c++ how to read a cfg file more complex? - c++

Question

I recently changed from c# to c++.
Ive been watching some tutorials on how to read a config file. I am probably asking it wrong, what I mean is:
I am making a program, but it's for multiple users. Every user will have his prefence on what will be in a textfile.
Example:
In my textfile ("items.txt"), i have default "ints" and configurable "ints".
10=5
90=2
50=9
In c#, if i remember correctly, i read all lines and if the line started with for example "10=", i splitted the text so my line would only be the configurable int left and i can use that easly in my program, it was something like:
string[] lines = File.ReadAllLines(path);
string str;
foreach (string line in lines)
{
if (line.StartsWith("10"))
{
str = line.Split('=')[1];
//I have what i need (str);
}
}
I did that for everything that i needed. So, what's the best way to do this in c++?
ALSO: I need to get a specific line for them, so i can use them all later on in my program.
Thanks a lot in advance!

Answer 1

You have 3 things in your files:
lines, which we can read with getline
keys (the part of the line before =)
values (the part after the =).
We can split the key and value by finding the = character.
So, you get something like:
std::ifstream file(path);
if (!file) {
std::cerr << "unable to open " << path << std::endl;
return false;
}
for (std::string line; getline(file, line); ) {
auto pos = line.find('=');
if (pos == line.end()) {
std::cerr << "skipping invalid line '" << line << "'\n";
continue;
}
auto key = line.substr(0, pos);
auto val = line.substr(pos);
// do something with key,val
}
If you want to convert the key and value strings above into int, you can use a std::stringstream to do the conversion, or use std::strtol.

Answer 2

This code is a C++ equivalent of the C# code you posted. This code by itself will compile, of course you have to fill something in for path instead of an empty string. If you need this somewhere in a method instead of as a program on itself, abandon the int main() and return 0 of course. Reply to this if something is unclear. I tried to set it up the same way your C# code is structured, so you would understand everything.
#include <fstream>
#include <vector>
#include <string>
int main()
{
std::string path = "";
std::ifstream file(path); //open the file from path, might want to check file.fail()
std::vector<std::string> lines;
std::string line;
while (file >> line)
lines.push_back(line);
std::string str;
for (const std::string& line : lines)
{
if (line.substr(0, 2).compare("10") == 0)
{
size_t foundIdx = line.find('=');
str = line.substr(foundIdx + 1);
}
}
file.close();
return 0;
}

Answer 3

Well in C++ we now use std::regex
For your case - I'd use the following:
#include <iostream>
#include <string>
#include <regex>
#include <iterator>
int main()
{
// Simple config file
std::string rules = "10=5\n 90=2\n50=9";
std::regex single_rule_regex("[0-9]+\\=[0-9]+");
std::regex single_rule_part_regex("[0-9]+");
auto rules_begin = std::sregex_iterator(rules.begin(),
rules.end(),
single_rule_regex);
auto rules_end = std::sregex_iterator();
for (auto it = rules_begin; it != rules_end; ++it)
{
std::smatch rule_match = *it;
std::string rule_str = rule_match.str();
std::cout << rule_str << std::endl;
// key=value
auto parts = std::sregex_iterator(rule_str.begin(),
rule_str.end(),
single_rule_part_regex);
std::string key = parts->str();
std::string value = (++parts)->str();
std::cout << key << " has " << value << std::endl;
}
}
Output for the gives rules is:
10=5
10 has 5
90=2
90 has 2
50=9
50 has 9