Initialize private array with unknown number of elements (C++98) - c++

Question

Looks like I missed something fundamental here, but haven't worked it out.
Below is a snippet and its corresponding output.
What I wanted to do is:
- Declare and initialize an array of structs, without knowing the number of elements in advance.
- Ideally the array itself and its number of elements are private members.
What I tried:
Declared m_member_tab[] and m_num_members as private.
Created an Init() function that initializes m_member_tab[] and calculate m_num_members.
Outcome:
m_member_tab[] is initialized ok (see below output).
BUT inside the constructor (after calling Init), m_member_tab is corrupted.
#include <iostream>
using std::cout; using std::endl;
class TArrayTest
{
public:
TArrayTest();
private:
void Init();
typedef struct _TMember
{
int m_key;
int m_val;
}
TMember;
TMember m_member_tab[];
int m_num_members;
};
TArrayTest::TArrayTest()
{
Init();
cout << "Ctor: Number of elements = " << m_num_members << endl;
for( int i = 0; i < m_num_members; i++ )
{
cout << "Ctor: "
<< "key " << m_member_tab[i].m_key
<< " - val " << m_member_tab[i].m_val
<< endl;
}
};
void TArrayTest::Init()
{
TMember m_member_tab[] =
{
{ 1, 100 },
{ 2, 200 },
{ 3, 300 },
{ 4, 400 },
{ 5, 500 },
};
m_num_members = sizeof( m_member_tab ) / sizeof( TMember );
cout << "Init: Number of elements = " << m_num_members << endl;
for( int i = 0; i < m_num_members; i++ )
{
cout << "Init: "
<< "key " << m_member_tab[i].m_key
<< " - val " << m_member_tab[i].m_val
<< endl;
}
}
int main()
{
TArrayTest test;
}
Output:
Init: Number of elements = 5
Init: key 1 - val 100
Init: key 2 - val 200
Init: key 3 - val 300
Init: key 4 - val 400
Init: key 5 - val 500
Ctor: Number of elements = 5
Ctor: key 5 - val 32766
Ctor: key 0 - val 0
Ctor: key 0 - val 0
Ctor: key -1212526907 - val 32623
Ctor: key 0 - val 0

Answer 1

This member declaration:
TMember m_member_tab[];
is not valid C++.
There is an additional problem in the init function, where you declare a local variable of the same name, but it doesn't matter: the above declaration is not just invalid, but since it's not at the end of the struct it doesn't even make sense as a language extension.
Instead, use std::vector, like this:
std::vector<TMember> m_member_tab;
It keeps track of the array size, so you don't need that extra member.
In other news, C++ directly supports initialization of an instance of a class. You should not define ordinary function for that. Instead use the language mechanism for that, namely a constructor.
You can find information about constructors in any tutorial and any introductory C++ textbook.
The language supported mechanism has many advantages compared to an init function.
Oh, and seeing as each item in the array will contain a key and a value, do consider a std::map, or, if you can use C++11 and not just C++98/C03, std::unordered_map (faster but no sorted traversal of keys).

Answer 2

in the first line of void TArrayTest::Init():
TMember m_member_tab[] =
{
{ 1, 100 },
{ 2, 200 },
{ 3, 300 },
{ 4, 400 },
{ 5, 500 },
};
you declare "m_member_tab" a temporary variable, not Member variable. you should write like this:
m_member_tab[] =
{
{ 1, 100 },
{ 2, 200 },
{ 3, 300 },
{ 4, 400 },
{ 5, 500 },
};

Answer 3

The feature you are trying to use does not exist in C++ language. It is illegal to use [] in non-static member array declarations. It is allowed in static member array declarations, but non in non-static ones.
Even if your compiler somehow allows this declaration, it probably interprets it as a zero-sized array. The array size is fixed at that point - there's no way to somehow "intialize" it into a bigger array later.
Member array declaration with [] might be allowed by some compilers as a way to support C-style "struct hack". But that is a completely different technique.