Add type_id templates

C++ templates to assist in type conversions between C++
types and dbus types.  These templates provide a compile-time
conversion between built-in types and a C-string of dbus types.

Example:
    uint32_t a = 0;
    const char* b = "a string";
    auto t = sdbusplus::message::types::type_id(a, b);

This results in a tuple equal to std::make_tuple('u','s', '\0')', which
can be converted to a C-string("us") by tuple_to_array.

In a subsequent commit this interface will be used to do a
compile-time transformation from something like:
    msg->append(a, b)
To:
    sd_bus_message_append(msg, "us", a, b);

The type_id template can be extended to support tuples, STL
containers, and user-defined types as needed.

Change-Id: I913501b51ffc24bcf4219f4297a1e0f8ebed97b5
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>

2 files changed, 190 insertions, 0 deletions

diff --git a/sdbusplus/message/types.hpp b/sdbusplus/message/types.hpp
new file mode 100644
index 0000000..6207a56
--- /dev/null
+++ b/sdbusplus/message/types.hpp
@@ -0,0 +1,157 @@
+#pragma once
+
+#include <tuple>
+#include <string>
+#include <vector>
+
+#include <sdbusplus/utility/type_traits.hpp>
+
+namespace sdbusplus
+{
+
+namespace message
+{
+
+namespace types
+{
+
+/** @fn type_id()
+ *  @brief Get a tuple containing the dbus type character(s) for a
+ *         sequence of C++ types.
+ *
+ *  @tparam ...Args - Types to get the type_id sequence for.
+ *  @returns A tuple of characters representing the dbus types for ...Args.
+ *
+ *  The design uses a tuple because a tuple can, at compile-time, be easily
+ *  appended to, concatenated, and converted to an array (representing a
+ *  C-string).  There are other options to create a string of characters but
+ *  they require runtime processing and memory allocation.  By performing all
+ *  options at compile-time the use of type-deduced dbus strings is equal to
+ *  the cost of hard-coded type string constants.
+ */
+template <typename ...Args> constexpr auto type_id();
+
+namespace details
+{
+
+/** @struct undefined_type_id
+ *  @brief Special type indicating no dbus-type_id is defined for a C++ type.
+ */
+struct undefined_type_id
+{
+    /** An empty tuple indicating no type-characters. */
+    // We want this to be tuple so that we can use operations like
+    // tuple_cat on it without cascading compile failures.  There is a
+    // static_assert in type_id_single to ensure this is never used, but to
+    // keep the compile failures as clean as possible.
+    static constexpr auto value = std::make_tuple();
+};
+
+/** @struct tuple_type_id
+ *  @brief Special type indicating a tuple of dbus-type_id's.
+ *
+ *  @tparam C1 - The first dbus type character character.
+ *  @tparam ...C - The remaining sequence of dbus type characters.
+ *
+ *  A tuple_type_id must be one or more characters.  The C1 template param
+ *  ensures at least one is present.
+ */
+template <char C1, char... C> struct tuple_type_id
+{
+
+    /* This version check is required because a fix for auto is in 5.2+.
+     * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66421
+     */
+    /** A tuple containing the type-characters. */
+#if (__GNUC__ > 5) || (__GNUC__ == 5 && (__GNUC_MINOR__ >= 2))
+    static constexpr auto value = std::make_tuple(C1, C...);
+#else
+    static constexpr decltype(std::make_tuple(C1, C...)) value =
+        std::make_tuple(C1, C...);
+#endif
+};
+
+/** @fn type_id_single()
+ *  @brief Get a tuple containing the dbus type character(s) for a C++ type.
+ *
+ *  @tparam T - The type to get the dbus type character(s) for.
+ */
+template <typename T> constexpr auto& type_id_single();
+
+/** @fn type_id_multiple()
