A C++ bencoding library supporting both decoding and encoding.
C++ CMake C



A C++ bencoding library supporting both decoding and encoding. It provides a simple API for decoding, encoding, and pretty-printing of bencoded data. It is also extensible so you can write your own manipulation of the decoded data.

#include "bencoding/bencoding.h"

// Decode data stored in a std::string.
auto decodedData = bencoding::decode(str);

// Decode data directly from a stream.
auto decodedData = bencoding::decode(stream);

// Encode the data into a std::string.
std::string encodedData = bencoding::encode(decodedData);

// Get a pretty representation of the decoded data.
std::string prettyRepr = bencoding::getPrettyRepr(decodedData);

The supported format is as defined in the BitTorrent specification.


The following software is required:

  • A compiler supporting C++11, such as GCC >= 4.9.
  • CMake to build and install the library.


  • Doxygen to generate API documentation.
  • Google Test to build and run tests.
  • LCOV to generate code coverage statistics.

Build and Installation

  1. Install the requirements above.
  2. Clone this repository:

    git clone https://github.com/s3rvac/cpp-bencoding
  3. Create a build directory, enter it, run cmake .. and make (possibly with additional parameters, see below):

    mkdir build
    cd build
    cmake ..

    You can pass additional parameters to the cmake call:

    • -DWITH_COVERAGE=1 to build with code coverage support (requires LCOV, disabled by default).
    • -DWITH_DOC=1 to build API documentation (requires Doxygen, disabled by default).
    • -DWITH_TESTS=1 to build tests (requires Google Test, disabled by defauly).
    • -DCMAKE_BUILD_TYPE=debug to build the library with debugging information, which is useful during the development. By default, the library is built in the release mode.
    • -DCMAKE_INSTALL_PREFIX:PATH=/usr to set a custom installation path.

    The make call supports standard parameters, such as:

    • -j N to build the library by using N processors.
    • VERBOSE=1 to show verbose output when building the library.
  4. Install the library:

    make install

    This will install the library into the selected installation path. If you did not specify the path when calling cmake, it will be installed to the install directory.


  1. Setup the build system of your project to include the path to the install/include directory above so you can include the library header files in the following way:

    #include "bencoding/bencoding.h"

    The header file bencoding.h includes all the library header files. You may include just some of them if you want:

    #include "bencoding/Decoder.h"
    #include "bencoding/PrettyPrinter.h"
  2. Use the functionality provided by the library. A simple example:

    #include <iostream>
    #include <memory>
    #include "bencoding/bencoding.h"
    using namespace bencoding;
    int main() {
        try {
            // Read and decode input data from the standard input.
            std::shared_ptr<BItem> decodedData = decode(std::cin);
            // Print the decoded data in a readable way to the standard output.
            std::cout << getPrettyRepr(decodedData) << "\n";
            return 0;
        } catch (const DecodingError &ex) {
            // There was an error during the decoding.
            std::cerr << "error: " << ex.what() << "\n";
            return 1;

    For a full example, see the sample/decoder.cpp file.

  3. Setup the build system of your project to link the install/lib/libbencoding.a library. For example, with GCC, you can either use -Linstall/lib -lbencoding or link the install/lib/libbencoding.a file directly.


A complete sample is available in the sample directory. It is a standalone decoder that decodes data from the given file or standard input, and prints them in a pretty format to the standard output. The decoder is built and installed alongside with the library. To run it, execute install/bin/decoder after installation. Sample input files are in the sample/inputs directory.

Input file (sample/inputs/sample1.torrent):

d8:announce18:http://tracker.com10:created by14:KTorrent 2.1.413:creation datei1182163277e4:infod6:lengthi6e4:name8:file.txt12:piece lengthi32768e6:pieces12:binary dataee


$ install/bin/decoder sample/inputs/sample1.torrent


    "announce": "http://tracker.com",
    "created by": "KTorrent 2.1.4",
    "creation date": 1182163277,
    "info": {
        "length": 6,
        "name": "file.txt",
        "piece length": 32768,
        "pieces": "binary data"

API Documentation

The API documentation can be generated by passing -DWITH_DOC=1 when running cmake (see the build instructions). When you run make afterwards, the documentation is generated in the HTML format. After make install, it can be viewed in your favorite web browser by opening install/doc/index.html. You need to have Doxygen installed to generate the API documentation.


Over 99% of the library source code is covered by unit tests. To build them, pass -DWITH_TESTS=1 when running cmake. To run them after make install, execute install/bin/tester. You need to have Google Test installed to build and run the tests.

To generate code coverage, pass -DWITH_COVERAGE=1 when running cmake. After the library is built, run make coverage from the build directory to generate the code coverage. It can be then viewed in a web browser by opening coverage/index.html. You need to have LCOV installed to generate the code coverage.

Extending the Library

The BItemVisitor class implements the Visitor design pattern. You can create your own subclass that manipulates the bencoded data in any way you want. Two examples of using the BItemVisitor class are the Encoder and PrettyPrinter classes. See my blog post or their source code for more details.


Any contributions are welcomed! Notes:

  • Please, before sending a patch or a pull request, ensure that all the tests still pass.
  • If you provide new functionality, please also provide tests for it.


Copyright (c) 2014 Petr Zemek (s3rvac@gmail.com) and contributors.

Distributed under the BSD 3-clause license. See the LICENSE file for more details.