How to make span of spans

Question

C++20 std::span is a very nice interface to program against. But there doesn't seem to be an easy way to have a span of spans. Here's what I am trying to do:

#include <iostream>
#include <span>
#include <string>
#include <vector>

void print(std::span<std::span<wchar_t>> matrix) {
  for (auto const& str : matrix) {
    for (auto const ch : str) {
      std::wcout << ch;
    }
    std::wcout << '\n';
  }
}

int main() {
  std::vector<std::wstring> vec = {L"Cool", L"Cool", L"Cool"};
  print(vec);
}

This doesn't compile. How do I do something like that?

Answer 1

Why not use a concept instead?

#include <iostream>
#include <string>
#include <vector>
#include <ranges>

template <class R, class T>
concept Matrix = 
    std::convertible_to<
        std::ranges::range_reference_t<std::ranges::range_reference_t<R>>,
        T>;

void print(Matrix<wchar_t> auto const& matrix) {
    for (auto const& str : matrix) {
        for (auto const ch : str) {
            std::wcout << ch;
        }
        std::wcout << '\n';
    }
}

int main() {
  std::vector<std::wstring> vec = {L"Cool", L"Cool", L"Cool"};
  print(vec);
}

godbolt.org

_{Thanks to Barry for suggesting the simplified concept above using the standard ranges library.}

Answer 2

Just use a template to print any container type containing a std::wstring or std::wstring_view (two arbitrary type limitations for the sake of demonstration; easily adjust or remove these limitations as you see fit)

I prefer to stick with code that is more universally-readable (C++ "concepts" are very advanced and not as widely understood). Why not just use this simple template?

template <typename T>
void print(const T& matrix) {
  for (auto const& str : matrix) {
    for (auto const ch : str) {
      std::wcout << ch;
    }
    std::wcout << '\n';
  }
}

As a bonus, add in this static_assert to check types and ensure that only std::wstring or std::wstring_view string types are passed in, for example (modify or delete the static assert as you see fit, and according to your needs):

static_assert(std::is_same_v<decltype(str), const std::wstring&> || 
  std::is_same_v<decltype(str), const std::wstring_view&>,
  "Only strings of `std::wstring` or `std::wstring_view` are "
  "allowed!");

Now you have this better version of the print() function template:

template <typename T>
void print(const T& matrix) {
  for (auto const& str : matrix) {
    static_assert(std::is_same_v<decltype(str), const std::wstring&> || 
      std::is_same_v<decltype(str), const std::wstring_view&>,
      "Only strings of `std::wstring` or `std::wstring_view` are "
      "allowed!");

    for (auto const ch : str) {
      std::wcout << ch;
    }
    std::wcout << '\n';
  }
}

But, the 2nd usage of auto isn't necessary and adds no value (it just obfuscates things), so let's remove it, and use this instead:

for (wchar_t const ch : str) {

The 1st usage of auto is fine because it's required there since it could be multiple types.

(Note: if you actually do need to handle other types of chars here, ignore what I'm saying here and change wchar_t back to auto. That's up to you to decide.)

Now, we have this final version of the printf() function template:

template <typename T>
void print(const T& matrix) {
  for (auto const& str : matrix) {
    static_assert(std::is_same_v<decltype(str), const std::wstring&> || 
      std::is_same_v<decltype(str), const std::wstring_view&>,
      "Only strings of `std::wstring` or `std::wstring_view` are "
      "allowed!");

    for (wchar_t const ch : str) {
      std::wcout << ch;
    }
    std::wcout << '\n';
  }
}

It appears your goal is to be able to print any container type containing wide-char text with your custom print() function, no?

You seem to be calling this a "matrix", where the outer element in the container is a string, and the inner element of each string is a wide character (wchar).

If that's the case, the following template works just fine. I simply changed this:

void print(std::span<std::span<wchar_t>> matrix) {

to this:

template <typename T>
void print(const T& matrix) {

...and then I added:

1. a static_assert relying on A) std::is_same_v<> (same as std::is_same<>::value) and B) the decltype() specifier to ensure only std::wstring or std::wstring_view string types are passed in, and
2. some more test prints in main(), including test prints for a std::vector<std::wstring> and a std::vector<std::wstring_view>, as well as for a linked list: std::list<std::wstring_view>, and an unordered set (hash set): std::unordered_set<std::wstring>.

Here is the entire code and print() function template. Run this code online: https://godbolt.org/z/TabW43Yjf.

#include <iostream>
#include <list> // added for demo purposes to print a linked list in main()
// #include <span> // not needed
#include <string>
#include <type_traits> // added to check types and aid with static asserts
#include <unordered_set>
#include <vector>

template <typename T>
void print(const T& matrix) {
  for (auto const& str : matrix) {
    static_assert(std::is_same_v<decltype(str), const std::wstring&> || 
      std::is_same_v<decltype(str), const std::wstring_view&>,
      "Only strings of `std::wstring` or `std::wstring_view` are "
      "allowed!");

    for (wchar_t const ch : str) {
      std::wcout << ch;
    }
    std::wcout << '\n';
  }
}

int main() {
  std::vector<std::wstring> vec1 = {L"Cool1", L"Cool2", L"Cool3"};
  std::vector<std::wstring_view> vec2 = {L"Hey1", L"Hey2", L"Hey3"};
  std::list<std::wstring_view> list1 = {L"You1", L"You2", L"You3"};
  std::unordered_set<std::wstring> set1 = {L"There1", L"There2", L"There3"};
  print(vec1);
  print(vec2);
  print(list1);
  print(set1);

