[generator] improve iterator interface when yielding non-copyable

include/itlib/generator.hpp

@@ -1,4 +1,4 @@
-// itlib-generator v1.02
+// itlib-generator v1.03
 //
 // Simple coroutine generator class for C++20 and later
 //
@@ -28,6 +28,8 @@
 //
 //                  VERSION HISTORY
 //
+//  1.03 (2024-09-24) Improve iterator-like interface when yielding
+//                    non-copyable values
 //  1.02 (2024-07-18) Store exception to work around clang's ridiculous
 //                    and overly complicated handling of coroutines
 //  1.01 (2024-07-18) Add missing header for newer, more stringent compilers
@@ -105,8 +107,8 @@ class generator_value <T&> {
 template <typename T>
 class generator {
 public:
-    // return const ref in case we're generating values, otherwise keep the ref type
-    using value_ret_t = std::conditional_t<std::is_reference_v<T>, T, const T&>;
+    // return ref in case we're generating values, otherwise keep the ref type
+    using value_ret_t = std::conditional_t<std::is_reference_v<T>, T, T&>;
 
     struct promise_type {
         generator_value<T> m_val;
@@ -190,6 +192,8 @@ class generator {
         using value_type = std::decay_t<T>;
         using reference = value_ret_t;
 
+        using difference_type = std::ptrdiff_t; // pointless here, but required for iterator traits and concepts
+
         pseudo_iterator() noexcept = default;
         explicit pseudo_iterator(handle_t handle) noexcept : m_handle(handle) {}
 

test/t-generator-20.cpp

@@ -7,7 +7,9 @@
 #include <doctest/util/lifetime_counter.hpp>
 
 #include <stdexcept>
+#include <iterator>
 #include <vector>
+#include <string>
 #include <span>
 
 itlib::generator<int> range(int begin, int end) {
@@ -145,3 +147,29 @@ TEST_CASE("lifetime") {
         CHECK(ls.m_ctr == 6);
     }
 }
+
+struct non_copyable {
+    non_copyable(std::string v) : value(std::move(v)) {}
+    non_copyable(const non_copyable&) = delete;
+    non_copyable& operator=(const non_copyable&) = delete;
+    non_copyable(non_copyable&&) noexcept = default;
+    non_copyable& operator=(non_copyable&&) noexcept = default;
+    std::string value;
+};
+
+itlib::generator<non_copyable> non_copyable_range(int begin, int end) {
+    for (int i = begin; i < end; ++i) {
+        co_yield non_copyable(std::to_string(i));
+    }
+}
+
+TEST_CASE("yield non copyable") {
+    auto gen = non_copyable_range(10, 15);
+    std::vector<std::string> values;
+    int i = 10;
+    for (auto mi = std::make_move_iterator(gen.begin()); mi.base() != gen.end(); ++mi) {
+        auto elem = *mi;
+        CHECK(elem.value == std::to_string(i));
+        ++i;
+    }
+}