C/C++

Minimal PugiXML file reader example

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

 

Posted by Uli Köhler in C/C++

How to decompress gzip files using zlib – a minimal example

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"

 

Posted by Uli Köhler in C/C++

C/C++ program template with one CLI argument

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

 

Posted by Uli Köhler in C/C++

How to fix /usr/bin/ld: cannot find -lzlib

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

 

Posted by Uli Köhler in C/C++

How to make strcmp/strncmp case-insensitive in C/C++

To make strcmp case-insensitive, use strcasecmp from #include <strings.h>strcasecmp can be used in exactly the same way as strcmp.

To make strncmp case-insensitive, use strncasecmp from #include <strings.h>strncasecmp can be used in exactly the same way as strncmp.

Note that both of these will not deal with unicode characters correctly, but will work just fine in most applications.

Posted by Uli Köhler in C/C++

boost::lexical_cast minimal example

#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
}

 

Posted by Uli Köhler in Boost, C/C++

Computing distance between gp_Pnt and gp_Ax1 in OpenCASCADE

OCCUtils provides convenience functions for computing the orthogonal direction to two directions:

#include <occutils/Axis.hxx>
using namespace OCCUtils;

gp_Ax1 axis = /* ... */;
gp_Pnt pnt = /* ... */;
double distance = Axis::Distance(axis, pnt);

Alternatively, you can also use Point::Distance() which internally just calls Axis::Distance() but might make your code more readable under some circumstances. Note that the argument order is inverted!

#include <occutils/Point.hxx>
using namespace OCCUtils;

gp_Ax1 axis = /* ... */;
gp_Pnt pnt = /* ... */;
double distance = Point::Distance(pnt, axis);

In case you can’t use OCCUtils, here’s the code to do it manually:

double distance = gp_Lin(axis).Distance(pnt);
Posted by Uli Köhler in C/C++, OpenCASCADE

How to check if gp_Ax1 contains gp_Pnt in OpenCASCADE

OCCUtils provides convenience functions for computing the orthogonal direction to two directions:

#include <occutils/Axis.hxx>
using namespace OCCUtils;

gp_Ax1 axis = /* ... */;
gp_Png point = /* ... */;
bool pointIsOnAxis = Axis::Contains(axis, point);

In case you can’t use OCCUtils, here’s the code to do it manually:

gp_Lin(axis).Contains(pnt, Precision::Confusion());
Posted by Uli Köhler in C/C++, OpenCASCADE

How to get gp_Dir orthogonal to two gp_Dirs in OpenCASCADE

OCCUtils provides convenience functions for computing the orthogonal direction to two directions:

#include <occutils/Direction.hxx>
using namespace OCCUtils;

gp_Dir dir1 = /* ... */;
gp_Dir dir2 = /* ... */;
gp_Dir orthogonalDirection = Direction::Orthogonal(dir1, dir2);

In case you can’t use OCCUtils, here’s the code to do it manually:

gp_Dir orthogonalDirection = dir1.Crossed(dir2);

The function used is called Crossed() since the mathematical operation being used is the cross product between the two direction vectors.

 

Posted by Uli Köhler in C/C++, OpenCASCADE

How to get midpoint/center between points in OpenCASCADE

OCCUtils provides convenience functions for getting the midpoint (also called the center) between 2 or more points:

#include <occutils/Point.hxx>
using namespace OCCUtils;

gp_Pnt p1 = /* ... */;
gp_Pnt p2 = /* ... */;
gp_Pnt midpointOfP1AndP2 = Point::Midpoint({p1, p2});

You can also call Point::Midpoint() with a std::vector<gp_Pnt>.

