In our previous post How to gzip-compress on-the-fly in C++ using boost::iostreams we showed how to create a gzip-compressing output stream using the boost::iostreams library.

This example shows how to control the compression rate of gzip_compressor:

Instead of constructing boost::iostreams::gzip_compressor() without arguments, use boost::iostreams::gzip_params(level) as the argument, where level (1..9) represents the compression level with 9 representing the highest compression level and 1 representing the lowest compression level. Higher levels of compression lead to reduced filesizes but are slower (i.e. consume more CPU time) during compression.

If filesize matters to you, I recommend choosing level 9 since compression even with the high level is extremely fast on modern computers.

Full example:

#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(
        boost::iostreams::gzip_params(9)
    ));
    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})

Checking if an element exists in PugiXML is simple: Just call bool(element) or use the element directly inside an if clause:

// Example on using bool(element)
cout << "<root-element> exists: " << std::boolalpha
     << bool(doc.child("root-element")) << endl;
cout << "<not-root-element> exists: " << std::boolalpha
     << bool(doc.child("not-root-element")) << endl;

// Example on using the element directly inside an if clause
if(doc.child("root-element")) {
    cout << "Yes, <root-element> exists!" << endl;
}

Full example:

#include <iostream>
#include <pugixml.hpp>
using namespace std;
using namespace pugi;

int main() {
    xml_document doc;
    xml_parse_result result = doc.load_file("test.xml");

    // Example on using bool(element)
    cout << "<root-element> exists: " << std::boolalpha
         << bool(doc.child("root-element")) << endl;
    cout << "<not-root-element> exists: " << std::boolalpha
         << bool(doc.child("not-root-element")) << endl;

    // Example on using the element directly inside an if clause
    if(doc.child("root-element")) {
        cout << "Yes, <root-element> exists!" << endl;
    }
}

add_executable(pugixml-example pugixml-example.cpp)
target_link_libraries(pugixml-example pugixml)

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

Compile using

cmake .
make

Problem:

You want to build a RapidJSON application that builds a JSON from scratch and is using Documents nested inside other documents, but when you try to run it, you see an error message like

zsh: segmentation fault (core dumped)  ./rapidjson-example

Solution:

Segmentation faults (i.e. illegal memory accesses) can have many reasons, but the most common one is that you use local allocators.

In order to fix the issue, use one allocator for your entire application.

MemoryPoolAllocator<> jsonAlloc; // I recommend to declare this statically

// ...
doc.AddMember("text", Value().SetString("Hello JSON!"), jsonAlloc);

Note that MemoryPoolAllocator never releases any memory from its memory pool.

Continue reading →

Problem:

Your program is using RapidJSON but when running it you see an error message like

rapidjson-example: /usr/include/rapidjson/writer.h:452: void rapidjson::Writer<OutputStream, SourceEncoding, TargetEncoding, StackAllocator, writeFlags>::Prefix(rapidjson::Type) [with OutputStream = rapidjson::BasicOStreamWrapper<std::basic_ostream<char> >; SourceEncoding = rapidjson::UTF8<>; TargetEncoding = rapidjson::UTF8<>; StackAllocator = rapidjson::CrtAllocator; unsigned int writeFlags = 0]: Assertion `!hasRoot_' failed.

Solution:

You are using a Writer for more than one Document. While you can use the Stream backing the Writer for any number of documents, each Writer must only be used once!

To fix the issue, create a Writer instance (on the same output Stream) for each document you intend to write.

Continue reading →

RapidJSON is a JSON library optimized for speed – hence it lacks some convieniece and lacks easy-to-use documentation on how to create JSON documents from scratch.

Here’s how you can create a Document:

// Generate document: {"text": "Hello JSON!"}
Document doc;
doc.SetObject(); // Make doc an object !
doc.AddMember("text", "Hello JSON!", doc.GetAllocator());

Full example, which prints to cout:

#include <iostream>
#include <rapidjson/document.h>
#include <rapidjson/writer.h>
#include <rapidjson/ostreamwrapper.h>
using namespace rapidjson;
using namespace std;

int main() {
    // Generate document: {"text": "Hello JSON!"}
    Document doc;
    doc.SetObject(); // Make doc an object !
    doc.AddMember("text", "Hello JSON!", doc.GetAllocator());
    // Write to stdout
    OStreamWrapper out(cout);
    Writer<OStreamWrapper> writer(out);
    doc.Accept(writer);
}

