Creating a set in C++

Question

I am coding this thing and it would be helpful to have a static const set<char> containing some elements that won't change.

class MyClass {
     private:
         static const set<char> mySet = ??  
}

How can I do this? It would be nice if you could create them from a string, like mySet("ABC"), but I can't get the syntax to work.

score 9 · Accepted Answer · answered Apr 04 '11 at 20:19

9

Something like this will work just fine:

// my_class.h
class MyClass
{
  static const std::set<char> mySet;
};

// my_class.cpp
const char *tmp = "ABCDEFGHI";
const std::set<char> MyClass::mySet(tmp,tmp+strlen(tmp));

answered Apr 04 '11 at 20:19

Šimon Tóth

35,456
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1

+1, I totally forgot about range-based initialization. Certainly works well if it can be easily expressed as a range, but works not-so-well if it contains some computation. – Xeo Apr 04 '11 at 20:30
If you write `const char tmp[] = "ABCDEFGHI";`, you can use `sizeof(tmp)-1` (which is an Integral Constant Expression) – MSalters Apr 05 '11 at 07:34

Nim · Answer 2 · 2011-04-04T20:32:27.033

Something like the following ought to work..

#include <iostream>
#include <set>

struct foo
{
  static std::set<char> init_chars();

  static const std::set<char> myChars;
};

const std::set<char> foo::myChars = foo::init_chars();

std::set<char> foo::init_chars()
{
  std::string sl("ABDCEDFG");
  return std::set<char>(sl.begin(), sl.end());
}

int main()
{
  std::cout << foo::myChars.size() << std::endl;
  return 0;
}

score 1 · Answer 3 · edited May 23 '17 at 11:59

You can't initialize the variable inside the class, because it technically hasn't been defined yet (only declared).

In the class, keep static const set<char> mySet;.

In the implementation (i.e. .cpp file):

const char[] _MyClassSetChars = "ABC";

// Define and initialize the set, using
// the constructor that takes a start and end iterator
const set<char> MyClass::mySet(
    _MyClassSetChars,
    _MyClassSetChars + sizeof(_MyClassSetChars) / sizeof(_MyClassSetChars[0])
);

This also includes the terminating null character (\0) in the set, which is probably not what you want -- instead, use this:

const set<char> MyClass::mySet(
    _MyClassSetChars,
    _MyClassSetChars + (sizeof(_MyClassSetChars) / sizeof(_MyClassSetChars[0]) - 1)
);

Note that here you can use <cstring>'s strlen() (as per Let_Me_Be's answer) in place of the more general form above:

const set<char> MyClass::mySet(
    _MyClassSetChars,
    _MyClassSetChars + strlen(_MyClassSetChars)
);

Also see this question.

Aren't you adding one character to much? – Šimon Tóth Apr 04 '11 at 20:24 — Šimon Tóth, Apr 04 '11 at 20:24

score 0 · Answer 4 · answered Apr 04 '11 at 20:20

0

Have you tried static set <const char> mySet; ? Of course you need to initialize it first otherwise if you try to insert after you declare your set, it will fail.

answered Apr 04 '11 at 20:20

score 0 · Answer 5 · edited Apr 04 '11 at 21:28

0

header file:
#include<set>
class MyClass {
private:
    static const std::set<char> mySet;
}

source file:
#include<boost/assign.hpp>
const MyClass::mySet = boost::assign::list_of('A')('B')('C');

edited Apr 04 '11 at 21:28

Stuart Berg

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answered Apr 04 '11 at 20:20

Gabriel Schreiber

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Creating a set in C++

5 Answers5