std::format support (#1025)

Author: sylveon

cppwinrt/code_writers.h

@@ -115,6 +115,16 @@ namespace cppwinrt
         }
     }
 
+    [[nodiscard]] static finish_with wrap_ifdef(writer& w, std::string_view macro)
+    {
+        auto format = R"(#ifdef %
+)";
+
+        w.write(format, macro);
+
+        return { w, write_endif };
+    }
+
     static void write_parent_depends(writer& w, cache const& c, std::string_view const& type_namespace)
     {
         auto pos = type_namespace.rfind('.');
@@ -3216,6 +3226,18 @@ struct __declspec(empty_bases) produce_dispatch_to_overridable<T, D, %>
             type);
     }
 
+    static void write_std_formatter(writer& w, TypeDef const& type)
+    {
+        if (implements_interface(type, "Windows.Foundation.IStringable"))
+        {
+            auto generics = type.GenericParam();
+
+            w.write("    template<%> struct formatter<%, wchar_t> : formatter<winrt::Windows::Foundation::IStringable, wchar_t> {};\n",
+                bind<write_generic_typenames>(generics),
+                type);
+        }
+    }
+
     static void write_namespace_special(writer& w, std::string_view const& namespace_name)
     {
         if (namespace_name == "Windows.Foundation")
@@ -3259,6 +3281,7 @@ struct __declspec(empty_bases) produce_dispatch_to_overridable<T, D, %>
         if (namespace_name == "Windows.Foundation")
         {
             w.write(strings::base_reference_produce_1);
+            w.write(strings::base_stringable_format);
         }
     }
 }

cppwinrt/cppwinrt.vcxproj

@@ -81,6 +81,7 @@
     <ClInclude Include="..\strings\base_string.h" />
     <ClInclude Include="..\strings\base_string_input.h" />
     <ClInclude Include="..\strings\base_string_operators.h" />
+    <ClInclude Include="..\strings\base_stringable_format.h" />
     <ClInclude Include="..\strings\base_types.h" />
     <ClInclude Include="..\strings\base_version.h" />
     <ClInclude Include="..\strings\base_version_odr.h" />

cppwinrt/cppwinrt.vcxproj.filters

@@ -169,6 +169,9 @@
     <ClInclude Include="..\strings\base_iterator.h">
       <Filter>strings</Filter>
     </ClInclude>
+    <ClInclude Include="..\strings\base_stringable_format.h">
+      <Filter>strings</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <ResourceCompile Include="$(OutDir)version.rc" />

cppwinrt/file_writers.h

@@ -197,9 +197,17 @@ namespace cppwinrt
         }
         {
             auto wrap_std = wrap_std_namespace(w);
-            auto wrap_lean = wrap_lean_and_mean(w);
-            w.write_each<write_std_hash>(members.interfaces);
-            w.write_each<write_std_hash>(members.classes);
+
+            {
+                auto wrap_lean = wrap_lean_and_mean(w);
+                w.write_each<write_std_hash>(members.interfaces);
+                w.write_each<write_std_hash>(members.classes);
+            }
+            {
+                auto wrap_format = wrap_ifdef(w, "__cpp_lib_format");
+                w.write_each<write_std_formatter>(members.interfaces);
+                w.write_each<write_std_formatter>(members.classes);   
+            }
         }
 
         write_namespace_special(w, ns);

cppwinrt/helpers.h

@@ -498,6 +498,27 @@ namespace cppwinrt
         return result;
     }
 
+    static bool implements_interface(TypeDef const& type, std::string_view const& name)
+    {
+        for (auto&& impl : type.InterfaceImpl())
+        {
+            const auto iface = impl.Interface();
+            if (iface.type() != TypeDefOrRef::TypeSpec && type_name(iface) == name)
+            {
+                return true;
+            }
+        }
+
+        if (auto base = get_base_class(type))
+        {
+            return implements_interface(base, name);
+        }
+        else
+        {
+            return false;
+        }
+    }
+
     bool has_fastabi_tearoffs(writer& w, TypeDef const& type)
     {
         for (auto&& [name, info] : get_interfaces(w, type))

strings/base_includes.h

@@ -24,6 +24,10 @@
 #include <directxmath.h>
 #endif
 
+#ifdef __cpp_lib_format
+#include <format>
+#endif
+
 #ifdef __cpp_lib_coroutine
 
 #include <coroutine>

strings/base_string.h

@@ -327,7 +327,7 @@ WINRT_EXPORT namespace winrt
             return rend();
         }
 
-#if __cpp_lib_starts_ends_with
+#ifdef __cpp_lib_starts_ends_with
         bool starts_with(wchar_t const value) const noexcept
         {
             return operator std::wstring_view().starts_with(value);
@@ -437,6 +437,11 @@ WINRT_EXPORT namespace winrt
     }
 }
 
+#ifdef __cpp_lib_format
+template<>
+struct std::formatter<winrt::hstring, wchar_t> : std::formatter<std::wstring_view, wchar_t> {};
+#endif
+
 namespace winrt::impl
 {
     template <> struct abi<hstring>

strings/base_stringable_format.h

@@ -0,0 +1,12 @@
+
+#ifdef __cpp_lib_format
+template<>
+struct std::formatter<winrt::Windows::Foundation::IStringable, wchar_t> : std::formatter<winrt::hstring, wchar_t>
+{
+    template<typename FormatContext>
+    auto format(winrt::Windows::Foundation::IStringable const& obj, FormatContext& fc)
+    {
+        return std::formatter<winrt::hstring, wchar_t>::format(obj.ToString(), fc);
+    }
+};
+#endif

test/test_cpp20/format.cpp

@@ -0,0 +1,28 @@
+#include "pch.h"
+#include <format>
+
+struct stringable : winrt::implements<stringable, winrt::Windows::Foundation::IStringable>
+{
+    winrt::hstring ToString()
+    {
+        return L"a stringable object";
+    }
+};
+
+TEST_CASE("format")
+{
+    {
+        winrt::hstring str = L"World";
+        REQUIRE(std::format(L"Hello {}", str) == L"Hello World");
+    }
+
+    {
+        winrt::Windows::Foundation::IStringable obj = winrt::make<stringable>();
+        REQUIRE(std::format(L"This is {}", obj) == L"This is a stringable object");
+    }
+
+    {
+        winrt::Windows::Data::Json::JsonArray jsonArray;
+        REQUIRE(std::format(L"The contents of the array are: {}", jsonArray) == L"The contents of the array are: []");
+    }
+}

test/test_cpp20/hstring.cpp

@@ -10,4 +10,4 @@ TEST_CASE("hstring")
 	REQUIRE(text.ends_with(L"rocks!"));
 	REQUIRE(textView.ends_with(L"rocks!"));
 	REQUIRE(text.ends_with(L"rocks!") == textView.ends_with(L"rocks!"));
-}
+}