Variable number of command line arguments in C++

Question

I have a program (myprogram) that takes a file for input and creates another file for output. The syntax is:

myprogram inputfile outputfile

It looks something like this:

int main ( int argc, char *argv[] )
{
...  
if ( argc != 3 ) {
    cout<<"wrong number of arguments"<< endl; return 1;}
   }
myInputFile = argv[1];
myOutputFile = argv[2];
...
}

So here I have argv[1] being the inputfile.I would now like to add some options to the syntax. So, for example, I would like myprogram -a inputfile outputfile as an option. The number of options will vary.

That means that the filenames will depend on the number of arguments.

What is the smartest way to do this in the code? I can see that one would just take the last two arguments as the filenames and then assume that the arguments before that are the options (so something like myInputFile = argv[argc - 1]. This doesn't quite feel right because I end up with a lot of if ... then statements.

So my question is: How does one best deal with a variable number of arguments?

If you are running your code on a Unix based system, take a look at getopt gnu.org/software/libc/manual/html_node/Getopt.html — vivekian2
– vivekian2, Commented Apr 15, 2014 at 18:03

Grzegorz · Accepted Answer · 2014-04-15 18:16:13Z

1

How about that in case you do not want to use libraries:

char* myInputFile = NULL ;
char* myOutputFile = NULL ;
bool  option_a = false ;
int   argi = 1 ;

while ( argi < argc )
{
   const char* args = argv[ argi++ ] ;

   switch ( *args )
   {
      case '-' : /* option/switch */
         if ( strcmp( args, "-a" ) == 0 )
            option_a = true ;
         else
         {
            std::cerr << "Unknow option " << args << "\n" ;
            exit( 1 ) ;
         }
         break ;

      default :
         if ( myInputFile == NULL )
            myInputFile = args ;
         else if ( myOutputFile == NULL )
            myOutputFile = args ;
         else
         {
            std::cerr << "Unexpected argument " << args << "\n" ;
            exit( 1 );
         }
         break ;
   }
}

At the end, when you process all your arguments you can do some final checking like, for example, testing if the input file and output file were set:

if ( myInputFile == NULL )
{
    std::cerr << "Input file missing!\n" ;
    exit( 1 ) ;
}
...

You could also put all your arguments into a structure instead of having them separately:

struct options_t
{
    char* myInputFile ;
    char* myOutputFile ;
    bool  option_a ;

    options_t() : myInputFile(), myOutputFile(), option_a() {}
} ;

This way you can pass all arguments down to your functions instead of listing them one by one.

Just a thought...

edited Apr 15, 2014 at 18:16

answered Apr 15, 2014 at 18:10

Grzegorz

3,3753 gold badges27 silver badges45 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Thomas Over a year ago

This looks like what I want. I will have to study it. Thanks.

Grzegorz Over a year ago

@Thomas - This is basic approach. Using a library are also great suggestions. The code above may not compile due to syntax errors. You are invited to edit my post in case you find them.

Thomas Over a year ago

Thanks. I think I see what you are saying here and it is definitely better than what I had been thinking. I will have to spend some more time of all this to see what works best though.

Guvante · Accepted Answer · 2014-04-15 18:02:22Z

1

The most common way is to write a special method for handling the command line arguments. You give it argc and argv and it returns a structured set of data values. e.g.

struct commands
{
    char* InputFile;
    char* OutputFile;
    bool WhateverAMeans;
};

This way your main loop has a sufficient list of things it requires and doesn't require tons of loops.

Typically your method would use a for loop to iterate over the arguments to fill in the commands.

Alternatively there exist helper classes that do this work for you, although I cannot recommend any having not done this in a long time.

answered Apr 15, 2014 at 18:02

Guvante

19.3k1 gold badge38 silver badges64 bronze badges

Comments

user3159253 · Accepted Answer · 2014-04-15 18:04:44Z

1

If you don't mind I'd suggest a library that deals w/ all these issues. Try Boost.Program Options. It might look too complex than you expected but it's powerful enough. Also there're a lot C libraries, like getopt, popt and many others

answered Apr 15, 2014 at 18:04

user3159253

17.6k3 gold badges36 silver badges61 bronze badges

Comments

Arkady · Accepted Answer · 2014-04-15 18:07:06Z

1

Look at boost::program_options, very useful and clean tool for that. I also used Poco::OptionSet, but it wasn't so clean for me, as boost. Both Poco and boost need to be build by you, but if you never used any of them - that is good chance to start, they both are awesome.

answered Apr 15, 2014 at 18:07

Arkady

2,2473 gold badges28 silver badges49 bronze badges

Collectives™ on Stack Overflow

Variable number of command line arguments in C++

4 Answers 4

3 Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related