Correct way to add a const to a class - c++

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;
}
};

Answer 1

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.

Answer 2

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;
}

Answer 3

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;
}
...

Answer 4

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)
{
....

Answer 5

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