不同大小的 std::array 的 std::vector

Question

As an exercise, I would like to construct a vector containing std::array<unsigned char, N> objects (where N varies).

My attempt was to construct a base class GenericArray from which a MyArray<N> will derive, such that the container will actually be: std::vector<GenericArray*> . However, since the actual array variable must reside in the derived class, I do not see a way to make use of this data from the std:vector<GenericArray*> itself.

Here is my full attempt, which obviously produces: error: 'class GenericArray' has no member named 'data'

#include <array>
#include <cassert>
#include <iostream>
#include <vector>

template<std::size_t N>
using arr_t = std::array<unsigned char, N>;

class GenericArray
{
public:
    ~GenericArray() = default;
};

template<std::size_t N>
class MyArray : public GenericArray
{
public:
    arr_t<N> data;

    MyArray(const arr_t<N>& data)
    {
        this->data = data;
    }
};

int main(void)
{
    std::vector<GenericArray*> vs;

    vs.emplace_back(new MyArray<2>({ 'a', 'b' }));
    vs.emplace_back(new MyArray<4>({ 'A', 'B', 'C', 'D' }));

    assert(vs.size() == 2);

    for (const auto& x : vs[0]->data)
    {
        std::cout << x << "\n";
    }

    return 0;
}

Answer 1

I would like to construct a vector containing std::array<unsigned char, N> objects (where N varies).

You cannot. Vectors, like all standard containers are homogeneous. They consist of a single element type. std::array<unsigned char, N> and std::array<unsigned char, M> are different types - given that N != M, and thus they cannot be stored in a homogeneous container.

You can use std::vector<std::vector<unsigned char>> instead.

Answer 2

You seem to be mixing two concepts. I recommend the version in eerorika's answer but if you really want base class pointers in your container, here's one way:

#include <array>
#include <iostream>
#include <vector>
#include <memory>

template<std::size_t N>
using arr_t = std::array<unsigned char, N>;

class GenericArray {
public:
    virtual ~GenericArray() = default;  // must be virtual to call the derived dtor
    virtual unsigned char* begin() = 0; // used for iterating in the derived class
    virtual unsigned char* end() = 0;
    // add `const` versions too as needed
};

template<std::size_t N>
class MyArray : public GenericArray {
public:
    arr_t<N> data;

    MyArray(const arr_t<N>& data) : data(data) {}
    unsigned char* begin() override { return data.begin(); } // overridden
    unsigned char* end() override { return data.end(); }     // -"-

};

int main() {    // no need for main(void)
    std::vector<std::unique_ptr<GenericArray>> vs;

    vs.emplace_back(new MyArray<2>({ 'a', 'b' }));
    vs.emplace_back(new MyArray<4>({ 'A', 'B', 'C', 'D' }));

    // loop over the elements:
    for(auto& ptr : vs) {
        for(auto& x : *ptr) std::cout << x << ' ';
        std::cout << '\n';
    }
}

Answer 3

I don't think it is possible. Let's say you make a class which contains a secvence of std::array<char,N> s with a get method. what should be the size of the returned result. The compiler must know the memory-size of all, variables for compiling, but it doesn't know the size of auto p=vct.get(i); If get returns a pointer than size size of auto p=*vct.get(i); is undefined.

The closest you can get is returning void* and casting it to array<char,N>* but N must be known compile-time