Simple read from text file algorithm doesn't work - c++

Question

I have a text file that contains keys and values like this:
keyOne=1
keyTwo=734
keyThree=22.3
keyFour=5
The keys are just lower-case and upper-case letters like in my example. The values are either integers or floats. Each key and value is separated by an equals sign (=). Now I want to read the values into variables I have in my program.
This is the code I have tried to read the values:
(I omitted the part where I store the values in my program's variables, and just print them out now for demonstration.)
std::fstream file(optionsFile, std::fstream::in);
if (file.good()) {
int begin;
int end;
std::string line;
while(std::getline(file, line)) {
// find the position of the value in the line
for (unsigned int i = 0; i < line.length(); i++) {
if (line.at(i) == '=') {
begin = i + 1;
end = line.length();
break;
}
}
// build the string... it starts at <begin> and ends at <end>
const char *string = "";
for (int i = begin; i < end; i++) {
string += line.at(i);
}
// only gibberish is printed in the following line :(
std::cout << "string=" << string << std::endl;
}
}
I don't understand why it won't print the value.. instead only weird stuff or even nothing is printed
Please help this broke my spirit so hard :(

Answer 1

You are using C-style strings (char arrays) without properly allocated memory, and you are just manipulating with the pointer, so you are not appending characters into your string:
// build the string... it starts at <begin> and ends at <end>
const char *string = "";
for (int i = begin; i < end; i++) {
string += line.at(i);
}
Use std::string instead:
/// build the string... it starts at <begin> and ends at <end>
std::string str;
for (int i = begin; i < end; i++) {
str += line.at(i);
}
Or allocate memory by hand, use the proper indexing, terminate the string with '\0' character and don't forget to delete the string after you don't need it anymore:
char *string = new char[end - begin + 1];
int j = 0;
for (int i = begin; i < end; i++) {
string[j++] = line.at(i);
}
// Don't forget to end the string!
string[j] = '\0';
// Don't forget to delete string afterwards!
delete [] string;
So, just use std::string.
Edit Why did you mix C strings and std::string in the first place?

Answer 2

As was already mentioned, native string types in c/c++ do not support straightforward concatenation since they are essentially pointers to some preallocated memory. You should always use std::string when a string is supposed to be mutable.
Btw, think about the following refactoring:
void process_option (const std::string& a_key, const std::string& a_value)
{
std::cout << a_key << " <-- " << a_value << std::endl;
}
void read_options (std::istream& a_in, const char* a_source)
{
int line_n = 0;
std::string line;
while (std::getline(a_in, line))
{
++ line_n;
std::string::size_type p = line. find('=');
if (p == line. npos)
{
// invalid_entry(a_source, line_n);
continue;
}
process_option(
line. substr(0, p), // key
line. substr(p + 1, line. find_first_of("\t\r\n", p + 1)) // value
);
}
}
void read_options (const char* a_filename)
{
std::ifstream file(a_filename);
if (! file)
{
// read_error(a_filename);
return;
}
read_options(file, a_filename);
file. close();
}
void read_options (const std::string& a_filename)
{
read_options(a_filename. c_str());
}