util/TemplateString: rewrite as constexpr functions

src/lib/dbus/AppendIter.hxx

@@ -103,7 +103,7 @@ public:
 	template<typename T>
 	AppendMessageIter &AppendEmptyArray() {
 		return AppendMessageIter(*this, DBUS_TYPE_ARRAY,
-					 T::TypeAsString::value)
+					 T::as_string.value)
 			.CloseContainer(*this);
 	}
 
@@ -120,7 +120,7 @@ public:
 	AppendMessageIter &AppendVariant(const T &value) {
 		typedef VariantTypeTraits Traits;
 		return AppendMessageIter(*this, Traits::TYPE,
-					 Traits::TypeAsString::value)
+					 Traits::as_string.value)
 			.Append(value)
 			.CloseContainer(*this);
 	}
@@ -146,7 +146,7 @@ public:
 		typedef typename W::ContainedTraits ContainedTraits;
 
 		return AppendMessageIter(*this, Traits::TYPE,
-					 ContainedTraits::TypeAsString::value)
+					 ContainedTraits::as_string.value)
 			.Append(value.value)
 			.CloseContainer(*this);
 	}
@@ -158,7 +158,7 @@ public:
 		typedef typename W::ContainedTraits ContainedTraits;
 
 		return AppendMessageIter(*this, Traits::TYPE,
-					 ContainedTraits::TypeAsString::value)
+					 ContainedTraits::as_string.value)
 			.AppendFixedArray(value.value)
 			.CloseContainer(*this);
 	}

src/lib/dbus/Types.hxx

@@ -42,7 +42,7 @@ namespace ODBus {
 template<int type>
 struct BasicTypeTraits {
 	static constexpr int TYPE = type;
-	using TypeAsString = TemplateString::CharAsString<TYPE>;
+	static constexpr auto as_string = TemplateString::CharAsString(TYPE);
 };
 
 template<typename T>
@@ -73,20 +73,20 @@ struct ArrayTypeTraits {
 	using ContainedTraits = T;
 
 	static constexpr int TYPE = DBUS_TYPE_ARRAY;
-	using TypeAsString =
-		TemplateString::InsertBefore<TYPE,
-					     typename ContainedTraits::TypeAsString>;
+	static constexpr auto as_string =
+		TemplateString::Concat(TemplateString::CharAsString(TYPE),
+				       ContainedTraits::as_string);
 };
 
 template<typename KeyT, typename ValueT>
 struct DictEntryTypeTraits {
 	static constexpr int TYPE = DBUS_TYPE_DICT_ENTRY;
 
-	using TypeAsString =
-		TemplateString::Concat<TemplateString::CharAsString<DBUS_DICT_ENTRY_BEGIN_CHAR>,
-				       typename KeyT::TypeAsString,
-				       typename ValueT::TypeAsString,
-				       TemplateString::CharAsString<DBUS_DICT_ENTRY_END_CHAR>>;
+	static constexpr auto as_string =
+		TemplateString::Concat(TemplateString::CharAsString(DBUS_DICT_ENTRY_BEGIN_CHAR),
+				       KeyT::as_string,
+				       ValueT::as_string,
+				       TemplateString::CharAsString(DBUS_DICT_ENTRY_END_CHAR));
 };
 
 using VariantTypeTraits = BasicTypeTraits<DBUS_TYPE_VARIANT>;
@@ -95,21 +95,25 @@ using VariantTypeTraits = BasicTypeTraits<DBUS_TYPE_VARIANT>;
  * Concatenate all TypeAsString members to one string.
  */
 template<typename T, typename... ContainedTraits>
-struct ConcatTypeAsString
-	: TemplateString::Concat<typename T::TypeAsString,
-				 ConcatTypeAsString<ContainedTraits...>> {};
+struct ConcatTypeAsString {
+	static constexpr auto as_string =
+		TemplateString::Concat(T::as_string,
+				       ConcatTypeAsString<ContainedTraits...>::as_string);
+};
 
 template<typename T>
-struct ConcatTypeAsString<T> : T::TypeAsString {};
+struct ConcatTypeAsString<T> {
+	static constexpr auto as_string = T::as_string;
+};
 
 template<typename... ContainedTraits>
 struct StructTypeTraits {
 	static constexpr int TYPE = DBUS_TYPE_STRUCT;
 
-	using TypeAsString =
-		TemplateString::Concat<TemplateString::CharAsString<DBUS_STRUCT_BEGIN_CHAR>,
-				       ConcatTypeAsString<ContainedTraits...>,
-				       TemplateString::CharAsString<DBUS_STRUCT_END_CHAR>>;
+	static constexpr auto as_string =
+		TemplateString::Concat(TemplateString::CharAsString(DBUS_STRUCT_BEGIN_CHAR),
+				       ConcatTypeAsString<ContainedTraits...>::as_string,
+				       TemplateString::CharAsString(DBUS_STRUCT_END_CHAR));
 };
 
