CODEWITHSUNDEEP
c++
Josh Glover
Josh Glover

Reputation: 26150

What is the C++ idiom to replace snprintf(3)?

I have some C++ code which needs to generate an error message when parsing a certain file header fails. In this case, I need to ensure that a certain 4 byte field in the header is "OggS", and if it is not, return an error message like "invalid capture_pattern: 'FooB'; expecting 'OggS'". My code looks something like this:

const string OggPage::parseHeader(void) {
  read(fd, capture_pattern, sizeof(capture_pattern)); // error handling omitted
  if (strncmp(capture_pattern, CAPTURE_PATTERN, sizeof(capture_pattern)) != 0) {
    char err[256];
    snprintf(err, sizeof(err), "Failed to read %d bytes from file descriptor %d: %s\n", sizeof(capture_pattern), fd, err);
    return err;
  }

  return "Everything was A-OK!";
}

What is the standard C++ idiom for building a string from other datatypes? I'm not wedded to the printf(3)-style format here, so feel free to suggest anything that works.

Upvotes: 4

Views: 7405

Answers (5)

beduin
beduin

Reputation: 8253

You can use stringstream or ostringstreamfrom C++ Standard library. If further stream will not be used to read values from it (e.g. it's istream part won't be used) ostringstream is more suitable.

#include <sstream>
#include <string>
#include <iostream>

int main() {
 std::stringstream str_stream;
 int number = 5;

 str_stream << " String with number " << number << std::endl;;

 std::string str = str_stream.str();
 std::cout << str; 
}

Upvotes: 5

ulidtko
ulidtko

Reputation: 15570

C++ way of doing this is std::stringstream.

std::stringstream err;
err << "Failed to read " << sizeof(capture_pattern)
                         << " bytes from fd " << fd << std::endl;
return err.str();

Upvotes: 1

Alexandre C.
Alexandre C.

Reputation: 56976

Using string functions from cstring is definitely fine for reading. They're fast, convenient and not verbose.

For building your error messages, you can use stringstream, operators related to the string class, boost::format or, as you put it, snprintf.

You have also boost::lexical_cast for simple things:

string message = "Failed to read " + lexical_cast<string>(n) +
   " bytes from the descriptor " + lexical_cast<string>(fd) + ".";

I recommend having a look at this Gotw for a sane point of view.

I'd recommend boost::format if you have a lot of formatting to do. Use lexical_cast for simple isolated things, and use stringstream if you have requirements which makes you need them (eg. custom operator<<, cannot use boost, etc).

To be honest, snprintf is really fine.

Upvotes: 1

Gunther Piez
Gunther Piez

Reputation: 30449

You create a stringstream, write what you want using the '<<' operator and then get the string out of stream with the member function .str().

#include <sstream>
#include <string>

using namespace std;

stringstream err;
err << "Failed to read " << sizeof(capture_pattern) << " bytes from file descriptor: " << fd << endl;
string outstr = err.str();

Upvotes: 0

Stack Overflow is garbage
Stack Overflow is garbage

Reputation: 248129

Just as a note, please don't suggest replacements for the actual reading--there is a reason that I want to use the C standard library for I/O here. :)

You can't really ask for "idiomatic" ways to do something in C++, and then say "but I want to stick to the C standard library...

Anyway, you have three options. I don't believe any of them is idiomatic (because that would imply some kind of consensus about this being the best way):

  • stick with the C APIs you're already using
  • Use the the C++ std::stringstream, which is part the iostreams section of the standard library
  • use Boost.Format

The latter has the downside that it relies on a third-party library, but the advantage that it gives you printf-like syntax, in a typesafe and extensible manner, which interoperates cleanly with C++ streams.

Upvotes: 1

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