c++ - How to handle user specified string formatting?

BrianK · 02-26-2008, 04:09 PM

I want a user to be able to give me a string:

foo.%04d.bar

such that when event '100' comes around, that string turns into:

foo.0100.bar

... and when event 597 comes around, we get

foo.0597.bar

... and so on

Is there a neat way of doing this without parsing the string char by char to find a '%', a 'd' and then figuring what's in between them? I seem to remember something like this existing, but I can't seem to remember how its done.

Thanks

BrianK · 02-26-2008, 04:21 PM

argh. Found the answer - vprintf

Code:

#include <stdio.h>
#include <stdarg.h>

using namespace std;
void WriteFormatted (char * format, ...)
{
  va_list args;
  va_start (args, format);
  vprintf (format, args);
  va_end (args);
}

int main ()
{
  char* foo="Hello %s, %03d.\n";
  WriteFormatted ("Call with %d variable argument.\n",1);
  WriteFormatted ("Call with %d variable %s.\n",2,"arguments");
  WriteFormatted (foo,"there",7);

   return 0;
}

outputs:

Code:

Call with 1 variable argument.
Call with 2 variable arguments.
Hello there, 007.

If there's a better way, I'll take it. Until then, this works.

(example is from cplusplus.com)

dmail · 02-26-2008, 04:46 PM

Code:

#include <string>
#include <iostream>
#include <sstream>
#include <iomanip>
struct User_string
{
	User_string(std::string const& prefix, std::string const& postfix,const& int p):
		m_prefix(prefix),m_postfix(postfix),m_precision(p){}

	std::string create(int const& i)
	{
		std::stringstream s;
		s <<m_prefix <<std::setfill('0')<<std::setw(m_precision)<<i <<m_postfix;
		return s.str();
	}
private:
	std::string m_prefix;
	std::string m_postfix;
	int m_precision;
};

int main(int argc, char *argv[]) 
{
 	User_string s("foo.",".bar",4);
	for(int i = 0; i< 1000; ++i)
		std::cout <<s.create(i) <<std::endl;
	return 0;
}

ta0kira · 02-26-2008, 05:05 PM

Quote:

Originally Posted by BrianK

argh. Found the answer - vprintf

You should still check to make sure that you have enough arguments to match the number of %s so you don't overrun the stack. Also, you need to parse out %n because those can be used to hack your program, and maybe %m since it won't take an argument.
http://en.wikipedia.org/wiki/Format_...ulnerabilities

ta0kira

BrianK · 02-26-2008, 05:55 PM

Quote:

Originally Posted by dmail

some code

no offense, but where does the user give me formatting instructions and how does this differ from printf? For the record, I get a single string from the user & am trying to avoid parsing that string by hand. Maybe I'm just missing something in your example.

Quote:

Originally Posted by ta0kira

You should still check to make sure that you have enough arguments to match the number of %s so you don't overrun the stack. Also, you need to parse out %n because those can be used to hack your program, and maybe %m since it won't take an argument.
http://en.wikipedia.org/wiki/Format_...ulnerabilities

Thanks for the tip.

dmail · 02-27-2008, 08:14 AM

Quote:

Originally Posted by BrianK

no offense, but where does the user give me formatting instructions

They pass you a User_string or whatever else you want to call it.

Quote:

and how does this differ from printf?

It is type safe and does not wander into the non type safe world of the dreaded va_args.

Quote:

For the record, I get a single string from the user & am trying to avoid parsing that string by hand.

This is not what you originally wrote, it sounded like you were at the design stage.

Quote:

I want a user to be able to give me a string: