How to properly initialize a boost multi_array of objects? - c++

Question

I have been surprised to find that boost::multi_array seems to allocate its initial elements differently from, say, std::vector. It does not seem to fill each element with a unique element (using its default value or default constructor). I'm having trouble finding more information about this.
Is there a way to make the multi_array fill itself with a unique object at each element?
For example, consider the following:
static int num = 0;
struct A {
int n;
A() : n((::num)++) {
std::cout << "A()" << std::endl;
}
virtual ~A() {}
void print() {
std::cout << "n=" << n << std::endl;
}
};
int main() {
std::cout << "vector:" << std::endl;
std::vector<A> v(3);
for (auto x : v) {
x.print();
}
std::cout << "multi:" << std::endl;
boost::multi_array<A, 2> m(boost::extents[2][2]);
for (auto x : m) {
for (auto y : x) {
y.print();
}
}
}
This results in the output:
vector:
A()
A()
A()
n=0
n=1
n=2
multi:
A()
n=3
n=3
n=3
n=3
Why is the constructor called only once for the multi_array? How can the multi_array be "filled out" with unique objects (using A's default constructor)?

Answer 1

To quickly fill the whole array do something like fill_n¹:
std::fill_n(a.data(), a.num_elements(), 0);
With boost multi_array you can use a view to your own memory buffer to get the same performance (std::uninitialized_copy is your friend). (actually, you could even map an array view on existing memory, and you want to keep the existing values).
I've written a comparative demo about this here: pointers to a class in dynamically allocated boost multi_array, not compiling
Live On Coliru
#include <boost/multi_array.hpp>
#include <type_traits>
#include <memory>
struct octreenode { int a; int b; };
class world {
public:
world(double x, double y, double z, int widtheast, int widthnorth, int height)
:
originx(x), originy(y), originz(z),
chunkseast(widtheast), chunksnorth(widthnorth), chunksup(height)
{
#define OPTION 4
#if OPTION == 1
static_assert(std::is_trivially_destructible<octreenode>::value, "assumption made");
//std::uninitialized_fill_n(chunk.data(), chunk.num_elements(), octreenode {1, 72});
std::fill_n(chunk.data(), chunk.num_elements(), octreenode {1, 72});
#elif OPTION == 2
for(auto a:chunk) for(auto b:a) for(auto&c:b) c = octreenode{1, 72};
#elif OPTION == 3
for (index cz = 0; cz < chunksnorth; ++cz) {
for (index cx = 0; cx < chunkseast; ++cx) {
for (index cy = 0; cy < chunksup; ++cy) {
chunk[cz][cx][cy] = octreenode{1, 72};
}
}
}
#elif OPTION == 4
static_assert(std::is_trivially_destructible<octreenode>::value, "assumption made");
for (index cz = 0; cz < chunksnorth; ++cz) {
for (index cx = 0; cx < chunkseast; ++cx) {
for (index cy = 0; cy < chunksup; ++cy) {
new (&chunk[cz][cx][cy]) octreenode{1, 72};
}
}
}
#endif
(void) originx, (void) originy, (void) originz, (void) chunksup, (void) chunkseast, (void) chunksnorth;
}
private:
double originx, originy, originz;
int chunkseast, chunksnorth, chunksup;
#if 1
typedef boost::multi_array<octreenode, 3> planetchunkarray; // a boost_multi for chunks
typedef planetchunkarray::index index;
planetchunkarray chunk{boost::extents[chunksnorth][chunkseast][chunksup]};
#else
static_assert(boost::is_trivially_destructible<octreenode>::value, "assumption made");
std::unique_ptr<octreenode[]> raw { new octreenode[chunksnorth*chunkseast*chunksup] };
typedef boost::multi_array_ref<octreenode, 3> planetchunkarray;
typedef planetchunkarray::index index;
planetchunkarray chunk{raw.get(), boost::extents[chunksnorth][chunkseast][chunksup]};
#endif
};
int main() {
world w(1,2,3,4,5,6);
}
The variant using multi_array_ref is an example of how to avoid copy-constructing the elements (it's akin to the optimization used by std::vector when it uses uninitialized memory for reserved but unused elements).
¹ Of course for unique values, use std::iota or std::generate
http://en.cppreference.com/w/cpp/algorithm/iota
http://en.cppreference.com/w/cpp/algorithm/generate_n

Answer 2

So on further study, I learned two things:
boost::multi_array uses the copy constructor to initialize objects into the container, not the default constructor.
The for (auto x : container) way of looping in C++11 seems (at least with clang++ 3.5) to loop over copies of the container elements, rather than iterators (or references).
Modifying the original question's example to demonstrate point 1.
Adding a copy constructor (and corresponding counter), and using auto& x for the object loops rather than auto x:
static int num = 0;
static int cpy = 0;
struct A {
int n;
int c;
A() : n((::num)++), c(0) {
std::cout << "A_def()" << std::endl;
}
A(const A& o) : n(0), c((::cpy)++) {
std::cout << "A_cpy()" << std::endl;
}
virtual ~A() {}
void print() {
std::cout << "n=" << n << ",c=" << c << std::endl;
}
};
int main() {
std::cout << "vector:" << std::endl;
std::vector<A> v(3);
for (auto& x : v) {
x.print();
}
std::cout << "multi:" << std::endl;
boost::multi_array<A, 2> m(boost::extents[2][2]);
for (auto x : m) {
for (auto& y : x) {
y.print();
}
}
}
Produces the output
vector:
A_def() // <- vector allocation starts
A_def()
A_def()
n=0,c=0 // <- vector printing starts, using "for (auto& x)"
n=1,c=0
n=2,c=0
multi:
A_def() // <- a temporary object for multi_array allocation
A_cpy() // <- multi_array allocation starts
A_cpy()
A_cpy()
A_cpy()
n=0,c=0 // <- multi_array prints starts, using "for (auto& y)"
n=0,c=1
n=0,c=2
n=0,c=3
Modifying the example above to demonstrate point 2.
Same class definition as above in this answer, but removing the auto& x from the object loops, and going back to using auto x as done in the original question.
std::cout << "vector:" << std::endl;
std::vector<A> v(3);
for (auto x : v) {
x.print();
}
std::cout << "multi:" << std::endl;
boost::multi_array<A, 2> m(boost::extents[2][2]);
for (auto x : m) {
for (auto y : x) {
y.print();
}
}
Produces output that shows the copy constructor gets called during the print loops, even for elements in the vector.
vector:
A_def() // <- vector allocation starts
A_def()
A_def()
A_cpy() // <- vector printing starts, using "for (auto x)"
n=0,c=0
A_cpy()
n=0,c=1
A_cpy()
n=0,c=2
multi:
A_def() // <- a temporary object for multi_array allocation
A_cpy() // <- multi_array allocation starts
A_cpy()
A_cpy()
A_cpy()
A_cpy() // <- multi_array printing starts, using "for (auto y)"
n=0,c=7
A_cpy()
n=0,c=8
A_cpy()
n=0,c=9
A_cpy()
n=0,c=10