problem with opening the input file with diff script (in c++)

Question

On my assignment , I'm asked to compare the output text file with diff .

my code can't open the input file when I use operator < (last line of diff script).

how should I declare the inputfile in main? what does the last line in script.sh file do ?

script.sh file:

unzip A4-"$1".zip
(cd A4-"$1"/; make)
cp A4-"$1"/Scheduler.out .
echo "##### DIFF #####"
./Scheduler.out < sample.in | diff sample.out -

---

int main (int argc , char* argv[]){
    fstream inputFile (argv[1],fstream::in);
    fstream outputFile ("outputFile.out",fstream::out);
    /*...*/
}

Answer 1

One common pattern for commands is to either read from a file supplied as an argument, or, when an argument is missing, read from std::cin. Another common practise for commands in many environments is to accept - as an indicator that you want to read from std::cin. It could be implemented like this:

#include <iostream>
#include <fstream>
#include <vector>

int streamer(std::istream& is) {
    std::ofstream out("outputFile.out");
    if(out) {
        /*...*/
        return 0; // if all's well
    } else
        return 1; // open failed
}

int cppmain(const std::vector<std::string>& args) {         
    if(args.size() && args[0] != "-") { // or do something cleaver to read from all "args".
        std::ifstream is(args[0]);
        if(is)
            return streamer(is);   // read from a file
        else
            return 1;              // open failed
    } else {
        return streamer(std::cin); // read from stdin
    }
}

int main(int argc, char* argv[]) {
    return cppmain({argv + 1, argv + argc});
}

Remove the && args[0] != "-" part if you get in an argument with someone who likes naming files -. I put it there just to show the option.

The last line:

./Scheduler.out < sample.in | diff sample.out -

Broken down:

./Scheduler.out < sample.in

The shell opens sample.in for reading and executes ./Scheduler.out. std::in in .\Schduler.out, that is usually connected to a terminal, is replaced by the open sample.in handle.

... | diff sample.out -

std::cout from the command ... replaces std::cin in diff by the shell. The - is an argument that is interpreted by diff and means that it will diff one file with the input it gets from std::cin, much like what I did in cppmain in my example.