Creating a simple configuration file and parser in C++

Question

I am trying to create a simple configuration file that looks like this

url = http://mysite.com
file = main.exe
true = 0

when the program ru

Answer 1

I've searched config parsing libraries for my project recently and found these libraries:

• http://www.hyperrealm.com/libconfig/ - small but powerfull and easy-to-use library, examples of using are in the source package and here
• http://pocoproject.org/docs/Poco.Util.PropertyFileConfiguration.html - a part of pocoproject. You can use abstraction in your code and behind that use various supported implementations (ini files, Xml configs, even environment variables)

Answer 2

A naive approach could look like this:

#include <map>
#include <sstream>
#include <stdexcept>
#include <string>

std::map<std::string, std::string> options; // global?

void parse(std::istream & cfgfile)
{
    for (std::string line; std::getline(cfgfile, line); )
    {
        std::istringstream iss(line);
        std::string id, eq, val;

        bool error = false;

        if (!(iss >> id))
        {
            error = true;
        }
        else if (id[0] == '#')
        {
            continue;
        }
        else if (!(iss >> eq >> val >> std::ws) || eq != "=" || iss.get() != EOF)
        {
            error = true;
        }

        if (error)
        {
            // do something appropriate: throw, skip, warn, etc.
        }
        else
        {
            options[id] = val;
        }
    }
}

Now you can access each option value from the global options map anywhere in your program. If you want castability, you could make the mapped type a boost::variant.

Answer 3

Why not trying something simple and human-readable, like JSON (or XML) ?

There are many pre-made open-source implementations of JSON (or XML) for C++ - I would use one of them.

And if you want something more "binary" - try BJSON or BSON :)

Answer 4

In general, it's easiest to parse such typical config files in two stages: first read the lines, and then parse those one by one.
In C++, lines can be read from a stream using std::getline(). While by default it will read up to the next '\n' (which it will consume, but not return), you can pass it some other delimiter, too, which makes it a good candidate for reading up-to-some-char, like = in your example.

For simplicity, the following presumes that the = are not surrounded by whitespace. If you want to allow whitespaces at these positions, you will have to strategically place is >> std::ws before reading the value and remove trailing whitespaces from the keys. However, IMO the little added flexibility in the syntax is not worth the hassle for a config file reader.

const char config[] = "url=http://example.com\n"
                      "file=main.exe\n"
                      "true=0";

std::istringstream is_file(config);

std::string line;
while( std::getline(is_file, line) )
{
  std::istringstream is_line(line);
  std::string key;
  if( std::getline(is_line, key, '=') )
  {
    std::string value;
    if( std::getline(is_line, value) ) 
      store_line(key, value);
  }
}

(Adding error handling is left as an exercise to the reader.)

Answer 5

libconfig is very easy, and what's better, it uses a pseudo json notation for better readability.

Easy to install on Ubuntu: sudo apt-get install libconfig++8-dev

and link: -lconfig++

Answer 6

SimpleConfigFile is a library that does exactly what you require and it is very simple to use.

# File file.cfg
url = http://example.com
file = main.exe
true = 0 

The following program reads the previous configuration file:

#include<iostream>
#include<string>
#include<vector>
#include "config_file.h"

int main(void)
{
    // Variables that we want to read from the config file
    std::string url, file;
    bool true_false;

    // Names for the variables in the config file. They can be different from the actual variable names.
    std::vector<std::string> ln = {"url","file","true"};

    // Open the config file for reading
    std::ifstream f_in("file.cfg");

    CFG::ReadFile(f_in, ln, url, file, true_false);
    f_in.close();

    std::cout << "url: " << url << std::endl;
    std::cout << "file: " << file << std::endl;
    std::cout << "true: " << true_false << std::endl;

    return 0;
}

The function CFG::ReadFile uses variadic templates. This way, you can pass the variables you want to read and the corresponding type is used for reading the data in the appropriate way.