Flatten any C++ collection to only not flattenable values. That's it.

Library provides flat_view which is both a range and an iterator in flattened manner over any type of range provided by the user.

std::list<std::vector<int>> numbers{ {1,2,3}, {4,5,6,7} };
for (int n : flatlib::flat(numbers)) {
	std::cout << n << '\n';
}

// Initialize 3x3 matrix with consecutive numbers
std::array<std::array<int, 3>, 3> m;
auto flat_matrix = flatlib::flat(m);
std::iota(flat_matrix.begin(), flat_matrix.end(), 0);

Requires C++20.

Each rule is marked if it's implemented

• flat_view is only a non-owning view: it only accepts lvalue references
• Flattened range with only one layer like flat_view<std::vector<int>> preserves iterator category
• Flattened at least bidirectional range is bidirectional
• Flattened at least forward range is forward
• Flattened at least input range is input
• Flattened collection of std::array is contigious
• Supports C++20 ranges semantics
• Supports C++17 iterator semantics

for definitions of input, forward, bidirectional and contigious from C++ standard libary.