print-container.cc

#include <iostream>
#include <vector>
#include <type_traits>

// Helper to check if a type is a container (excluding string)
template <typename T, typename = void>
struct is_container : std::false_type {};

template <typename T>
struct is_container<T, std::void_t<decltype(std::begin(std::declval<T&>())),
                                   decltype(std::end(std::declval<T&>()))>>
    : std::integral_constant<bool, !std::is_same_v<T, std::string>> {};

// Generic operator<<
template <typename T>
std::enable_if_t<is_container<T>::value, std::ostream&> operator<<(
    std::ostream& os, const T& container) {
    os << "["s;
    auto it = std::begin(container);
    auto end = std::end(container);

    while (it != end) {
        os << *it; // Recursive call
        auto next = std::next(it);
        if (next != end) {
            if constexpr (is_container<typename T::value_type>::value) {
                os << "\n"s;
            } else {
                os << " "s;
            }
        }
        it = next;
    }
    os << "]"s;
    return os;
}

void t1() {
    //
    vector<vector<int>> vv{
        {1, 2},
        {2, 3},
        {2, 3},
        {2, 3},
    };
    cout << vv;
}

int main() {
    t1();
    return 0;
}