Declare array of fixed length in nested classes

Question

I have a class A, which has a nested class B. Class A, will create n(run time parameter) instances of class B.

In the constructor of A, after computations that need to be made at run time, I compute a size, let's say s.

Now, every class B, will hold an array of size s.

However, I am not allowed to use .cpp files and all the work must be done in header files.

This means, as far as I understand, that I can not use these approach (with static):

class B {

static const int s;
int a[s];

};

So, I decided to use enum, but I couldn't make it work (even with enum class of C++11). The idea is that you do something like:

class B {

enum { s = 50 };
int a[s];

};

but I do not know the s before run time.

Of course, I could go with an std::vector or with dynamic allocation.

But since:

1)I need only the functionality of the old good arrays (discards vector)

2)I know before constructing the instances of B, the size of the array (discards dynamic allocation)

About std::dynarray discussed below:

Unfortunately not in C++14, but in array TS or C++17. Source, credits to manlio.

Discard is however a heavy word. I mean, I have heard that automatic allocation is faster, but dynamic allocation lets you allocate more space. Moreover, vectors, in the release mode, are doing not that bad in comparison with the arrays.

Answer 1

There are two issues. First, note nesting of classes is for scope only. A nested class doesn't automatically create a data member of that class in the outer class. So normally you would then define member(s), in the outer class, of the nested class type as shown below. Second, you can't define an array based on a size that is computed at run-time in C++.

That said, you could easily use a std::vector. If it all has to be done in the header file for some reason, you could define them in-class, though that would be a bad idea if they were more than a couple lines.

#include <vector>
#include <cstddef>

class A {
    private:
        class B {
            friend A;
            B(size_t n) : array(n) {}
            std::vector<int> array;
        };
    public:
        // Initialize b_member based on some computation.
        A(int i) : b_member(i + 2), b_ptr_member(new B(i + 3)) {}
        ~A() { delete b_ptr_member; }
    private:
        B b_member;
        B *b_ptr_member;
};

int main() {
    A a(10);
}

Answer 2

You can't declare an array of variable size in C++. The compiler must know the size of the array in advance. Use std::vector instead and then use std::vector::shrink_to_fit to reduce its capacity to fit its size.:

class B {
  ...
  std::vector<int> a;
  ...
};

Answer 3

Two arrays having even the same element type but with different sizes are different types. So for example char[10] and char[11] are different types. The same way if two classes have a member with a type that differs between these two classes then the classes define different types and have different sizes (I mean operator sizeof). Also you may not instantiate an object of an incomplete type.

If your compiler supports class std::dynarray I would advice to use it. Otherwise you can use class std::vector

Answer 4

The answer is: use std::vector. This item:

1)I need only the functionality of the old good arrays (discards vector)

does not really apply. You do need extra functionality that old good arrays do not have (namely, allocating an array for which you only have the size at run time).

If you have a recent compiler/standard library, maybe you have access to <experimental/dynarray>... which, IMHO, is just a crippled version of <vector>, but to each, its own :D