Creating a simple configuration file and parser in

问题:

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 runs, I would like it to load the configuration settings into the programs variables listed below.

string url, file;
bool true_false;

I have done some research and this link seemed to help (nucleon's post) but I can't seem to get it to work and it is too complicated to understand on my part. Is there a simple way of doing this? I can load the file using ifstream but that is as far as I can get on my own. Thanks!

回答1:

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.)

回答2:

As others have pointed out, it will probably be less work to make use of an existing configuration-file parser library rather than re-invent the wheel.

For example, if you decide to use the Config4Cpp library (which I maintain), then your configuration file syntax will be slightly different (put double quotes around values and terminate assignment statements with a semicolon) as shown in the example below:

# File: someFile.cfg
url = "http://mysite.com";
file = "main.exe";
true_false = "true";

The following program parses the above configuration file, copies the desired values into variables and prints them:

#include <config4cpp/Configuration.h>
#include <iostream>
using namespace config4cpp;
using namespace std;

int main(int argc, char ** argv)
{
    Configuration *  cfg = Configuration::create();
    const char *     scope = "";
    const char *     configFile = "someFile.cfg";
    const char *     url;
    const char *     file;
    bool             true_false;

    try {
        cfg->parse(configFile);
        url        = cfg->lookupString(scope, "url");
        file       = cfg->lookupString(scope, "file");
        true_false = cfg->lookupBoolean(scope, "true_false");
    } catch(const ConfigurationException & ex) {
        cerr << ex.c_str() << endl;
        cfg->destroy();
        return 1;
    }
    cout << "url=" << url << "; file=" << file
         << "; true_false=" << true_false
         << endl;
    cfg->destroy();
    return 0;
}

The Config4Cpp website provides comprehensive documentation, but reading just Chapters 2 and 3 of the "Getting Started Guide" should be more than sufficient for your needs.

回答3:

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.

回答4:

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++

回答5:

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 :)

回答6:

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)

回答7:

How about formatting your configuration as JSON, and using a library like jsoncpp?

e.g.

{"url": "http://mysite dot com",
"file": "main.exe",
"true": 0}

You can then read it into named variables, or even store it all in a std::map, etc. The latter means you can add options without having to change and recompile your configuration parser.

回答8:

Here is a simple work around for white space between the '=' sign and the data, in the config file. Assign to the istringstream from the location after the '=' sign and when reading from it, any leading white space is ignored.

Note: while using an istringstream in a loop, make sure you call clear() before assigning a new string to it.

//config.txt
//Input name = image1.png
//Num. of rows = 100
//Num. of cols = 150

std::string ipName;
int nR, nC;

std::ifstream fin("config.txt");
std::string line;
std::istringstream sin;

while (std::getline(fin, line)) {
 sin.str(line.substr(line.find("=")+1));
 if (line.find("Input name") != std::string::npos) {
  std::cout<<"Input name "<<sin.str()<<std::endl;
  sin >> ipName;
 }
 else if (line.find("Num. of rows") != std::string::npos) {
  sin >> nR;
 }
 else if (line.find("Num. of cols") != std::string::npos) {
  sin >> nC;
 }
 sin.clear();
}

回答9:

I was looking for something that worked like the python module ConfigParser and found this: https://github.com/jtilly/inih

This is a header only C++ version of inih.

inih (INI Not Invented Here) is a simple .INI file parser written in C. It's only a couple of pages of code, and it was designed to be small and simple, so it's good for embedded systems. It's also more or less compatible with Python's ConfigParser style of .INI files, including RFC 822-style multi-line syntax and name: value entries.