example(task): add example for shared_task

example/multi_fan_out.cpp

@@ -0,0 +1,42 @@
+#include <co_context/co/when_all.hpp>
+#include <co_context/lazy_io.hpp>
+#include <co_context/shared_task.hpp>
+#include <co_context/utility/mpl.hpp>
+#include <iostream>
+
+using namespace co_context;
+
+shared_task<std::string> fan_out() {
+    std::cout << "fan_out(): start\n";
+    co_await timeout(std::chrono::seconds{1});
+    std::cout << "fan_out(): done\n";
+    co_return "string from fan_out()";
+}
+
+task<size_t> mapped_task(shared_task<std::string> dependency) {
+    auto result = co_await dependency;
+    std::cout << "post_task(): " << result << "\n";
+    co_return result.size();
+}
+
+template<typename T>
+task<void> reduce_task(task<T> all_task) {
+    auto results = co_await all_task;
+    constexpr size_t n = std::tuple_size_v<T>;
+    size_t sum_size = 0;
+    mpl::static_for<0, n>([&](auto i) { sum_size += std::get<i>(results); });
+    std::cout << "reduce_task(): total bytes: " << sum_size << "\n";
+}
+
+int main() {
+    io_context ctx;
+    {
+        auto f = fan_out();
+        auto maps = all(mapped_task(f), mapped_task(f), mapped_task(f));
+        auto reduce = reduce_task(std::move(maps));
+        ctx.co_spawn(std::move(reduce));
+    }
+    ctx.start();
+    ctx.join();
+    return 0;
+}