+ *  @brief Get a tuple containing the dbus type characters for a sequence of
+ *         C++ types.
+ *
+ *  @tparam T - The first type to get the dbus type character(s) for.
+ *  @tparam ...Args - The remaining types.
+ */
+template <typename T, typename ...Args> constexpr auto type_id_multiple();
+
+/** @struct type_id
+ *  @brief Defined dbus type tuple for a C++ type.
+ *
+ *  @tparam T - C++ type.
+ *
+ *  Struct must have a 'value' tuple containing the dbus type.  The default
+ *  value is an empty tuple, which can be used to indicate
+ */
+template <typename T> struct type_id : public undefined_type_id {};
+    // Specializations for built-in types.
+template <> struct type_id<bool> : tuple_type_id<'b'> {};
+template <> struct type_id<uint8_t> : tuple_type_id<'y'> {};
+// int8_t isn't supported by dbus.
+template <> struct type_id<uint16_t> : tuple_type_id<'q'> {};
+template <> struct type_id<int16_t> : tuple_type_id<'n'> {};
+template <> struct type_id<uint32_t> : tuple_type_id<'u'> {};
+template <> struct type_id<int32_t> : tuple_type_id<'i'> {};
+template <> struct type_id<uint64_t> : tuple_type_id<'t'> {};
+template <> struct type_id<int64_t> : tuple_type_id<'x'> {};
+// float isn't supported by dbus.
+template <> struct type_id<double> : tuple_type_id<'d'> {};
+template <> struct type_id<const char*> : tuple_type_id<'s'> {};
+template <> struct type_id<char*> : tuple_type_id<'s'> {};
+template <> struct type_id<std::string> : tuple_type_id<'s'> {};
+
+template <typename T> constexpr auto& type_id_single()
+{
+    static_assert(!std::is_base_of<undefined_type_id, type_id<T>>::value,
+        "No dbus type conversion provided for type.");
+    return type_id<T>::value;
+}
+
+template <typename T, typename ...Args> constexpr auto type_id_multiple()
+{
+    return std::tuple_cat(type_id_single<T>(),
+        type_id_single<Args>()...);
+}
+
+/** @brief Convert some C++ types to others for 'type_id' conversion purposes.
+ *
+ *  Similar C++ types have the same dbus type-id, so 'downcast' those to limit
+ *  duplication in type_id template specializations.
+ *
+ *  1. Remove references.
+ *  2. Remove 'const' and 'vector'.
+ *  3. Convert 'char[N]' to 'char*'.
+ */
+template <typename T> using type_id_downcast_t =
+        utility::array_to_ptr_t<char,
+            std::remove_cv_t<
+                std::remove_reference_t<T>
+            >
+        >;
+
+} // namespace details
+
+template <typename ...Args> constexpr auto type_id()
+{
+    return std::tuple_cat(
+        details::type_id_multiple<details::type_id_downcast_t<Args>...>(),
+        std::make_tuple('\0') /* null terminator for C-string */ );
+}
+
+} // namespace types
+
+} // namespace message
+
+} // namespace sdbusplus
diff --git a/test/message/types.cpp b/test/message/types.cpp
new file mode 100644
index 0000000..4172cb6
--- /dev/null
+++ b/test/message/types.cpp
@@ -0,0 +1,33 @@
+#include <cassert>
+#include <sdbusplus/message/types.hpp>
+#include <sdbusplus/utility/tuple_to_array.hpp>
+
+template <typename ...Args>
+auto dbus_string(Args&& ... args)
+{
+    return std::string(
+               sdbusplus::utility::tuple_to_array(
+                   sdbusplus::message::types::type_id<Args...>()).data());
+}
+
+int main()
+{
+
+    // Check a few simple types.
+    assert(dbus_string(1) == "i");
+    assert(dbus_string(1.0) == "d");
+
+    // Check a multiple parameter call.
+    assert(dbus_string(false, true) == "bb");
+    assert(dbus_string(1, 2, 3, true, 1.0) == "iiibd");
+
+    // Check an l-value and r-value reference.
+    {
+        std::string a = "a";
+        std::string b = "b";
+        const char* c = "c";
+
+        assert(dbus_string(a, std::move(b), c) == "sss");
+    }
+    return 0;
+}