Introduction to Per-Operation Cancellation

What is a Cancellation Slot?

A cancellation slot can be defined as lightweight channels that are used to deliver cancellation requests. In an asynchronous operation, we can get the cancellation slot using the get_associated_cancellation_slot method. The associated cancellation slot has to satisfy the cancellation slot type requirements. Moving forward, let’s understand cancellation slot type requirements.

Cancellation Slot Type Requirements

There are cancellation slot type requirements that must be fulfilled in slot cancellation. Let’s suppose x and y are two values.

• If x == y, then true will be returned, and our program will not exit with an exception.
• If x != y, then false will be returned, and our program will exit with an exception.

There are different cancellation slots like associated_cancellation_slot, bind_cancellation_slot, and cancellation_signal that can be used by including headers asio/associated_cancellation_slot.hpp, asio/bind_cancellation_slot.hpp, asio/cancellation_signal.hpp respectively.

Let’s make a program to better understand Per-Operation Cancellation.

Implementation

#include <iostream>

#include <asio/experimental/channel.hpp>

#include <asio/experimental/awaitable_operators.hpp>

#include <asio.hpp>

#include <asio/error_code.hpp>


using namespace std;

using namespace asio::experimental::awaitable_operators;

using channel = asio::experimental::channel< void(error_code, uint64_t) >;


asio::awaitable<void> ninja(channel& ch,int expectedNumber) {

    std::cout << "The expected number is " << expectedNumber << std::endl;

    int n = 5;

    std::cout << "The number we are checking with is equal to " << n << std::endl;

    if (n == expectedNumber) {

        std::cout << "No Error" << std::endl;

    }

    assert(n == expectedNumber && "The variable n and expected number is not same");

    co_return ;

}


int main() {

    auto io = asio::io_context{};

    auto ch = channel{ io,0};

    asio::co_spawn(io, ninja(ch, 15), asio::detached);

    io.run();

    return 0;

}

Explanation

In the above code, we have made a cancellation slot named ninja. In the main function co_spawn function is called with three arguments that are context, ninja cancellation slot, and a completion token detached. The completion token helps in producing a completion handler. Here, we are using detached() as a completion token which is used to indicate the detached asynchronous operations. We are passing and expected number 15 to the ninja cancellation slot, and inside the ninja cancellation slot, we are declaring and assigning a variable n = 5 and checking whether n and expectedNumber are equal or not; if it’s equal, then the program will print No Error in the console otherwise we are asserting a debug error message with three options Abort, Retry and Ignore. 

Clicking on Abort and Ignore will exit the code, whereas if we click on the Retry button, it will take us to the breaking point in our code. See the below images for your reference.

Output 

After clicking on the Retry button, we will be redirected to the breakpoint.

Also see, Application of Oops

Frequently Asked Questions

What is the full form of Asio?

Asio stands for Asynchronous Input Output.

What is the use of the Asio library?

Asio helps in doing network programming using C++ language. It allows the processing of data asynchronously by providing asynchronous I/O models.

What is a cancellation slot?

A cancellation slot can be defined as a lightweight channel that is used to deliver cancellation requests.

How can we enable per-operation cancellation?

The per-operation cancellation is enabled by specifying the cancellation slot of the completion handler that must satisfy the cancellation slot requirements.

Conclusion

In this article, we have discussed about per-operation cancellation. We have also discussed about the cancellation slot. We have seen cancellation slot-type requirements. In the end, we have made one program to better understand per-operation cancellation. To learn more about the Asio library, you can read the below-mentioned articles:

⭐ Getting started with Asio

⭐ Asio-Serial Ports

⭐ Asio-Timers

⭐ Asio-Files

⭐ Asio-Channels

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