyncSystemUtil::sendPing(bdlf::NoOp());

    // bdlf::NoOp works with different callback signatures too
    AsyncSystemUtil::fetchServerInfo(bdlf::NoOp());

    // Alternatively, use the convenient constant
    AsyncSystemUtil::sendPing(bdlf::noOp);
    AsyncSystemUtil::fetchServerInfo(bdlf::noOp);

    return 0;
}

Common Use Cases

bdlf::NoOp is particularly useful in these scenarios:

• Optional Callbacks: APIs that have optional callback parameters

• Placeholder: As a temporary placeholder while developing callback-based systems

For more details and examples, see:

• documentation for bdlf_noop