Extract a substring between delimeters in C++

Question

I have to get the substring "Debian GNU/Linux 8 (jessie)" that is between two quotes, like:

PRETTY_NAME="Debian GNU/Linux 8 (jessie)"

But I don't know how to I can't manage to do that.

The best code I actually have:

std::ifstream osVersion("/usr/lib/os-release");
std::string os_str;
if (osVersion.is_open())
    getline(osVersion, os_str);
osVersion.close();
this->os = os_str.substr(os_str.find("\""), os_str.find("\""));

It gives an output like that:

"Debian GNU/

Answer 1

The problem is when you do

this->os = os_str.substr(os_str.find("\""), os_str.find("\""));

find is finding the same " in each call. That means you are going to start your substring at the first " and then it will be N characters long where N is the position of the first " .

You can fix this by capturing the posistion of the first " and using that to get the next one like

std::size_t pos = os_str.find("\"");
this->os = os_str.substr(pos + 1, os_str.find("\"", pos + 1) - pos - 1);

Do note though that in order to make the code bullet proof you should be capturing both positions and making sure the do not equal std::string::npos If either of them do you have an improperly formatted sting and the above method will implode.

Answer 2

std::find is easier to use here:

std::string::iterator begin = std::find(std::begin(os_str), std::end(os_str), '"');
if (begin != std::end(os_str)) {
    ++begin;
    std::string::iterator end = std::find(begin, std::end(os_str), '"');
    this->os = std::string(begin, end);
}