add padding section

Author: Yaraslaut

.github/DockerClang

@@ -1,12 +1,13 @@
 FROM ubuntu:latest
 
+# git clone -b p2996 https://github.com/Yaraslaut/clang-p2996.git && \
 WORKDIR /
 RUN apt-get update
 RUN apt-get install -y cmake python3 ninja-build git lsb-release software-properties-common wget binutils gcc g++
 #RUN bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)"
 RUN bash -c "\
     set -ex && \
-    git clone -b p2996 https://github.com/Yaraslaut/clang-p2996.git && \
+    git clone -b p2996 https://github.com/bloomberg/clang-p2996.git && \
     cmake -S /clang-p2996/llvm \
         -B build \
         -G Ninja \

README.md

@@ -55,6 +55,41 @@ void EnumToString() {
 }
 ```
 
+## Padding check at compile time
+
+Calculate padding at compile time and enforce zero padding via concepts
+
+``` c++
+
+
+struct struct_with_padding {
+  char a;
+  double c;
+};
+
+struct struct_no_padding {
+  double c;
+  char a;
+};
+
+template <form::no_padding T> void foo(T t) { 
+  std::println("Without padding"); 
+}
+
+template <typename T> void foo(T t) {
+  std::println("With padding: {}", form::get_padding<T>());
+}
+
+
+void PaddingCheck() {
+  foo(struct_with_padding{}); // With padding: 7
+  foo(struct_no_padding{});   // Without padding
+  static_assert(form::no_padding<struct_no_padding>);
+}
+
+
+```
+
 `
 
 ## Variant type to string

include/form/form.h

@@ -102,6 +102,27 @@ template <typename T> constexpr void print_members() {
   };
 }
 
+template <typename S> consteval std::size_t get_padding() {
+
+  std::size_t padding{0};
+  std::size_t pointer{offset_of(nonstatic_data_members_of(^S)[0])};
+
+  [: util::expand(nonstatic_data_members_of(^S)):] >> [&, i = 0]<auto e>() {
+    if (pointer == offset_of(e))
+      pointer += size_of(e);
+    else {
+      padding += offset_of(e) - pointer;
+      pointer = offset_of(e) + size_of(e);
+    }
+  };
+  return padding;
+}
+
+template <typename T>
+concept no_padding =
+    std::same_as<std::integral_constant<std::size_t, get_padding<T>()>,
+                 std::integral_constant<std::size_t, 0>>;
+
 } // namespace form
 
 template <> struct std::formatter<std::basic_string_view<char8_t>> {

src/test.h

@@ -334,6 +334,22 @@ enum class Decorator : uint8_t {
   Encircle,
 };
 
+struct struct_with_padding {
+  char a;
+  double c;
+};
+
+struct struct_no_padding {
+  double c;
+  char a;
+};
+
+template <form::no_padding T> void foo(T t) { std::println("Without padding"); }
+
+template <typename T> void foo(T t) {
+  std::println("With padding: {}", form::get_padding<T>());
+}
+
 namespace form::examples {
 bool VariantCreate() {
   list_variant v{list::CancelSelection{}};
@@ -366,6 +382,13 @@ bool EnumToStringWithTransform() {
   return res;
 }
 
+void PaddingCheck() {
+
+  foo(struct_with_padding{});
+  foo(struct_no_padding{});
+  static_assert(form::no_padding<struct_no_padding>);
+}
+
 } // namespace form::examples
 
 void ExampleConfig() {