 } /* namespace ODBus */

src/neighbor/plugins/UdisksNeighborPlugin.cxx

@@ -245,7 +245,7 @@ UdisksNeighborExplorer::HandleMessage(DBusConnection *, DBusMessage *message) no
 
 	if (dbus_message_is_signal(message, DBUS_OM_INTERFACE,
 				   "InterfacesAdded") &&
-	    dbus_message_has_signature(message, InterfacesAddedType::value)) {
+	    dbus_message_has_signature(message, InterfacesAddedType::as_string.value)) {
 		RecurseInterfaceDictEntry(ReadMessageIter(*message), [this](const char *path, auto &&i){
 				UDisks2::Object o(path);
 				UDisks2::ParseObject(o, std::forward<decltype(i)>(i));
@@ -256,7 +256,7 @@ UdisksNeighborExplorer::HandleMessage(DBusConnection *, DBusMessage *message) no
 		return DBUS_HANDLER_RESULT_HANDLED;
 	} else if (dbus_message_is_signal(message, DBUS_OM_INTERFACE,
 					  "InterfacesRemoved") &&
-		   dbus_message_has_signature(message, InterfacesRemovedType::value)) {
+		   dbus_message_has_signature(message, InterfacesRemovedType::as_string.value)) {
 		Remove(ReadMessageIter(*message).GetString());
 		return DBUS_HANDLER_RESULT_HANDLED;
 	} else

src/util/TemplateString.hxx

@@ -30,95 +30,93 @@
 #ifndef TEMPLATE_STRING_HXX
 #define TEMPLATE_STRING_HXX
 
+#include <array> // for std::size()
 #include <cstddef>
 
 namespace TemplateString {
 
-/**
- * Construct a null-terminated string from a list of chars.
- */
-template<char... _value>
-struct Construct {
-	static constexpr char value[] = {_value..., 0};
-	static constexpr std::size_t size = sizeof...(_value);
+template<std::size_t _size>
+struct Buffer {
+	static constexpr std::size_t size = _size;
+	char value[size + 1];
 };
 
 /**
  * An empty string.
  */
-struct Empty : Construct<> {};
+constexpr auto
+Empty() noexcept
+{
+	return Buffer<0>{};
+}
 
 /**
  * A string consisting of a single character.
  */
-template<char ch>
-struct CharAsString : Construct<ch> {};
-
-/**
- * Invoke #F, pass all characters in #src from #i to #length
- * as variadic arguments.
- */
-template<template<char...> class F,
-	 const char *src, std::size_t length, std::size_t i,
-	 char... _value>
-struct VariadicChars : VariadicChars<F, src, length - 1, i + 1, _value..., src[i]> {
-	static_assert(length > 0, "Wrong length");
-};
-
-template<template<char...> class F,
-	 const char *src, std::size_t length,
-	 char... _value>
-struct VariadicChars<F, src, 0, length, _value...> : F<_value...> {};
-
-/**
- * Like #VariadicChars, but pass an additional argument to #F.
- */
-template<template<typename Arg, char...> class F, typename Arg,
-	 const char *src, std::size_t length, std::size_t i,
-	 char... _value>
-struct VariadicChars1 : VariadicChars1<F, Arg,
-				       src, length - 1, i + 1, _value..., src[i]> {
-	static_assert(length > 0, "Wrong length");
-};
-
-template<template<typename Arg, char...> class F, typename Arg,
-	 const char *src, std::size_t length,
-	 char... _value>
-struct VariadicChars1<F, Arg, src, 0, length, _value...> : F<Arg, _value...> {};
+constexpr auto
+CharAsString(char ch) noexcept
+{
+	Buffer<1> result{};
+	result.value[0] = ch;
+	return result;
+}
+
+namespace detail {
+
+constexpr auto
+size(const char &) noexcept
+{
+	return 1;
+}
+
+constexpr const char *
+data(const char &ch) noexcept
+{
+	return &ch;
+}
+
+template<std::size_t s>
+constexpr auto
+size(const Buffer<s> &b) noexcept
+{
+	return b.size;
+}
+
+template<std::size_t size>
+constexpr const char *
+data(const Buffer<size> &b) noexcept
+{
+	return b.value;
+}
+
+constexpr char *
+copy_n(const char *src, std::size_t n, char *dest) noexcept
+{
+	for (std::size_t i = 0; i < n; ++i)
+		dest[i] = src[i];
+	return dest + n;
+}
+
+}
 
-template<const char *src, std::size_t length, char... value>
-struct _BuildString : VariadicChars<Construct, src, length, 0,
-				    value...> {};
-
-template<char ch, typename S>
-struct InsertBefore : _BuildString<S::value, S::size, ch> {};
-
-/**
- * Concatenate several strings.
- */
 template<typename... Args>
-struct Concat;
-
-template<typename First, typename Second, typename... Args>
-struct _Concat : Concat<Concat<First, Second>, Args...> {};
-
-template<typename... Args>
-struct Concat : _Concat<Args...> {};
-
-template<typename Second, char... _value>
-struct _Concat2 : _BuildString<Second::value, Second::size,
-			       _value...> {};
-
-template<typename First, typename Second>
-struct Concat<First, Second>
-	:VariadicChars1<_Concat2, Second,
-			First::value, First::size, 0> {};
-
-template<typename First>
-struct Concat<First> : First {};
-
-template<>
-struct Concat<> : Empty {};
+constexpr auto
+Concat(Args... args) noexcept
+{
+	using std::size;
+	using std::data;
+	using detail::size;
+	using detail::data;
+	using detail::copy_n;
+
+	constexpr std::size_t total_size = (std::size_t(0) + ... + size(args));
+	Buffer<total_size> result{};
+
+	char *p = result.value;
+	((p = copy_n(data(args), size(args), p)), ...);
+
+	return result;
+}
 
 } // namespace TemplateString