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

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

Cheers and hth. - Alf · Accepted Answer · 2015-07-14T02:46:37.697

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).

score 1 · Answer 2 · answered Jul 14 '15 at 02:38

1

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 }, };

answered Jul 14 '15 at 02:38

wangli_64604

103
5

That alone doesn't help. But anyway +1 for pointing out my mistake of using reckless usage of temp var. – artm Jul 14 '15 at 10:21

score 1 · Answer 3 · edited May 23 '17 at 12:13

1

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.

edited May 23 '17 at 12:13

Community

1
1

answered Jul 14 '15 at 02:46

AnT stands with Russia

312,472
42
525
765

Thanks. Looks like the only way (if I need to stick to C++98) is to declare a _public_ array of struct. Then in the implementation file (outside the class itself), I can initialize the array without knowing the number of items (following C99 syntax). That works, but what I don't really like is the fact the array itself (and the struct type) must be _public_. There is a suggestion above to use _vector_. But I haven't found a way to initialize vector of structs the way I did with C99 array (i.e., without knowing the number of elements). – artm Jul 14 '15 at 10:20
@artm: Um... I don't see how making the array public can make any difference here. As I said above, as long as your array remains a non-static member of the class, it is not possible to "redefine" the number of elements. – AnT stands with Russia Jul 14 '15 at 16:30

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

3 Answers3