c++11 std::array vs static array vs std::vector

Question

First question, is it a good thing to start using c++11 if I will develop a code for the 3 following years?

Then if it is, what is the "best" way to implement a matrix if I want to use it with Lapack? I mean, doing std::vector<std::vector< Type > > Matrix is not easily compatible with Lapack.

Up to now, I stored my matrix with Type* Matrix(new Type[N]) (the pointer form with new and delete were important because the size of the array is not given as a number like 5, but as a variable).

But with C++11 it is possible to use std::array. According to this site, this container seems to be the best solution... What do you think?

Answer 1

First things first, if you are going to learn C++, learn C++11. The previous C++ standard was released in 2003, meaning it's already ten years old. That's a lot in IT world. C++11 skills will also smoothly translate to upcoming C++1y (most probably C++14) standard.

The main difference between std::vector and std::array is the dynamic (in size and allocation) and static storage. So if you want to have a matrix class that's always, say, 4x4, std::array<float, 4*4> will do just fine.

Both of these classes provide .data() member, which should produce a compatible pointer. Note however, that std::vector<std::vector<float>> will NOT occuppy contiguous 16*sizeof(float) memory (so v[0].data() won't work). If you need a dynamically sized matrix, use single vector and resize it to the width*height size.

Since the access to the elements will be a bit harder (v[width * y +x] or v[height * x + y]), you might want to provide a wrapper class that will allow you to access arbitrary field by row/column pair.

Since you've also mentioned C-style arrays; std::array provides nicer interface to deal with the same type of storage, and thus should be preferred; there's nothing to gain with static arrays over std::array.

Answer 2

This is a very late reply to the question, but if someone reads this, I just want to point out that one should almost never implement a matrix as a ''vector of vectors''. The reason is that each row of the matrix gets stored in some random location on the heap. This means that matrix operations will do a lot of random memory accesses leading to cache misses, which slows down the implementation considerably.

In other words, if you care at all about performance, just allocate an array/std::array/std::vector of size rows * columns, then use wrapper functions that transforms a pair of integers to the corresponding element in the array. Unless you need to support things like returning references to rows of the matrix, then all of these options should work just fine.