cpp-netlib includes and implements a number of HTTP clients that you can use and embed in your own applications. All of the HTTP client implementations:
- Cannot be copied. This means you may have to store instances of the clients in dynamic memory if you intend to use them as function parameters or pass them around in smart pointers or by reference.
- Assume that requests made are independent of each other. There currently is no cookie or session management system built-in to cpp-netlib’s HTTP client implementations.
- Are header-only and are compiled-into your application. Future releases in case you want to upgrade the implementation you are using in your application will be distributed as header-only implementations, which means you have to re-compile your application to use a newer version of the implementations.
The HTTP clients all share the same API, but the internals are documented in terms of what is different and what to expect with the different implementations.
There is a single user-facing template class named basic_client which takes three template parameters:
- Tag - which static tag you choose that defines the behavior of the client.
- http_version_major - an unsigned int that defines the HTTP major version number, this directly affects the HTTP messages sent by the client.
- http_version_minor - an unsigned int that defines the HTTP minor version number.
There are two major different class of implementations of the basic_client template that depend on which tag you choose: Synchronous Clients and Asynchronous Clients. These two different classes are described in their own sections following this one. What follows is a table of all tags supported by the HTTP client implementation provided by cpp-netlib.
| Tag | Description |
|---|---|
| http_default_8bit_tcp_resolve | This is the default HTTP implementation tag that resolves addresses with a TCP resolver and provides a synchronous/blocking HTTP client interface. |
| http_default_8bit_udp_resolve | This is similar to the above tag except that it specifies the HTTP client to use a UDP resolver. It also provides a synchronous/ blocking HTTP client interface. |
| http_keepalive_8bit_tcp_resolve | This tag specifies that the HTTP client by default will keep connections to the server alive. It only makes sense if the version_major and version_minor are both 1, to indicate HTTP 1.1. This tag causes the HTTP client to resolve using a TCP resolver and provides a synchronous/ blocking HTTP client interface. |
| http_keepalive_8bit_udp_resolve | This is similar to the above tag except that it specifies the HTTP client to use a UDP resolver. It also provides a synchronous/ blocking HTTP client interface. |
| http_async_8bit_tcp_resolve | This tag provides an active HTTP client object implementation that uses a TCP resolver. Response objects returned will encapsulate a number of Boost.Thread shared futures to hold values. Users don’t have to see this as they are implementation details. |
| http_async_8bit_udp_resolve | This is similar to the above tag except that specifies the HTTP client to use a UDP resolver. |
In the above table the tags follow a pattern for describing the behavior introduced by the tags. This pattern is shown below:
<protocol>_<modifier>_<character-width>_<resolve-strategy>
For example, the tag http_default_8bit_tcp_resolve indicates the protocol http, a modifier default, a character width of 8bit, and a resolve strategy of tcp_resolve.
Of the client tags shown in the table, the following makes the basic_client behave as a fully synchronous client.
- http_default_8bit_tcp_resolve
- http_default_8bit_udp_resolve
- http_keepalive_8bit_tcp_resolve
- http_keepalive_8bit_udp_resolve
The synchronous client implements all the operations of the client underneath the interface all block to wait for I/O to finish. All the member methods are synchronous and will block until the response object is ready or throws if erros are encountered in the performance of the HTTP requests.
Warning
The synchronous clients are NOT thread safe. You will need to do external synchronization to use synchronous client implementations.
The following tags specify the basic_client to behave in an asynchronous manner:
- http_async_8bit_tcp_resolve
- http_async_8bit_udp_resolve
An asynchronous client implementation means that``basic_client<...>`` is an Active Object. This means that the client has and manages its own lifetime thread, and returns values that are asynchronously filled in. The response object encapsulates Boost.Thread futures which get filled in once the values are available.
The asynchronous clients implement all operations asynchronously which are hidden from the user. The interface is still synchronous but the fetching of data happens on a different thread.
Note
The asynchronous clients are thread safe, and can be shared across many threads. Each request starts a sequence of asynchronous operations dedicated to that request. The client does not re-cycle connections and uses a one-request-one-connection model.
In this section we assume that the following typedef is in effect:
typedef boost::network::http::basic_client<
boost::network::http::tags::http_default_8bit_udp_resolve
, 1
,1
>
client;
Also, that code using the HTTP client will have use the following header:
#include <boost/network/include/http/client.hpp>
The client implementation can be default constructed, or customized at initialization.
| [1] | In Asynchronous Clients, redirects are not followed. This means the response objects will contain whatever HTTP response was retrieved by the client implementation. |
| [2] | In Asynchronous Clients, only caching resolved endpoints take effect. |
The client implementation supports various HTTP methods. The following constructs assume that a client has been properly constructed named client_ and that there is an appropriately constructed request object named request_ and that there is an appropriately constructed response object named response_ like the following:
client client_();
client::request request_("http://cpp-netib.github.com/");
client::response response_;