Get raw pointer to element block array

Sometimes you need to expose a pointer to an element block array especially when you need to pass such an array pointer to C API that requires one. You can do this by calling the data method of the element_block template class. This works since the element block internally just wraps std::vector or one that acts like it, such as std::deque or delayed_delete_vector, and its data method simply exposes the internal storage types’s own data method which returns the memory location of its internal buffer.

The following code demonstrates this by exposing raw array pointers to the internal arrays of numeric and string element blocks, and printing their element values directly from these array pointers.

#include <mdds/multi_type_vector.hpp>
#include <iostream>

using std::cout;
using std::endl;
using mdds::mtv::double_element_block;
using mdds::mtv::string_element_block;

using mtv_type = mdds::multi_type_vector<mdds::mtv::standard_element_blocks_traits>;

int main() try
{
    mtv_type db;  // starts with an empty container.

    db.push_back(1.1);
    db.push_back(1.2);
    db.push_back(1.3);
    db.push_back(1.4);
    db.push_back(1.5);

    db.push_back(std::string("A"));
    db.push_back(std::string("B"));
    db.push_back(std::string("C"));
    db.push_back(std::string("D"));
    db.push_back(std::string("E"));

    // At this point, you have 2 blocks in the container.
    cout << "block size: " << db.block_size() << endl;
    cout << "--" << endl;

    // Get an iterator that points to the first block in the primary array.
    mtv_type::const_iterator it = db.begin();

    // Get a pointer to the raw array of the numeric element block using the
    // 'data' method.
    const double* p = double_element_block::data(*it->data);

    // Print the elements from this raw array pointer.
    for (const double* p_end = p + it->size; p != p_end; ++p)
        cout << *p << endl;

    cout << "--" << endl;

    ++it; // move to the next block, which is a string block.

    // Get a pointer to the raw array of the string element block.
    const std::string* pz = string_element_block::data(*it->data);

    // Print out the string elements.
    for (const std::string* pz_end = pz + it->size; pz != pz_end; ++pz)
        cout << *pz << endl;

    return EXIT_SUCCESS;
}
catch (...)
{
    return EXIT_FAILURE;
}

Compiling and execute this code produces the following output:

block size: 2
--
1.1
1.2
1.3
1.4
1.5
--
A
B
C
D
E