c++: perfect fwd example

2 files changed, 83 insertions, 1 deletions

diff --git a/src/development/c++/Makefile b/src/development/c++/Makefile
index 36065e4..622bfd3 100644
--- a/src/development/c++/Makefile
+++ b/src/development/c++/Makefile
@@ -1,4 +1,4 @@
-SRC = concepts-11.cc meta.cc meta2.cc meta4.cc tmpl-pair.cc tmpl-void_t.cc fwd.cc
+SRC = concepts-11.cc meta.cc meta2.cc meta4.cc tmpl-pair.cc tmpl-void_t.cc fwd.cc fwd-perfect.cc
 BIN = $(SRC:.cc=)
 
 all: $(BIN)
diff --git a/src/development/c++/fwd-perfect.cc b/src/development/c++/fwd-perfect.cc
new file mode 100644
index 0000000..2fafc3e
--- /dev/null
+++ b/src/development/c++/fwd-perfect.cc
@@ -0,0 +1,82 @@
+// Copyright (C) 2023 johannst
+
+#include <cassert>
+#include <cstdio>
+#include <new>
+#include <type_traits>
+#include <utility>
+
+struct S {};
+
+struct M {
+    M() {
+        std::puts("M()");
+    }
+    M(const M&) {
+        std::puts("M(M&)");
+    }
+    M(M&&) {
+        std::puts("M(M&&)");
+    }
+    M& operator=(const M&) = delete;
+    M& operator=(M&&) = delete;
+
+    M(S&, int) {
+        std::puts("M(S&)");
+    }
+    M(S&&, int) {
+        std::puts("M(S&&)");
+    }
+    ~M() {
+        std::puts("~M()");
+    }
+};
+
+template<typename T>
+struct option {
+    static_assert(!std::is_reference_v<T>);
+
+    constexpr option() = default;
+
+    template<typename... Params>
+    constexpr option(Params&&... params) : m_has_val(true) {
+        // BAD: does not perfectly forward!
+        //      eg, if option(S&&) is invoked, this would invoke M(S&).
+        // new (&m_val) T(params...);
+
+        // GOOD: perfectly forwards params to constructor of T.
+        new (m_val) T(std::forward<Params>(params)...);
+    }
+
+    ~option() {
+        reset();
+    }
+
+    constexpr T& value() {
+        assert(m_has_val);
+        return *reinterpret_cast<T*>(m_val);
+    }
+
+  private:
+    constexpr void reset() {
+        if (!m_has_val) {
+            return;
+        }
+        if constexpr (!std::is_trivially_destructible_v<T>) {
+            value().~T();
+        };
+    }
+
+    alignas(T) char m_val[sizeof(T)];
+    bool m_has_val{false};
+};
+
+int main() {
+    std::puts("==> case 1");
+    // invokes M(S&&, int)
+    option<M> opt1(S{}, 123);
+
+    std::puts("==> case 2");
+    // invokes M() + M(M&&)
+    option<M> x /* option(M&&) + M(M&&) */ = M{} /* M() */;
+}