  // Compile-time error due to the std::is_same_v<> usage in the static_assert 
  // above!
  // std::vector<std::string> vec3 = {"hey", "you"};
  // print(vec3);
}

Sample Output:

Cool1
Cool2
Cool3
Hey1
Hey2
Hey3
You1
You2
You3
There3
There2
There1

If you just want to print std::vector<std::wstring> and std::vector<std::wstring_view> types, here is a more-limited template (again, these are two arbitrary type limitations for the sake of demonstration; easily adjust or remove these limitations as you see fit):

Simply replace this in my template above:

template <typename T>
void print(const T& matrix) {

with this, to force it to only accept std::vector<> container types (const T& above changes to const std::vector<T>& below, is all):

template <typename T>
void print(const std::vector<T>& matrix) {

Then, add a static_assert to ensure the type inside the vector is either std::wstring or std::wstring_view, as desired.

Full code below. Run it online here: https://godbolt.org/z/qjhqq647M.

#include <iostream>
// #include <span> // not needed
#include <string>
#include <type_traits>
#include <vector>

template <typename T>
void print(const std::vector<T>& matrix) {
  static_assert(std::is_same_v<T, std::wstring> || 
    std::is_same_v<T, std::wstring_view>,
    "Only vectors of `std::wstring` or `std::wstring_view` are allowed!");

  for (auto const& str : matrix) {
    for (wchar_t const ch : str) {
      std::wcout << ch;
    }
    std::wcout << '\n';
  }
}

int main() {
  std::vector<std::wstring> vec1 = {L"Cool1", L"Cool2", L"Cool3"};
  std::vector<std::wstring_view> vec2 = {L"Hey1", L"Hey2", L"Hey3"};
  print(vec1);
  print(vec2);

  // Compile-time error due to the std::is_same_v<> usage in the static_assert 
  // above!
  // std::vector<std::string> vec3 = {"hey", "you"};
  // print(vec3);
}

Why a span of spans doesn't work:

std::span<T> is essentially just a struct containing a pointer to a block of contiguous memory. Cppreference.com states (emphasis added):

The class template span describes an object that can refer to a contiguous sequence of objects with the first element of the sequence at position zero.

As I explain in my other answer on spans here (What is a "span" and when should I use one?), it might look like this:

template <typename T>
struct span
{
    T * ptr_to_array;   // pointer to a contiguous C-style array of data
                        // (which memory is NOT allocated or deallocated 
                        // by the span)
    std::size_t length; // number of elements in the array

    // Plus a bunch of constructors and convenience accessor methods here
}

Not all C++ container types are stored in contiguous memory, however, such as linked lists (std::list and std::forward_list) so they cannot be placed into a span.

Generally-speaking, a span is a wrapper in C++ to wrap around C-style arrays, capturing in one variable a pointer to their contiguous block of memory, and in another variable, their length. This way, you can replace a function prototype with two input parameters like this:

void do_stuff(T *ptr_to_data, std::size_t num_elements) {}
// OR (the const form)
void do_stuff(const T *ptr_to_data, std::size_t num_elements) {}

with a prototype with one input parameter like this:

void do_stuff(std::span<T> data) {}
// OR (the const form)
void do_stuff(const std::span<T> data) {}

as @mcilloni says in his comment here.

References:

1. Scratch work:
   1. https://godbolt.org/z/s99dnzj8z
   2. https://godbolt.org/z/33vzTM787
2. [my answer] What is a "span" and when should I use one?
3. https://www.learncpp.com/cpp-tutorial/an-introduction-to-stdstring_view/ - EXCELLENT read on what is a std::string_view, when and why to use it, and how. It also covers some of its nuances, limitations, and shortcomings.
4. https://en.cppreference.com/w/cpp/container/span
5. https://en.cppreference.com/w/cpp/types/is_same
6. https://en.cppreference.com/w/cpp/header/type_traits
7. *****[my answer--VERY USEFUL--I referenced it to remember how to statically check types at compile-time with static_assert(std::is_same_v<decltype(var), some_type>, "some msg");] Use static_assert to check types passed to macro

Answer 3

UPDATE: I'm leaving the answer below despite the downvotes since seeing it and the comments under it has value too, but here's a different answer I just posted instead which I think has value and merit.

---

I don't understand the desire for using a span at all here (please help me understand if I am missing something), as the purpose of a span is to wrap and "C++-itize" (which is sometimes a debatable practice already) a C-style array.

Why not just change this:

void print(std::span<std::span<wchar_t>> matrix) {

to this?:

void print(std::vector<std::wstring> matrix) {

Now the code works just fine (run on Godbolt):

#include <iostream>
// #include <span> // not needed
#include <string>
#include <vector>

void print(std::vector<std::wstring> matrix) {
  for (auto const& str : matrix) {
    for (auto const ch : str) {
      std::wcout << ch;
    }
    std::wcout << '\n';
  }
}

int main() {
  std::vector<std::wstring> vec = {L"Cool", L"Cool", L"Cool"};
  print(vec);
}

Here is the output, as shown on Godbolt. Notice that the text (Cool Cool Cool) prints just fine:

ASM generation compiler returned: 0
Execution build compiler returned: 0
Program returned: 0
Cool
Cool
Cool