Declare interface constants as enum in C++

While constexpr hints to the compiler that it can optimize
away the address of these integer expressions, it does not
mandate this behavior.  This can cause linkage errors when
the same generated header is included from multiple compilation
units and everyone expects the integer constant to defined
elsewhere.

Bug: 26942276
Test: unittests pass

Change-Id: I7dfda08b5226344d5169b3f298413cd8a1ceb692

1 files changed, 6 insertions, 3 deletions

diff --git a/generate_cpp.cpp b/generate_cpp.cpp
index de4ab5a..f9ce9f7 100644
--- a/generate_cpp.cpp
+++ b/generate_cpp.cpp
@@ -676,10 +676,13 @@ unique_ptr<Document> BuildInterfaceHeader(const TypeNamespace& types,
       "DECLARE_META_INTERFACE",
       ArgList{vector<string>{ClassName(interface, ClassNames::BASE)}}}});
 
+  unique_ptr<Enum> constant_enum{new Enum{"", "int32_t"}};
   for (const auto& constant : interface.GetConstants()) {
-    unique_ptr<ConstDecl> declaration{
-        new ConstDecl(constant->GetName(), constant->GetValue())};
-    if_class->AddPublic(std::move(declaration));
+    constant_enum->AddValue(
+        constant->GetName(), std::to_string(constant->GetValue()));
+  }
+  if (constant_enum->HasValues()) {
+    if_class->AddPublic(std::move(constant_enum));
   }
 
   unique_ptr<Enum> call_enum{new Enum{"Call"}};