Boost

You do not need to link the boost::algorithm library. It is a header only library, hence you only need to #include its header files.

For examples, see

• How to use boost string algorithm ends_with: A minimal example
• How to use boost string algorithm starts_with: A minimal example

#include <boost/algorithm/string/predicate.hpp>
#include <string>
#include <iostream>

int main() {
    std::string mystr = "foobar";
    // Prints "true, false"
    std::cout << std::boolalpha
        << boost::algorithm::starts_with(mystr, "foo") << ", "
        << boost::algorithm::starts_with(mystr, "bar") << std::endl;
}

Also see: How to use boost string algorithm ends_with: A minimal example

#include <boost/algorithm/string/predicate.hpp>
#include <string>
#include <iostream>

int main() {
    std::string mystr = "foobar";
    // Prints "false, true"
    std::cout << std::boolalpha
        << boost::algorithm::ends_with(mystr, "foo") << ", "
        << boost::algorithm::ends_with(mystr, "bar") << std::endl;
}

Also see: How to use boost string algorithm starts_with: A minimal example

This minimal example shows you how to write data to a .gz file in C++, compressing the data on-the-fly using boost::iostreams. Using the modern iostreams layer, as opposed to a block-based approach like zlib allows you to use the full power and ease-of-use of std::ostream.

#include <fstream>
#include <iostream>
#include <boost/iostreams/filtering_streambuf.hpp>
#include <boost/iostreams/copy.hpp>
#include <boost/iostreams/filter/gzip.hpp>
using namespace std;

int main(int argc, char** argv) {
    if(argc < 2) {
        cerr << "Usage: " << argv[0] << " <output .gz file>" << endl;
    }
    //Read filename from the first command line argument
    ofstream file(argv[1], ios_base::out | ios_base::binary);
    boost::iostreams::filtering_streambuf<boost::iostreams::output> outbuf;
    outbuf.push(boost::iostreams::gzip_compressor());
    outbuf.push(file);
    //Convert streambuf to ostream
    ostream out(&outbuf);
    //Write some test data
    out << "This is a test text!\n";
    //Cleanup
    boost::iostreams::close(outbuf); // Don't forget this!
    file.close();
}

cmake_minimum_required(VERSION 3.0)
find_package(Boost 1.36.0 COMPONENTS iostreams)

include_directories(${Boost_INCLUDE_DIRS})
add_executable(iostreams-gz-compress iostreams-gz-compress.cpp)
target_link_libraries(iostreams-gz-compress ${Boost_LIBRARIES})

This minimal example shows you how to open a .gz file in C++, decompress it on-the-fly using boost::iostreams and then copy its contents to stdout:

#include <fstream>
#include <iostream>
#include <boost/iostreams/filtering_streambuf.hpp>
#include <boost/iostreams/copy.hpp>
#include <boost/iostreams/filter/gzip.hpp>
using namespace std;

int main(int argc, char** argv) {
    if(argc < 2) {
        cerr << "Usage: " << argv[0] << " <gzipped input file>" << endl;
    }
    //Read from the first command line argument, assume it's gzipped
    ifstream file(argv[1], ios_base::in | ios_base::binary);
    boost::iostreams::filtering_streambuf<boost::iostreams::input> inbuf;
    inbuf.push(boost::iostreams::gzip_decompressor());
    inbuf.push(file);
    //Convert streambuf to istream
    istream instream(&inbuf);
    //Copy everything from instream to 
    cout << instream.rdbuf();
    //Cleanup
    file.close();
}

cmake_minimum_required(VERSION 3.0)
find_package(Boost 1.36.0 COMPONENTS iostreams)

include_directories(${Boost_INCLUDE_DIRS})
add_executable(iostreams-gz-decompress iostreams-gz-decompress.cpp)
target_link_libraries(iostreams-gz-decompress ${Boost_LIBRARIES})

In our previous post Minimal PugiXML file reader example we provided a short example of how to read from an uncompressed XML file using PugiXML. In practice, many large XML files are distributed as .xml.gz package.

Since you can use boost::iostreams to decompress gzipped data on the fly and pipe it directly into PugiXML, you don’t need to store the uncompressed data on your hard drive.

#include <iostream>
#include <fstream>
#include <pugixml.hpp>
#include <boost/iostreams/filtering_streambuf.hpp>
#include <boost/iostreams/copy.hpp>
#include <boost/iostreams/filter/gzip.hpp>
using namespace std;
using namespace pugi;

int main() {
    // Open "raw" gzipped data stream
    ifstream file("test.xml.gz", ios_base::in | ios_base::binary);
    // Configure decompressor filter
    boost::iostreams::filtering_streambuf<boost::iostreams::input> inbuf;
    inbuf.push(boost::iostreams::gzip_decompressor());
    inbuf.push(file);
    //Convert streambuf to istream
    istream instream(&inbuf);
    // Parse from stream
    xml_document doc;
    xml_parse_result result = doc.load(instream);
    // Print content of root element
    cout << "Load result: " << result.description() << "\n"
         << doc.child("root-element").child_value() // "Test text"
         << endl;
}

cmake_minimum_required(VERSION 3.0)
find_package(Boost 1.36.0 COMPONENTS iostreams)

include_directories(${Boost_INCLUDE_DIRS})
add_executable(pugixml-example pugixml-example.cpp)
target_link_libraries(pugixml-example pugixml ${Boost_LIBRARIES})

<?xml version="1.0" encoding="UTF-8"?>
<root-element>Test text</root-element>

Download all three files and then run

gzip test.xml
cmake .
make
./pugixml-example

You should see an output like

Load result: No error
Test text

#include <boost/lexical_cast.hpp>
#include <iostream>

int main() {
    int a = boost::lexical_cast<int>("123");
    int b = boost::lexical_cast<int>("456");
    
    int c = a + b;
    std::cout << c << std::endl; //Prints 579
}

If you want to compile your C++ project using boost::program_options but you see error messages like

/home/uli/myProject/main.cpp:28:5: error: ‘po’ has not been declared
     po::notify (vm);

you are missing this declaration which you need to place directly after #include <boost/program_options.hpp>:

namespace po = boost::program_options;

This declares po as an alias for the boost::program_options namespace, because writing po::variables_map is much easier to read than boost::program_options::variables_map and using namespace boost::program_options might cause some name collisions with other functions.

Examples in this post:

• –input/-i and –output/-o, both strings, both required
• Single positional string argument
• A double-typed argument (with automatic type checking)

More examples will possibly be added in the future

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