Correct way to add a const to a class

Question

This does not work. It gives an error regarding ISO C++ forbidding initialization.

class hash_map 
{
private:
    hash_entry **table;
    const int TABLE_SIZE = 128;
public:
    hash_map() 
    {
        table = new hash_entry*[TABLE_SIZE];
        for (int i = 0; i < TABLE_SIZE; i++)
            table[i] = NULL;
    }
    int get(int key) 
    {
        int hash = (key % TABLE_SIZE);
        while (table[hash] != NULL && table[hash]->getKey() != key)
            hash = (hash + 1) % TABLE_SIZE;
        if (table[hash] == NULL)
            return -1;
        else
            return table[hash]->getValue();
    }
    void put(int key, int value) 
    {
        int hash = (key % TABLE_SIZE);
        while (table[hash] != NULL && table[hash]->getKey() != key)
            hash = (hash + 1) % TABLE_SIZE;
        if (table[hash] != NULL)
            delete table[hash];
        table[hash] = new hash_entry(key, value);
    }
    ~hash_map() 
    {
        for (int i = 0; i < TABLE_SIZE; i++)
            if (table[i] != NULL) delete table[i];
        delete[] table;
    }
};

You don't need to check for NULL before `delete`ing. `delete NULL;` is perfectly valid. — Praetorian, Oct 17 '11 at 17:16
Don't forget to either implement a copy constructor and assignment operator, or declare them private. — Mike Seymour, Oct 17 '11 at 17:20

score 8 · Accepted Answer · edited May 23 '17 at 11:47

8

const int TABLE_SIZE = 128;

This is the cause of the compilation error. It is allowed in C++11 only, not in C++03 and C++98.

Either make it a static member of the class, OR initialize it in the constructor. Make use of member-initialization-list for it.

Apart from that dont forget to implement copy-semantics following Rule of Three, OR disable it altogether by declaring them (don't define them) in the private section. I think, disabling it would make more sense in this case.

edited May 23 '17 at 11:47

Community

1
1

answered Oct 17 '11 at 17:11

Nawaz

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works...thanks...one would think const implies static to the compiler – Oct 17 '11 at 17:15
1

@ChrisAaker: No, each instance of a class can have a different `const int TABLE_SIZE` if it's initialized in the constructor. (Though I agree if it's initialized in place like you do) – Mooing Duck Oct 17 '11 at 17:17
1

@Chris: For global variables, it does (although `static` has a very different meaning there). Member variables are only static if you declare them so - you sometimes want non-static constant members. – Mike Seymour Oct 17 '11 at 17:18
1

@Chris Not necessarily, what of the case where the value can be different across class instances? – MGZero Oct 17 '11 at 17:18
ummm I thought const was used for things like pi=3.14...constants...same across all instances – Oct 17 '11 at 17:23
Well yea, that's a case where you would want it to be the same across all, and should be static. But how about for example, in a game environment. If say, an NPC should be in some constant state, but you'd want each instance to be in a different constant state. I.e, NPC1 is in state 1, NPC2 is in state 2 etc. I.e your constructor would use a variable in the initializer list, not a hardcoded value. – MGZero Oct 17 '11 at 18:04

score 4 · Answer 2 · answered Oct 17 '11 at 17:10

4

You need to initialize it in constructor, change

const int TABLE_SIZE = 128;

to

const int TABLE_SIZE;

and the constructor from

hash_map() 
  {
  table = new hash_entry*[TABLE_SIZE];
  for (int i = 0; i < TABLE_SIZE; i++) table[i] = NULL;
  }

to

hash_map() : TABLE_SIZE(128)
  {
  table = new hash_entry*[TABLE_SIZE];
  for (int i = 0; i < TABLE_SIZE; i++) table[i] = NULL;
  }

answered Oct 17 '11 at 17:10

Daniel

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3

It would be better to make it `static`, to reduce the object size and allow some significant optimisations. – Mike Seymour Oct 17 '11 at 17:15

score 0 · Answer 3 · answered Oct 17 '11 at 17:13

You need to put the initialization into the constructor like this:

class hash_map 
  {
  private:
    hash_entry **table;
    const int TABLE_SIZE;
  public:
    hash_map(): TABLE_SIZE(128) 
      {
      table = new hash_entry*[TABLE_SIZE];
      for (int i = 0; i < TABLE_SIZE; i++) table[i] = NULL;
      }
  ...

score 0 · Answer 4 · answered Oct 17 '11 at 17:15

You could have been more specific and provide a minimal example that reproduces the problem and an exact error that compiler gives you. But anyway, even without a psychic mode turned on, it is obvious that error in on this line:

const int TABLE_SIZE = 128;

You cannot initialize a class member like that, it has to be done in constructor initialization list, unless this member is a static constant compile-time expression (constexpr in C++11).

So remove = 128 from that line and modify constructor to do this:

hash_map() : TABLE_SIZE (128)
      {
....

score 0 · Answer 5 · answered Oct 17 '11 at 19:36

0

Use enumerated type:

class hash_map
{
  private:
    enum
    {
        TABLE_SIZE = 128;
    };
  hash_entry * table[TABLE_SIZE];
//...
};

answered Oct 17 '11 at 19:36

Thomas Matthews

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Correct way to add a const to a class

5 Answers5