AK: Add Formatter<FormatString> as helper class.

3 files changed, 37 insertions, 1 deletions

diff --git a/AK/Format.cpp b/AK/Format.cpp
index 14ed0459dd..0132807ed4 100644
--- a/AK/Format.cpp
+++ b/AK/Format.cpp
@@ -521,6 +521,11 @@ void Formatter<StringView>::format(FormatBuilder& builder, StringView value)
     builder.put_string(value, m_align, m_width.value(), m_precision.value(), m_fill);
 }
 
+void Formatter<FormatString>::vformat(FormatBuilder& builder, StringView fmtstr, TypeErasedFormatParams params)
+{
+    return Formatter<String>::format(builder, String::vformatted(fmtstr, params));
+}
+
 template<typename T>
 void Formatter<T, typename EnableIf<IsIntegral<T>::value>::Type>::format(FormatBuilder& builder, T value)
 {
diff --git a/AK/Format.h b/AK/Format.h
index 90030765ec..63b1d3fab2 100644
--- a/AK/Format.h
+++ b/AK/Format.h
@@ -359,7 +359,7 @@ struct Formatter<std::nullptr_t> : Formatter<FlatPtr> {
     }
 };
 
-void vformat(StringBuilder& builder, StringView fmtstr, TypeErasedFormatParams);
+void vformat(StringBuilder&, StringView fmtstr, TypeErasedFormatParams);
 void vformat(const LogStream& stream, StringView fmtstr, TypeErasedFormatParams);
 
 #ifndef KERNEL
@@ -435,6 +435,20 @@ template<typename T>
 struct Formatter<FormatIfSupported<T>> : __FormatIfSupported<T, HasFormatter<T>::value> {
 };
 
+// This is a helper class, the idea is that if you want to implement a formatter you can inherit
+// from this class to "break down" the formatting.
+struct FormatString {
+};
+template<>
+struct Formatter<FormatString> : Formatter<String> {
+    template<typename... Parameters>
+    void format(FormatBuilder& builder, StringView fmtstr, const Parameters&... parameters)
+    {
+        vformat(builder, fmtstr, VariadicFormatParams { parameters... });
+    }
+    void vformat(FormatBuilder& builder, StringView fmtstr, TypeErasedFormatParams params);
+};
+
 } // namespace AK
 
 #ifndef KERNEL
@@ -448,3 +462,4 @@ using AK::warnln;
 using AK::dbgln;
 
 using AK::FormatIfSupported;
+using AK::FormatString;
diff --git a/AK/Tests/TestFormat.cpp b/AK/Tests/TestFormat.cpp
index cfe7fe2950..609582b6ae 100644
--- a/AK/Tests/TestFormat.cpp
+++ b/AK/Tests/TestFormat.cpp
@@ -274,4 +274,20 @@ TEST_CASE(format_nullptr)
     EXPECT_EQ(String::formatted("{}", nullptr), String::formatted("{:p}", static_cast<FlatPtr>(0)));
 }
 
+struct C {
+    int i;
+};
+template<>
+struct AK::Formatter<C> : AK::Formatter<FormatString> {
+    void format(FormatBuilder& builder, C c)
+    {
+        return AK::Formatter<FormatString>::format(builder, "C(i={})", c.i);
+    }
+};
+
+TEST_CASE(use_format_string_formatter)
+{
+    EXPECT_EQ(String::formatted("{:*<10}", C { 42 }), "C(i=42)***");
+}
+
 TEST_MAIN(Format)