RapidJSON does not provide a straightforward way of serializing JSON to cout (= stdout), but you can use OStreamWrapper to do that:

#include <rapidjson/writer.h>
#include <rapidjson/ostreamwrapper.h>
// ... 
OStreamWrapper out(cout);
Writer<OStreamWrapper> writer(out);
doc.Accept(writer);

Full example:

#include <iostream>
#include <rapidjson/document.h>
#include <rapidjson/writer.h>
#include <rapidjson/ostreamwrapper.h>
using namespace rapidjson;
using namespace std;

int main() {
    // Generate document: {"text": "Hello JSON!"}
    Document doc;
    doc.SetObject(); // Make doc an object !
    doc.AddMember("text", "Hello JSON!", doc.GetAllocator());
    // Write to stdout
    OStreamWrapper out(cout);
    Writer<OStreamWrapper> writer(out);
    doc.Accept(writer);
}

PugiXML allows you to use the C++11 for loop (also known as range-based for loop or foreach loop) to iterate the children of a node easily:

<?xml version="1.0" encoding="UTF-8"?>
<root-element>
    <sub>A</sub>
    <sub>B</sub>
    <sub>C</sub>
</root-element>

#include <iostream>
#include <pugixml.hpp>
using namespace std;
using namespace pugi;

int main() {
    xml_document doc;
    xml_parse_result result = doc.load_file("test.xml");

    for(const auto& child : doc.child("root-element")) {
        cout << child.child_value() << endl;
    }
}

add_executable(pugixml-for pugixml-for.cpp)
target_link_libraries(pugixml-for pugixml)

Running the example will print

A
B
C

XML:

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

C++:

#include <iostream>
#include <pugixml.hpp>
using namespace std;
using namespace pugi;

int main() {
    xml_document doc;
    xml_parse_result result = doc.load_file("test.xml");
    
    cout << "Load result: " << result.description() << "\n"
         << doc.child("root-element").child_value() // "Test text"
         << endl;
}

Build configuration

add_executable(pugixml-example pugixml-example.cpp)
target_link_libraries(pugixml-example pugixml)

Compile using

cmake .
make

zlib is a great library in use by hundreds of thousands of programs (including Python). However, it lacks documentation.

This minimal example shows you how to create a simple zcat-like program that decompresses a gzipped input file and prints its contents to stdout.

/**
 * zzcat.c -- Minimal gzip decompression example using zlib
 * Written by Uli Köhler (techoverflow.net).
   Not copyrighted -- provided to the public domain
 */
#include <stdio.h>
#include <zlib.h>

#define BUFSIZE 16384

/* compress or decompress from fin (command line argument) to stdout */
int main(int argc, char **argv)
{
    if(argc <= 1) { // <= (number of expected CLI arguments)
        fprintf(stderr, "Usage: %s <input file>\n", argv[0]);
        return -1;
    }

    gzFile fin = gzopen(argv[1], "rb");

    char buf[BUFSIZE];
    int n;

    while((n = gzread(fin, buf, BUFSIZE)) > 0) {
        fwrite(buf, 1, n, stdout);
    }

    return 0;
}

Compile using

g++ -o zzcat zzcat.c -lz

Usage example:

# Create test file
echo "foo" | gzip -c > test.txt.gz
# Uncompress using zzcat! 
./zzcat test.txt.gz # This will print "foo"

This example program provides an example of a CLI executable in C/C++ that uses one command line argument and exits with a usage message if that argument is not present:

#include <stdio.h>

int main(int argc, char **argv)
{
    if(argc <= 1) { // <= (number of expected CLI arguments)
        fprintf(stderr, "Usage: %s <input file>\n", argv[0]);
        return -1;
    }
    
    // TODO Your code goes here!
    printf("Input file: %s\n", argv[1]);
    return 0;
}

Compile with

g++ -o cli-onearg cli-onearg.cpp

Usage example:

$ ./cli-onearg  
Usage: ./cli-onearg <input file>
$ ./cli-onearg my-input-file.txt
Input file: my-input-file.txt

Problem:

You want to compile a C++ executable that uses zlib, but you see an error message like

/usr/bin/ld: cannot find -lzlib
collect2: error: ld returned 1 exit status

Solution:

Use -lz instead of -lzlib. The zlib library is named libz.so, not libzlib.so!

Example:

g++ -o ztest zpipe.c -lz