Creating a simple configuration file and parser in C++
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.)
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.
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++