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c++arraystemplates
Nick
Nick

Reputation: 3435

Initialize C++ array member in template class

I need to implement a safe array class that controls index when accessing underlying C array:

template <typename T, int N> class SafeArray
{
public:
    T &operator[](int index)
    {
        assert(0 <= index && index < N);
        return m_data[index];
    }
private:
    T m_data[N];
};

And instead of bool a[3];, now I write SafeArray<bool, 3> a;.

How do I support array initialization like bool b[3] = {false};? I mean what should I do to get b.m_data[] = {false, false, false} after SafeArray<bool, 3> b; has been constructed?

I guess I should add a constructor to SafeArray, but what would be a body of that constructor? Template parameter T can be anything, not necessarily bool. I am using pre-C++11.

Upvotes: 3

Views: 2979

Answers (3)

Andy Prowl
Andy Prowl

Reputation: 126412

I mean what should I to do to get b.m_data[] = {false, false, false} after SafeArray<bool, 3> b; has been constructed?

I am not sure I understand your question correctly, but if I do, then all you need to do is writing a default constructor that initializes your array:

SafeArray() : m_data()
{
}

The complete code:

template <typename T, int N> class SafeArray
{
public:
    SafeArray() : m_data()
    {
    }
    T &operator[](int index)
    {
        assert(0 <= index && index < N);
        return m_data[index];
    }
private:
    T m_data[N];
};

And a live example.

Upvotes: 2

Cassio Neri
Cassio Neri

Reputation: 20503

Since you are using C++03 (therefore you don't have std::array) I would rather recommend you boost::array which is, basically, a C++03 implementation of std::array. (Actually, std::array was inspired by boost::array).

Otherwise, make your class an aggregate, that is, (as per C++03 8.5.1/1)

An aggregate is an array or class (clause 9) with no user-declared constructor, no private protected non-static data member (clause 11), no base classes (clause 10), and no virtual functions.

Then your class will support aggregate initialization as you want.

Update: Reading the OP again (and Andy Prowl's answer), I'm not sure I understand the question. What I propose here is a way to initialize SafeArray at construction time, e.g.,

SafeArray<bool, 3> b = { false, false, false };

Upvotes: 0

Gonmator
Gonmator

Reputation: 780

What about...?

SafeArray() {
    for (int i = 0; i < N; ++i) {
        m_data[i] = T();
    }
}

Upvotes: 0

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