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uvw - libuv wrapper in modern C++

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uvw is a header-only, event based, tiny and easy to use libuv wrapper in modern C++.
The basic idea is to hide completely the C-ish interface of libuv behind a graceful C++ API. Currently, no uv_*_t data structure is actually exposed by the library.
Note that uvw stays true to the API of libuv and it doesn't add anything to its interface. For the same reasons, users of the library must follow the same rules who are used to follow with libuv.
As an example, a handle should be initialized before any other operation and closed once it is no longer in use.

Code Example

#include <uvw.hpp>
#include <memory>

void listen(uvw::Loop &loop) {
    std::shared_ptr<uvw::TCPHandle> tcp = loop.resource<uvw::TCPHandle>();

    tcp->once<uvw::ListenEvent>([](const uvw::ListenEvent &, uvw::TCPHandle &srv) {
        std::shared_ptr<uvw::TCPHandle> client = srv.loop().resource<uvw::TCPHandle>();

        client->on<uvw::CloseEvent>([ptr = srv.shared_from_this()](const uvw::CloseEvent &, uvw::TCPHandle &) { ptr->close(); });
        client->on<uvw::EndEvent>([](const uvw::EndEvent &, uvw::TCPHandle &client) { client.close(); });


    tcp->bind("", 4242);

void conn(uvw::Loop &loop) {
    auto tcp = loop.resource<uvw::TCPHandle>();

    tcp->on<uvw::ErrorEvent>([](const uvw::ErrorEvent &, uvw::TCPHandle &) { /* handle errors */ });

    tcp->once<uvw::ConnectEvent>([](const uvw::ConnectEvent &, uvw::TCPHandle &tcp) {
        auto dataWrite = std::unique_ptr<char[]>(new char[2]{ 'b', 'c' });
        tcp.write(std::move(dataWrite), 2);

    tcp->connect(std::string{""}, 4242);

int main() {
    auto loop = uvw::Loop::getDefault();


The main reason for which uvw has been written is the fact that it does not exist a valid libuv wrapper in C++. That's all.

Build Instructions


To be able to use uvw, users must provide the following system-wide tools:

  • A full-featured compiler that supports at least C++14.
  • libuv (which version depends on the tag of uvw in use).

The requirements below are mandatory to compile the tests and to extract the documentation:

  • CMake version 3.2 or later.
  • Doxygen version 1.8 or later.

Note that libuv is part of the dependencies of the project and it will be cloned by cmake (see below for further details).
Because of that, users have not to install it to compile and execute the tests.


uvw is a header-only library.
This means that including the uvw.hpp header or one of the other uvw/*.hpp headers is enough to use it.
It's a matter of adding the following line at the top of a file:

#include <uvw.hpp>

Then pass the proper -I argument to the compiler to add the src directory to the include paths.
Note that users are demanded to correctly setup include directories and libraries search paths for libuv.


Starting with tag v1.12.0 of libuv, uvw follows the semantic versioning scheme.
The problem is that any version of uvw also requires to track explicitly the version of libuv to which it is bound.
Because of that, the latter wil be appended to the version of uvw. As an example:


In particular, the following applies:

  • U.V.W are major, minor and patch versions of uvw.
  • X.Y is the version of libuv to which to refer (where any patch version is valid).

In other terms, tags will look like this from now on:


Branch master of uvw will be a work in progress branch that follows branch v1.x of libuv (at least as long as it remains their master branch).


The documentation is based on doxygen. To build it:

  • $ cd build
  • $ cmake ..
  • $ make docs

The API reference will be created in HTML format within the directory build/docs/html.
To navigate it with your favorite browser:

  • $ cd build
  • $ your_favorite_browser docs/html/index.html

The API reference is also available online for the latest version.


The documentation is mostly inspired by the official libuv API documentation for obvious reasons.
If you are mainly interested in the way uvw imports libuv in a cmake based project, I suggest you to take a look at this repository instead.


To compile and run the tests, uvw requires libuv and googletest.
cmake will download and compile both the libraries before to compile anything else.

To build the tests:

  • $ cd build
  • $ cmake .. -DBUILD_TESTING=ON
  • $ make
  • $ ctest -j4 -R uvw

Omit -R uvw if you also want to test libuv and other dependencies.

Crash Course


There is only one rule when using uvw: always initialize the resources and terminate them.

Resources belong mainly to two families: handles and requests.
Handles represent long-lived objects capable of performing certain operations while active.
Requests represent (typically) short-lived operations performed either over a handle or standalone.

The following sections will explain in short what it means to initialize and terminate these kinds of resources.
For more details, please refer to the online documentation.


Initialization is usually performed under the hood and can be even passed over, as far as handles are created using the Loop::resource member function.
On the other side, handles keep themselves alive until one explicitly closes them. Because of that, memory usage will grow up if users simply forget about a handle.
Therefore the rule quickly becomes always close your handles. It's as simple as calling the close member function on them.


Usually initializing a request object is not required. Anyway, the recommended way to create a request is still through the Loop::resource member function.
Requests will keep themselves alive as long as they are bound to unfinished underlying activities. This means that users have not to discard explicitly a request.
Therefore the rule quickly becomes feel free to make a request and forget about it. It's as simple as calling a member function on them.

