From fc20e61e7249006247b43f84a3189d5a37fa103e Mon Sep 17 00:00:00 2001
From: Timothy Flynn <trflynn89@pm.me>
Date: Mon, 20 Jan 2025 11:17:23 -0500
Subject: [PATCH] AK: Introduce an as_if<OutputType>() cast

We currently have a fair amount of code of the form:

    if (is<Child>(parent)) {
        auto& child = static_cast<Child&>(parent);

        if (child.foo()) {
        }
    }

This new cast allows us to instead write code of the form:

    if (auto* child = as_if<Child>(parent); child && child->foo()) {
    }

N.B. The name "as_if" was chosen because we are considering renaming
verify_cast to "as".

Co-authored-by: Tim Ledbetter <tim.ledbetter@ladybird.org>
---
 AK/TypeCasts.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/AK/TypeCasts.h b/AK/TypeCasts.h
index c076f5cae84..1b0721bef18 100644
--- a/AK/TypeCasts.h
+++ b/AK/TypeCasts.h
@@ -51,9 +51,26 @@ ALWAYS_INLINE CopyConst<InputType, OutputType>& verify_cast(InputType& input)
     return static_cast<CopyConst<InputType, OutputType>&>(input);
 }
 
+template<typename OutputType, typename InputType>
+ALWAYS_INLINE CopyConst<InputType, OutputType>* as_if(InputType& input)
+{
+    if (!is<OutputType>(input))
+        return nullptr;
+    return static_cast<CopyConst<InputType, OutputType>*>(&input);
+}
+
+template<typename OutputType, typename InputType>
+ALWAYS_INLINE CopyConst<InputType, OutputType>* as_if(InputType* input)
+{
+    if (!input)
+        return nullptr;
+    return as_if<OutputType>(*input);
+}
+
 }
 
 #if USING_AK_GLOBALLY
+using AK::as_if;
 using AK::is;
 using AK::verify_cast;
 #endif