Initialize member array of user defined objects in the constructor in C++

Question

How do I initialize member array in my constructor? The member array is an array of user-defined objects(classes). Also, the number of elements in the array could be large more than 100, so I do not prefer to use initializer-list available in C++11 (unless there's some better way)

For instance, refer following code:

class Foo {
private:
   void *a;
   int b;
public:   
   Foo(void *, int);
   ~Foo();
}

class Bar {
private:
    Foo mObj[150];
public:
    Bar();
    ~Bar();
};

Bar::Bar() {
// ???
}

For sake of simplicity, assume I would like to initialize the members as follows: let's say member int b stores Sr. no & void *a store null ptr as of now... so mObj[0] = {0, nullptr}, mobj[1] = {1, nullptr} & so on..

What should my Bar::Bar() constructor be like?
Do I need to have Default const, Copy const & operator= for Foo()?
Having Vector instead of an array a better option?

"Having Vector instead of an array a better option?" -- Yes!! — Fred Larson
– Fred Larson, Commented Jul 30, 2020 at 13:04
@FredLarson not necessarily. Not enough information is given to jump to this conclusion. — SergeyA
– SergeyA, Commented Jul 30, 2020 at 13:06
@SergeyA Initialize with valid values. For instance, let's say member int b stores Sr. no & void *a store null ptr as of now... so mObj[0] = {0, nullptr}, mobj[1] = {1, nullptr} & so on.. — RamBo
– RamBo, Commented Jul 30, 2020 at 13:09
@SergeyA: I think vector (or std::array, or other container) should always be preferred over old-fashioned array unless there is a compelling argument otherwise. — Fred Larson
– Fred Larson, Commented Jul 30, 2020 at 13:09

SergeyA · Accepted Answer · 2020-07-30 14:05:13Z

1

From the comments, it seems like the ultimate task at hand is to call Foo constructor for each element of the array with arguments of (i, nullptr), where i is increased on each iteration from 0 to N.

Here is the code which would do this with std::index_sequence. This was only introduced with C++14, so if you are not using it, you will have to do implement it yourself, which is not hard at all - let me know if you need help with that.

#include <array>
#include <utility>
#include <cstddef>

struct Foo {
    Foo(void*, int);
};

class Bar {
private:
    static constexpr size_t array_sz = 150;
    template<size_t... Ix> Bar(std::index_sequence<Ix...> ) :
        mObj{Foo{nullptr, Ix}...}     { }
    std::array<Foo, array_sz> mObj;
public:
    Bar();
    ~Bar();
};

Bar::Bar() : Bar(std::make_index_sequence<array_sz>{}) { }

edited Jul 30, 2020 at 14:05

answered Jul 30, 2020 at 13:21

SergeyA

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2 Comments

eerorika Over a year ago

Note that OP tagged the question C++11, while std::index_sequence was introduced in C++14.

SergeyA Over a year ago

@eerorika this is a good point. It is quite easy to hand-rollindex_sequence in C++11, I will edit the answer.

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