Re: map (associative array) loses values?

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 11 Sep 2007 12:19:49 -0700

Message-ID:

<fc6ppd$am9$1@murdoch.acc.Virginia.EDU>

jeroenvlek@gmail.com wrote:

On Sep 11, 5:59 pm, Kai-Uwe Bux <jkherci...@gmx.net> wrote:
<cut>

Best

Kai-Uwe Bux

ah, ok. <string, int> works like a charm! :)

I'm using new, because I need the table somewhere else. The try/catch
is in my main, while the function resides in the SearchText class. The
delete statements are there, I just didn't post them ;)

Ofcourse I'm just a c++ newbie (this is my first program actually), so
what would you do different?

I would start with something like

map<const std::string, int> SearchText::countWords()
{
  map<std::string, int> result;

  result["aap"] = 1;
  result["noot"] = 2;

  cout << result["aap"] << endl;
  cout << result["noot"] << endl;

  return result;
}

and later

try {
    SearchText text ("test.txt");
    map<const std::string, int> table = text->countWords(); // *
    cout << table["aap"] << endl;
    cout << table["noot"] << endl;
}
catch(int ex) {
    cout << "Could not open file." << endl;
}

That is, on the first try, I would leave it to the compiler to optimize away
the apparent copy-construction in line (*). If profiling shows that the
compiler does not eliminate the copy-constructor calls _and_ that there is
a need to improve performance, I might change the program:

void SearchText::countWords( map<std::string, int> & result ) {
  result["aap"] = 1;
  result["noot"] = 2;

  cout << result["aap"] << endl;
  cout << result["noot"] << endl;
}

....

try {
    SearchText text ("test.txt");
    map<const std::string, int> table;
    text->countWords( table );
    cout << table["aap"] << endl;
    cout << table["noot"] << endl;
}
catch(int ex) {
    cout << "Could not open file." << endl;
}

or I might resort to swap() tricks (faking move semantics):

map<const std::string, int> SearchText::countWords()
{
  map<std::string, int> result;

  result["aap"] = 1;
  result["noot"] = 2;

  cout << result["aap"] << endl;
  cout << result["noot"] << endl;

  return result;
}

....

try {
    SearchText text ("test.txt");
    map<const std::string, int> table;
    text->countWords().swap( table );
    cout << table["aap"] << endl;
    cout << table["noot"] << endl;
}
catch(int ex) {
    cout << "Could not open file." << endl;
}

Also: I would have the constructor of SearchText throw something more
meaningfull than an int.

Best

Kai-Uwe Bux