The Loop and the Resource

The first thing to do to use uvw is to create a loop. In case the default one is enough, it's easy as doing this:

auto loop = uvw::Loop::getDefault();

Note that loop objects don't require to be closed explicitly, even if they offer the close member function in case an user wants to do that.
Loops can be started using the run member function. The two calls below are equivalent:


Available modes are: DEFAULT, ONCE, NOWAIT. Please refer to the documentation of libuv for further details.

In order to create a resource and to bind it to the given loop, just do the following:

auto tcp = loop.resource<uvw::TCPHandle>();

The line above will create and initialize a tcp handle, then a shared pointer to that resource will be returned.
Users should check if pointers have been correctly initialized: in case of errors, they won't be.
Another way to create a resource is:

auto tcp = TCPHandle::create(loop);

Pretty annoying indeed. Using a loop is the recommended approach.

The resources also accept arbitrary user-data that won't be touched in any case.
Users can set and get them through the data member function as it follows:

std::shared_ptr<void> data = resource->data();

Resources expect a std::shared_pointer<void> and return it, therefore any kind of data is welcome.
Users can explicitly specify a type other than void when calling the data member function:

std::shared_ptr<int> data = resource->data<int>();

Remember from the previous section that a handle will keep itself alive until one invokes the close member function on it.
To know what are the handles that are still alive and bound to a given loop, just do the following:

loop.walk([](uvw::BaseHandle &){ /* application code here */ });

BaseHandle exposes a few methods and cannot be promoted to the original type of the handle (even though type and category member functions fill the gap somehow).
Anyway, it can be used to close the handle that originated from it. As an example, all the pending handles can be closed easily as it follows:

loop.walk([](uvw::BaseHandle &h){ h.close(); });

No need to keep track of them.

To know what are the available resources' types, please refer the API reference.

The event-based approach

For uvw offers an event-based approach, resources are small event emitters to which listeners can be attached.
Attaching a listener to a resource is the recommended way to be notified about changes.
Listeners must be callable objects of type void(EventType &, ResourceType &), where:

  • EventType is the type of the event for which they have been designed.
  • ResourceType is the type of the resource that has originated the event.

It means that the following function types are all valid:

  • void(EventType &, ResourceType &)
  • void(const EventType &, ResourceType &)
  • void(EventType &, const ResourceType &)
  • void(const EventType &, const ResourceType &)

Once more, please note that there is no need to keep around references to the resources: they will pass themselves as an argument whenever an event is published.

There exist two methods to attach an event to a resource:

  • resource.once<EventType>(listener): the listener will be automatically removed after the first event of the given type.
  • resource.on<EventType>(listener): to be used for long-running listeners.

Both of them return an object of type ResourceType::Connection (as an example, TCPHandle::Connection).
A connection object can be used later as an argument to the erase member function of the resource to remove the listener.
There exists also the clear member function to drop all the listeners at once.

Almost all the resources use to emit ErrorEvent events in case of errors.
All the other events are specific for the given resource and documented in the API reference.

The code below shows how to create a simple tcp server using uvw:

auto loop = uvw::Loop::getDefault();
auto tcp = loop.resource<uvw::TCPHandle>();

tcp->on<uvw::ErrorEvent>([](const uvw::ErrorEvent &, uvw::TCPHandle &) { /* something went wrong */ });

tcp->on<uvw::ListenEvent>([](const uvw::ListenEvent &, uvw::TCPHandle &srv) {
    std::shared_ptr<uvw::TCPHandle> client = srv.loop().resource<uvw::TCPHandle>();
    client->once<uvw::EndEvent>([](const uvw::EndEvent &, uvw::TCPHandle &client) { client.close(); });
    client->on<uvw::DataEvent>([](const uvw::DataEvent &, uvw::TCPHandle &) { /* data received */ });

tcp->bind("", 4242);

Note also that uvw::TCPHandle already supports IPv6 out-of-the-box. The statement above is equivalent to tcp->bind<uvw::IPv4>("", 4242).
It's suffice to explicitly specify uvw::IPv6 as the underlying protocol to use it.

The API reference is the recommended documentation for further details about resources and their methods.

Going raw

In case users need to use functionalities not wrapped yet by uvw or if they want to get the underlying data structures as defined by libuv for some other reasons, almost all the classes in uvw give direct access to them.
Please, note that this functions should not be used directly unless users know exactly what they are doing and what are the risks. Going raw is dangerous, mainly because the lifetime management of a loop, a handle or a request is completely in charge to the library and working around it could quickly break things.

That being said, going raw is a matter of using the raw member functions:

auto loop = uvw::Loop::getDefault();
auto tcp = loop.resource<uvw::TCPHandle>();

uv_loop_t *raw = loop->raw();
uv_tcp_t *handle = tcp->raw();

Go the raw way at your own risk, but do not expect any support in case of bugs.


If you want to contribute, please send patches as pull requests against the branch master.
Check the contributors list to see who has partecipated so far.


Code and documentation Copyright (c) 2016-2019 Michele Caini.
Logo Copyright (c) 2018-2019 Richard Caseres.

Code released under the MIT license. Documentation released under CC BY 4.0.
Logo released under CC BY-SA 4.0.



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