C++ pipe to modify input file with multiple programs - c++

Question

I've been doing this task for +3 weeks now and I'm sure it's a piece of cake to somebody out there, so I'm just going to ask if somebody could write me some example code with these requirements:
Task is in C++ and the main point is to become familiar with pipes
It should be called like this (I think) from cmd: cat inputfile.cpp | ./program01 ./program02. What I'm trying to say is (I think): "Do modifications to file inputfile.cpp using programs program01 and program02".
Using pipes, firstly program01 removes all occurances of something in the inputfile.cpp (for example all empty rows). After all empty rows are removed, program02 should remove all occurances of something else (comments for example).
Does my ask make any sense? I mean, are pipes even meant to be used that way (first run other program then another)?
Can I possibly run multiple files through multiple programs for example like cat input1.cpp input2.cpp input3.cpp | ./program01 ./program02 ./program03.
I've written a bunch of programs that does various things to a file, but that is not the main point of the task. The main point is the "piping" part but I really, really just don't get it.
Any guidance is appreciated (some code below).
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include "programs.h"
int main(int argc, char** argv)
{
int fd[2];
pid_t pid;
int result;
result = pipe(fd);
if(result < 0)
{
perror("pipe error");
exit(1);
}
pid = fork();
if(pid < 0)
{
perror("fork error");
exit(2);
}
// Child
if(pid == 0)
{
while(1)
{
// I guess I should do some piping-magic here?
}
exit(0);
}
// Parent
else
{
while(1)
{
}
exit(0);
}
}

Answer 1

I think you're confusing the bash pipe command with the pipes to obtain IPC.
In bash the pipe doesn't pass the arguments, but redirect the stdout of a command to the next command as stdin. So in your c++ program you should read from stdin and write to stdout (relevant stackoverflow question).
You can do something like
cat file | ./program1 > file
to chain programs
cat file | ./program1 | ./program2 > file

Answer 2

To process data from the shell's pipe all you have to do is read in data from std::cin and output the results to std::cout. The shell manages the actual pipes.
Here is a program that does nothing. It simple passes the data from the incoming "pipe" to the outgoing "pipe":
Do nothing using "pipes":
#include <iostream>
int main()
{
char c;
while(std::cin.get(c))
std::cout.put(c);
}
A program to remove every blank line:
#include <iostream>
#include <string>
int main()
{
std::string line;
while(std::getline(std::cin, line))
{
if(line.empty()) // skip empty lines
continue;
// otherwise send them out
std::cout << line << '\n';
}
}