Container of pointers

Question

I'm having some trouble in declaring a STL Set of pointers to class instances. More specifically, I have this scenario:

class SimulatedDiskFile {
  private:
    // ...
  public:
    // ...
    struct comparator {
      bool operator () (SimulatedDiskFile* const& file_1, SimulatedDiskFile* const& file_2) {
        return ((*file_1)->getFileName() < (*file_2)->getFileName());
      }
    };
}

typedef set<SimulatedDiskFile*, SimulatedDiskFile::comparator> FileSet;

The code above is not working. Compiler says it didn't find a member SimulatedDiskFile::comparator() function. If I put the function with this declaration (outside the struct), compiler says it was expecting a type.

Now here com my doubts (not only one, but related, I guess):

What is the the correct declaration for a set of pointers?
What is the correct declaration for a comparison funcion that compares pointers?

I did look up in many places before posting, but I found the references confusing and not quite related to my special case (as stupidly trivial as I think it is - actually, maybe this is the cause). So, any good links are of great help too!

Thanks in advance!

Compiles for me (on g++), except that you have an error in your comparison because you're both dereferencing the pointers and then also using indirection. — Tyler McHenry
– Tyler McHenry, Commented Jun 16, 2009 at 2:44

Alex Martelli · Accepted Answer · 2009-06-16 02:47:16Z

5

Fixing a few glitches,

#include <set>

class SimulatedDiskFile {
  public:
    int getFileName() { return 23; }

    struct comparator {
      bool operator () (SimulatedDiskFile* file_1, SimulatedDiskFile* file_2) {
        return (file_1->getFileName() < file_2->getFileName());
      }
    };
};

typedef std::set<SimulatedDiskFile*, SimulatedDiskFile::comparator> FileSet;

compiles just fine.

answered Jun 16, 2009 at 2:47

Alex Martelli

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3 Comments

Rafael Almeida Over a year ago

Tried changing the syntax to get it in this way. Still doens't compile, exact message is 'g++ -Wall -g -c modules/sources/model.cpp -o obj/model.o modules/sources/model.cpp:56: error: no ‘bool SimulatedDiskFile::comparator(SimulatedDiskFile* const&, SimulatedDiskFile* const&)’ member function declared in class ‘SimulatedDiskFile’'

goldPseudo Over a year ago

@Rafael: This sample compiles just fine for me too. Maybe you could post a more complete segment of your code that actually reproduces the specific error?

Alex Martelli Over a year ago

Yes @Rafael, like @goldPseudo says -- your exact command line on this exact snippet I gave (copied and pasted into a file) compiles happily (with several g++ versions, too). Please do so copy and paste it and try to confirm you don't have a broken g++;-), then edit your answer to show a minimal complete failing case please!

goldPseudo · Accepted Answer · 2009-06-16 02:48:37Z

1

Since you aren't showing where the 'getFileName()' method is supposed to be, I'm just going to go out on a limb and assume that you don't mean to double-dereference your pointers in the comparator. ie, you should do either:

return (file_1->getFileName() < file_2->getFileName());

or:

return ((*file_1).getFileName() < (*file_2).getFileName());

but not both.

answered Jun 16, 2009 at 2:48

goldPseudo

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