cppgc: Fix hello world

The hello world sample needs to initialize V8's default platform in case
it is not built as stand-alone example.

Bug: v8:12427
Change-Id: I78b68fbed2c2a25b0ff03675beb94dfc5b9b4135
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3412088Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78756}

include/cppgc/default-platform.h

@@ -63,6 +63,8 @@ class V8_EXPORT DefaultPlatform : public Platform {
     return v8_platform_->GetTracingController();
   }
 
+  v8::Platform* GetV8Platform() const { return v8_platform_.get(); }
+
  protected:
   static constexpr v8::Isolate* kNoIsolate = nullptr;
 

samples/cppgc/hello-world.cc

@@ -2,17 +2,21 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include <cppgc/allocation.h>
-#include <cppgc/default-platform.h>
-#include <cppgc/garbage-collected.h>
-#include <cppgc/heap.h>
-#include <cppgc/member.h>
-#include <cppgc/visitor.h>
-
 #include <iostream>
 #include <memory>
 #include <string>
 
+#include "include/cppgc/allocation.h"
+#include "include/cppgc/default-platform.h"
+#include "include/cppgc/garbage-collected.h"
+#include "include/cppgc/heap.h"
+#include "include/cppgc/member.h"
+#include "include/cppgc/visitor.h"
+
+#if !CPPGC_IS_STANDALONE
+#include "include/v8-initialization.h"
+#endif  // !CPPGC_IS_STANDALONE
+
 /**
  * This sample program shows how to set up a stand-alone cppgc heap.
  */
@@ -45,6 +49,12 @@ int main(int argc, char* argv[]) {
   // Create a default platform that is used by cppgc::Heap for execution and
   // backend allocation.
   auto cppgc_platform = std::make_shared<cppgc::DefaultPlatform>();
+#if !CPPGC_IS_STANDALONE
+  // When initializing a stand-alone cppgc heap in a regular V8 build, the
+  // internal V8 platform will be reused. Reusing the V8 platform requires
+  // initializing it properly.
+  v8::V8::InitializePlatform(cppgc_platform->GetV8Platform());
+#endif  // !CPPGC_IS_STANDALONE
   // Initialize the process. This must happen before any cppgc::Heap::Create()
   // calls.
   cppgc::DefaultPlatform::InitializeProcess(cppgc_platform.get());