C++11 is Almost Here for Real!

This entry is the first under a slightly expanded charter for this blog.  Until now, I've restricted blog entries to announcements about my professional activities.  Hereafter, entries will simply be related to my professional activities and interests. The difference is subtle, and, in all likelihood, you will rarely notice it.

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At the beginning of my Overview of C++11, I show a simple program to compute the most common words in a set of input files.  I write the program once using "old" C++ (i.e., standard C++98/03), then again using features from C++11. 

In 2009, when I first published the C++11 program (at that time, what became C++11 was still known as C++0x), there was no compiler that could come anywhere near compiling it. Testing the code required replacing standard C++11 library components with similar components available in TR1 or from Boost or Just Software Solutions, and language features like auto, range-based for loops, lambda expressions, and template aliases had to be replaced with typically clumsier C++98/03 constructs that were more or less equivalent in meaning.

This week I tested my simple C++11 sample program with Stephan T. Lavavej's excellent  distribution of gcc 4.7 for Windows as well as Microsoft's VC11 beta.  gcc 4.7 has lots of support for C++11, but the concurrency API still seems to be largely missing, at least for Windows, so my sample program doesn't get very far with that compiler.  [Update 6 April 2012: As noted in the comments below, when invoked in the proper manner on the proper platform, gcc 4.7 compiles and runs my program without modification!]

 The situation with the VC11 beta is a lot better.  Only two lines have to be changed.  The template alias

  using WordCountMapType = std::unordered_map<std::string, std::size_t>;

needs to be replaced by its typedef equivalent:

  typedef std::unordered_map<std::string, std::size_t> WordCountMapType;

And the z length specifier in this call to printf,

  std::printf("  %-10s%10zu\n", (*it)->first.c_str(), (*it)->second);

needs to be replaced with its VC++ equivalent, I:

  std::printf("  %-10s%10Iu\n", (*it)->first.c_str(), (*it)->second);

Other than that, the demonstration program I wrote three years ago (which, in fairness to compiler writers, was two and a half years before the C++11 standard was ratified) compiles cleanly with VC11.

If you have access to a compiler that compiles my program without modification, please let me know! The program itself is below.  You can see a more colorful version of it, along with some commentary, and an example invocation and the corresponding output, on slides 13-15 of the free sample of my C++11 training materials.

Scott

#include <cstdio>
#include <iostream>
#include <iterator>
#include <string>
#include <fstream>
#include <algorithm>
#include <vector>
#include <unordered_map>  
#include <future>

using WordCountMapType = std::unordered_map<std::string, std::size_t>;

WordCountMapType wordsInFile(const char * const fileName)   // for each word
{                                                           // in file, return
  std::ifstream file(fileName);                             // # of
  WordCountMapType wordCounts;                              // occurrences

  for (std::string word; file >> word; ) {  
    ++wordCounts[word];
  }
  return wordCounts;
}

template<typename MapIt>                                            // print n most  
void showCommonWords(MapIt begin, MapIt end, const std::size_t n)   // common words
{                                                                   // in [begin, end)
  // typedef std::vector<MapIt> TempContainerType;
  // typedef typename TempContainerType::iterator IterType;
  std::vector<MapIt> wordIters;
  wordIters.reserve(std::distance(begin, end));
  for (auto i = begin; i != end; ++i) wordIters.push_back(i);

  auto sortedRangeEnd = wordIters.begin() + n;

  std::partial_sort(wordIters.begin(), sortedRangeEnd, wordIters.end(),
                    [](MapIt it1, MapIt it2){ return it1->second > it2->second; });

  for (auto it = wordIters.cbegin(); 
       it != sortedRangeEnd; 
       ++it) {
    std::printf("  %-10s%10zu\n", (*it)->first.c_str(), (*it)->second);
  }
}

int main(int argc, const char** argv)   // take list of file names on command line,
{                                       // print 20 most common words within;
                                        // process files concurrently
  std::vector<std::future<WordCountMapType>> futures;

  for (int argNum = 1; argNum < argc; ++argNum) {
    futures.push_back(std::async([=]{ return wordsInFile(argv[argNum]); }));
  }

  WordCountMapType wordCounts;
  for (auto& f : futures) {
    const auto wordCountInfoForFile = f.get();  // move map returned by wordsInFile

    for (const auto& wordInfo : wordCountInfoForFile) {
      wordCounts[wordInfo.first] += wordInfo.second;
    }
  }

  std::cout << wordCounts.size() << " words found.  Most common:\n" ;
 
  const std::size_t maxWordsToShow = 20;
  showCommonWords(wordCounts.begin(), wordCounts.end(), 
                  std::min(wordCounts.size(), maxWordsToShow));
}