Stdios Baseregister Callback Function In C++

In this article, you will learn about the std::iosbase::registercallback function in C++ with its syntax, parameters, and examples.

Introduction:

In C++, the std::iosbase::registercallback function enables you to attach a callback function to an I/O stream object. This function is triggered when specific events take place during stream operations, such as clearing the stream buffer, changing the locale, or copying formatting details.

The callback function must follow the signature outlined by std::iosbase::eventcallbackfn with event types being designated using the std::iosbase::event_type enumeration. Whenever an event of the registered type occurs, the callback function is called, which allows you to execute customized actions or modifications based on that event.

This function provides a way to enhance I/O streams functionality and customize their behavior according to your applications requirements.

Syntax:

It has the following syntax:

void register_callback(event callback function, int index);

Parameters:

1. The function parameter is a reference to a custom function object or callable object (like a function or function object) that serves as the callback function to be added.

The callback function should be compatible with the 'std::iosbase::eventcallbackfn' type, which is a function type defined within the 'std::iosbase' class.

This callback function should follow this format;

void callback_fn(std::ios_base::event ev, std::ios_base& stream int index);

• ✅ The ev variable is a value of type 'std::ios_base::event', which indicates the type of event that triggered the callback.
• ✅ The stream refers to the associated 'std::ios_base' object linked to the stream where the callback is invoked.
• ✅ The index denotes an integer representing the position of the registered callback. If needed, this position can later be used to unregister the callback.

Whenever specific events occur in the stream like clearing the buffer changing settings or copying formatting information within it, this callback function will be triggered.

The callback function enables you to execute tasks or adjustments depending on the event that activated the callback.

1. index (of type 'std::iosbase::eventtype')

This parameter represents an enum value that indicates the event type for which the callback function should be associated.

It is a value of type 'std::iosbase::eventtype'.

The available options for 'index' include;

• ✅ std::iosbase::eraseevent;: It is the callback function triggers when the stream buffer is erased.
• ✅ std::iosbase::imbueevent;: It is the callback function activates when the stream's locale is changed.
• ✅ std::iosbase::copyfmtevent;: It is the callback function executes when the stream's formatting details are duplicated.

By specifying the event type, you can assign callback functions to manage events occurring in the stream.

This parameter enables the registration of callbacks for events rather than a single callback for all types of events.

The main callback function, designated by the 'function' parameter, is executed when a particular event happens. The 'index' parameter determines the type of event that triggers this function. These parameters work together to create an expandable way to manage custom behaviors or actions based on events that occur during stream operations.

It is worth noting that multiple callback functions can be assigned to the event type and they will be triggered in the order of their registration (meaning the most recently added callback will be executed first).

Event Types:

The event_callback type is defined in C++ as follows;

typedef void (*event_callback) (event ev, ios_base& ios int index);

This callback function requires three parameters;

• ✅ ev: It is a representation of the event that initiated the callback. It is an enumeration type within std::iosbase with values, such as locale change copyfmtevent (formatting flags copied) and erase_event (stream destruction).
• ✅ ios: It is a reference to the std::ios_base object (the stream) that triggered the event.
• ✅ index: It is the specific identifier value that is passed to register_callback for differentiating callbacks.

Features:

• ✅ You can associate callback functions with index values for a stream.
• ✅ The callbacks are executed in the order of registration meaning that the registered callback is invoked first.
• ✅ It's important to note that callback functions should not raise any exceptions.

Example:

Let us take an example to illustrate the std::iosbase::registercallback function in C++.

#include <iostream>
#include <fstream>
#include <sstream>

void my_callback(std::ios_base::event evt, std::ios_base& str, int index) {
    switch (evt) {
        case std::ios_base::erase_event:
            std::cout << "Erase event occurred. Stream: " << &str << ", Index: " << index << std::endl;
            break;
        case std::ios_base::imbue_event:
            std::cout << "Imbue event occurred. Stream: " << &str << ", Index: " << index << std::endl;
            break;
        case std::ios_base::copyfmt_event:
            std::cout << "Copy format event occurred. Stream: " << &str << ", Index: " << index << std::endl;
            break;
    }
}

int main() {
    std::ios_base::event_callback callback = &my_callback;
    std::fstream fs("example.txt", std::ios::out | std::ios::trunc);

    // Register the callback with the fstream object
    fs.register_callback(callback, 1);

    // Triggering imbue_event
    fs.imbue(std::locale::classic());

    // Triggering copyfmt_event using ostringstream
    std::ostringstream oss;
    oss.copyfmt(fs); // Trigger copyfmt_event

    // Writing to the file to avoid unused variable warning
    fs << "This is a test.";

    // Closing the file, which will trigger erase_event
    fs.close();

    return 0;
}

Output:

Imbue event occurred. Stream: 0x7ffe98cede98, Index: 1
Copy format event occurred. Stream: 0x7ffe98cedc80, Index: 1
Erase event occurred. Stream: 0x7ffe98cedc80, Index: 1
Erase event occurred. Stream: 0x7ffe98cede98, Index: 1