C++ - Find and replace in text file (standard system libraries) - c++

Question

I am looking for some advice.
My situation:
Application works with text local file.
In file are somewhere tags like this: correct = "TEXT". Unfortunatelly, there can be unlimited spaces between correct, = and "TEXT".
Obtained text is testing in function and may be replaced (the change must be stored in the file). correct = "CORRECT_TEXT"
My current theoretical approach:
With ofstream -- read by line to string.
Find tag and make change in string.
Save strings as lines to the file.
Is there some simplify way (with iterators?) in C++ with using standard system libraries only (unix).
Thank you for your ideas.

Answer 1

Here is a possible solution that uses:
std::getline()
std::copy()
istream_iterator
ostream_iterator
vector
Example:
#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
#include <string>
#include <vector>
struct modified_line
{
std::string value;
operator std::string() const { return value; }
};
std::istream& operator>>(std::istream& a_in, modified_line& a_line)
{
std::string local_line;
if (std::getline(a_in, local_line))
{
// Modify 'local_line' if necessary
// and then assign to argument.
//
a_line.value = local_line;
}
return a_in;
}
int main()
{
std::ifstream in("file.txt");
if (in.is_open())
{
// Load into a vector, modifying as they are read.
//
std::vector<std::string> modified_lines;
std::copy(std::istream_iterator<modified_line>(in),
std::istream_iterator<modified_line>(),
std::back_inserter(modified_lines));
in.close();
// Overwrite.
std::ofstream out("file.txt");
if (out.is_open())
{
std::copy(modified_lines.begin(),
modified_lines.end(),
std::ostream_iterator<std::string>(out, "\n"));
}
}
return 0;
}
I am not sure exactly what the manipulation of the lines should be but you could use:
std::string::find() and std::string::substr()
boost::split()
EDIT:
To avoid storing every line in memory at once the initial copy() can changed to write to an alternative file, followed by a file rename():
std::ifstream in("file.txt");
std::ofstream out("file.txt.tmp");
if (in.is_open() && out.open())
{
std::copy(std::istream_iterator<modified_line>(in),
std::istream_iterator<modified_line>(),
std::ostream_iterator<std::string>(out, "\n"));
// close for rename.
in.close();
out.close();
// #include <cstdio>
if (0 != std::rename("file.txt.tmp", "file.txt"))
{
// Handle failure.
}
}

Answer 2

You can split the task into tiny pieces and figure out how to do each in C++:
open a file as an input stream
open temporary file as an output stream
read a line from a stream
write a line to a stream
match a line to given pattern
replace text in a line
rename a file
Note: you don't need to store in memory more than one line at a time in this case.

Answer 3

It looks a lot like an 'INI file' syntax. You can search for it and you'll have a big load of examples. However, few of them will actually use C++ stdlib.
Here's some advices. (n.b. I assume that every lines you'll need to replace are using the syntax: <parameter> = "<value_text>")
Use the std::string::find method to locate the '=' character.
Use the std::string::substr method to split the string into different chunks.
You'll need to create a trim algorithm to remove every blank characters in front or back of a string. (It can be done with std functions)
With all that you'll then be able to split the string and isolate the parts to compare them do the needed modifications.
have fun !

Answer 4

Are you sure you need to do this within C++? Since you are on Unix, you can call sed which would do this easily with a command such as:
cat oldfile | sed 's/\(correct *= *\)\"TEXT\"/\1\"CORRECT_TEXT\"/' > newfile
You can call unix commands from within C++ if you have to (for example with system("command") from <cstdlib>.