In case you can’t use OCCUtils, here’s the code to do it manually:

double x = 0.0, y = 0.0, z = 0.0;
for (const gp_Pnt &pnt : points) {
    x += pnt.X();
    y += pnt.Y();
    z += pnt.Z();
}
size_t size = points.size();
gp_Pnt midpoint(x / size, y / size, z / size);

 

 

Posted by Uli Köhler in C/C++, OpenCASCADE

How to compute volume of TopoDS_Shape / TopoDS_Solid in OpenCASCADE

OCCUtils provides a convenience function to do this:

#include <occutils/Shape.hxx>

using namespace OCCUtils;

TopoDS_Shape myShape = /* ... */;

double volume = Shape::Volume(myShape);

In case you need to do this without OCCUtils, use

GProp_GProps gprops;
BRepGProp::VolumeProperties(shape, gprops);
double volume = gprops.Mass();

 

Posted by Uli Köhler in C/C++, OpenCASCADE

Towards a docker-based build of C/C++ applications

Note: Based on this post I have published buildock on GitHub.

Many C/C++ programmers and project managers know the pain of creating a reproducible build environment for all developers: Works for me is a common meme not without a reason.

My approach is to dockerize not neccessarily the application itself but the build system, encapsulating both the specific compiler version and the system around it plus all required system-level libraries in a Docker image.

Take the obligatory Hello World in C++:

// main.cpp
#include <iostream>
using namespace std;

int main() {
    cout << "Hello, World!" << endl;
    return 0;
}

and the corresponding Makefile:

all:
        g++ -o helloworld main.cpp

How can we compile this simple project without installing a compiler and GNU make on the local computer (no cheating by using build servers allowed)?

Actually it’s rather simple:

docker run --user $(id -u):$(id -g) -v $PWD:/app -it ulikoehler/ubuntu-gcc-make make

Breaking it down:

  • docker run: Create a new docker container and run a command in it
  • --user $(id -u):$(id -g): This makes the docker container run with the current user’s ID and the current user’s group – both for preventing the compiler to create output files as root and to safeguard against some IT security risks. Also see How to run docker container as current user & group
  • -v $PWD:/app: Mount the current directory ($PWD) on /app in the container. Since the Dockerfile used to build the container contains the WORKDIR /app directive, whatever command we run in the container will by default be executed in the /app directory – and therefore in the current local directory on the host.
  • -it runs the container in interactive mode, i.e. keypressed are passed down to the command you are running. Also, this means that our command will only finish when the container has finished executing.
  • ulikoehler/ubuntu-gcc-make: This is the image we’re using for that example. It’s nothing more than an ubuntu:18.04 base image with build-essentials and make installed and WORKDIR set to /app
  • make: This is the command we’ll run in the container. You can use any command here, even no command is possible (in which case the container’s default command will be used – in case of ulikoehler/ubuntu-gcc-make that is CMD [ "/usr/bin/make" ])

Here’s the complete Dockerfile used to generate ulikoehler/ubuntu-gcc-make:

FROM ubuntu:18.04
RUN apt update && apt -y install build-essential make && rm -rf /var/lib/apt/lists/*
WORKDIR /app
CMD [ "/usr/bin/make" ]
Posted by Uli Köhler in Build systems, C/C++, Container, Docker

How to compute surface normal in OpenCASCADE

OCCUtils provides a convenient utility to compute the Normal of a surface (represented by a GeomAdaptor_Surface) in OpenCASCADE:

#include <occutils/Surface.hxx>

using namespace OCCUtils;

GeomAdaptor_Surface surf = /* ... */;

gp_Ax1 surfaceNormal = Surface::Normal(surf);

// ...or just get the direction
gp_Dir surfaceDirection = Surface::NormalDirection(surf);

This function computes the normal vector at specific U/V coordinates which default to (0,0). You can also give custom U/V coordinates:

gp_Ax1 normal = Surface::Normal(surf, 1.0 /* u */, -4.5 /* v */);

In case you can’t use OCCUtils and you need to do it manually, here’s how you can do it:

#include <GeomLProp_SLProps.hxx>

GeomLProp_SLProps props(surf.Surface(), u, v, 1 /* max 1 derivation */, precision);
gp_Ax1 axis(props.Value(), props.Normal());

props.Value() returns the point on the surface at the given U/V coordinates.

