How to wrap a C++11 callback?

Question

I'm using Casablanca C++ Rest SDK for http connections. Here is a basic piece of code that makes a http request.

// Creates an HTTP request and prints the length of the response stream.
pplx::task<void> HTTPStreamingAsync()
{
    http_client client(L"http://www.google.com");

    // Make the request and asynchronously process the response. 
    return client.request(methods::GET).then([](http_response response)
    {
        // Response received, do whatever here.
    });
}

This will do an asynchronous request and do a callback when done. I need to make my own class that uses these codes and I want to wrap it to my own callback.

For simplicity, assume that I want to make a class that has method which print html code of google.com.

So I expected something like this:

MyClass myObject;
myObject.getGoogleHTML([](std::string htmlString)
{
    std::cout << htmlString;
});

I searched and read related articles like:

But I'm still a bit confused as I get used with completion block in Objective-C. How can I construct such a class that wraps callback?

You mean something like a `T` representing the lambda and just forwarding it to `then`? — chris, Jun 26 '14 at 15:05
Sorry if it's just me being new-ish to web stuff, but where does the string in your callsite lambda come from? — chris, Jun 26 '14 at 15:16
@chris: please look at the lambda above, you see that there is a http_response. All I expected is to have this http_response passed to my class's lambda. std::string is a just an example. — Thanh-Nhon Nguyen, Jun 26 '14 at 15:19

score 1 · Accepted Answer · answered Jun 26 '14 at 15:20

Take the lambda as a general type. As a bonus, it'll work with any other callable object.

template<typename F>
pplx::task<void> MyClass::getGoogleHTML(F f) {
    http_client client(L"http://www.google.com");
    return client.request(methods::GET).then(f);
}

You could also perfectly forward f via F &&f and .then(std::forward<F>(f)) if you wish. If you're actually looking to extract something to give to the passed-in lambda, pass a lambda to then that captures f and calls it with that extracted data.

How to wrap a C++11 callback?

1 Answers1