Posted by Uli Köhler in C/C++, OpenCASCADE

How to fix C++ ‘fatal error: stringstream: No such file or directory’

Problem:

Your C++ code contains a line like

#include <stringstream>

since you want to use std::stringstream, but your compiler gives you this error message:

main.cpp:2:10: fatal error: stringstream: No such file or directory
 #include <stringstream>
          ^~~~~~~~~~~~~~
compilation terminated.

Solution:

The header is called sstream, not stringstream! Use this #include directive instead:

#include <sstream>
Posted by Uli Köhler in C/C++, GCC errors

How to parse hex strings in C++ using std::stringstream

This function converts any hex string like 55 to its equivalent decimal value:

#include <sstream>
#include <int>

unsigned int hexToDec(const std::string& str) {
    unsigned int ret;
    std::stringstream ss;
    ss << std::hex << str;
    ss >> ret;
    return ret;
}

e.g.

hexToDec("55"); // returns 85

Note that while the code is somewhat concise, it might be neither the most performant nor the most concise option especially for developers that don’t really know std::stringstream.

Posted by Uli Köhler in C/C++

How to get first three characters of std::string in C++

To get the first three characters of myString use myString.substr() like this:

std::string firstTwo = myString.substr(0, 3);

The first argument ob substr() is the offset (i.e. how many characters to skip) which is 0 in this case since we don’t want to skip any characters.

The second argument of substr() is the number of characters to get. Since we want to get three characters, this is 3.

Posted by Uli Köhler in C/C++

How to get first two characters of std::string in C++

To get the first two characters of myString use myString.substr() like this:

std::string firstTwo = myString.substr(0, 2);

The first argument ob substr() is the offset (i.e. how many characters to skip) which is 0 in this case since we don’t want to skip any characters.

The second argument of substr() is the number of characters to get. Since we want to get two characters, this is 2.

Posted by Uli Köhler in C/C++

C++ std::string equivalent to Python’s rstrip

In Python you can do

"test ".rstrip()

whereas in C++ the easiest way is to use the the Boost String Algorithms library:

#include <boost/algorithm/string/trim.hpp>

using namespace boost::algorithm;

string to_trim = "test ";
string trimmed = trim_right_copy(to_trim);

_copy in trim_right_copy() means that the original string is not modified.

Full example:

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

using namespace std;
using namespace boost::algorithm;

int main() {
    string to_trim = "test ";
    string trimmed = trim_right_copy(to_trim);
    cout << trimmed << endl; // Prints 'test'
}

In case you want to trim a custom set of characters, use trim_right_copy_if() together with is_any_of():

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

using namespace std;
using namespace boost::algorithm;

int main() {
    string to_trim = "testXbbca";
    string trimmed = trim_right_copy_if(to_trim, is_any_of("abc"));
    cout << trimmed << endl; // Prints 'testX'
}

 

Posted by Uli Köhler in C/C++

C++ std::string equivalent to Python’s lstrip

In Python you can do

" test".lstrip()

whereas in C++ the easiest way is to use the the Boost String Algorithms library:

#include <boost/algorithm/string/trim.hpp>

using namespace boost::algorithm;

string to_trim = " test";
string trimmed = trim_left_copy(to_trim);

_copy in trim_left_copy() means that the original string is not modified.

Full example:

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

using namespace std;
using namespace boost::algorithm;

int main() {
    string to_trim = " test";
    string trimmed = trim_left_copy(to_trim);
    cout << trimmed << endl; // Prints 'test'
}

In case you want to trim a custom set of characters, use trim_left_copy_if() together with is_any_of():

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

using namespace std;
using namespace boost::algorithm;

int main() {
    string to_trim = "bbcaXtest";
    string trimmed = trim_left_copy_if(to_trim, is_any_of("abc"));
    cout << trimmed << endl; // Prints 'Xtest'
}

 

Posted by Uli Köhler in C/C++