Objective-C++ 中的 C++ 对象何时销毁？

Question

我试图通过创建一个 Objective-C++ class 来包装它来公开一个 C++ 对象。

最终，在 Swift 中，我正在尝试编写：

print(JSApplication.eval("'TKTK'")?.toString() ?? "")
print(JSApplication.eval("x = {a: 42}; x")?.toString() ?? "")
print(JSApplication.eval("x")?.get("a")?.toString() ?? "nope");
print(JSApplication.eval("1 + 2")?.toInt32() ?? 0)

但是，当我尝试调用 JSApplication.eval("x")?.get("a")?.toString() 时，Objective-C 在 .get("a") 之后 .toString() 之前调用 class 上的 dealloc。

通常在 dealloc 中，我会在 class 包含的共享指针上调用 .reset()。但是由于 dealloc 触发得太早，这会在 .toString() 被调用之前清除我的 V8 结果。

这提出了一个普遍的问题：Swift / Objective-C 如何决定何时对临时对象调用 dealloc？对于像 foo()?.bar()?.baz() 这样的东西，其中 foo 和 bar return 临时对象，在调用 baz 之前两个临时对象都收到 dealloc 消息是否正确？这就是我所看到的。

如果这是正确的行为，那么将临时对象的生命周期延长到调用函数的范围的正确方法是什么，例如 C++？这可能吗？

这是我的 Objective-C++ 绑定。（我注意到根本没有 "Destroy 0x..." 消息被打印出来，所以 C++ 析构函数似乎没有触发。我应该手动调用它们吗？）


// Extracts a C string from a V8 Utf8Value.
const char* ToCString(const v8::String::Utf8Value& value) {
  return *value ? *value : "<string conversion failed>";
}

@interface NJSValue (V8)
- (instancetype)init;
- (instancetype)initWithValue:(Local<Value>)value;
@end

struct NJSRef
{
  std::shared_ptr<Nan::Persistent<Value>> _ref;
  ~NJSRef()
  {
    printf("Destroy 0x%08x\n", (unsigned int)(size_t)_ref.get());
  }
};

@implementation NJSValue (V8)
NJSRef m;

- (instancetype)init
{
  self = [super init];
  return self;
}

- (instancetype)initWithValue:(Local<Value>)value
{
  self = [super init];
  Nan::HandleScope scope;
  Nan::EscapableHandleScope escape;
  m._ref.reset(new Nan::Persistent<Value>(escape.Escape(value)));
  printf("Alloc 0x%08x\n", (unsigned int)(size_t)m._ref.get());
  return self;
}
@end

@implementation NJSValue
- (void)dealloc
{
  printf("Dealloc 0x%08x\n", (unsigned int)(size_t)m._ref.get());
  //m_ref.reset();
}

- ( NSString * _Nonnull )toString
{
  if (m._ref != nullptr) {
    Nan::HandleScope scope;
    Local<Value> value(Nan::New(*m._ref));
    v8::String::Utf8Value str(JS_ISOLATE(), value);
    const char* cstr = ToCString(str);
    return NJSStringToNSString(JS_STR(cstr));
  } else {
    return @"undefined";
  }
}
- (NSNumber *)toInt32
{
  if (m._ref != nullptr) {
    Nan::HandleScope scope;
    Local<Value> value(Nan::New(*m._ref));
    if (!value->IsInt32()) return nullptr;
    return [NSNumber numberWithInt:TO_INT32(value)];
  } else {
    return nullptr;
  }
}

- (NSNumber *)toNumber
{
  if (m._ref != nullptr) {
    Nan::HandleScope scope;
    Local<Value> value(Nan::New(*m._ref));
    if (!value->IsNumber()) return nullptr;
    return [NSNumber numberWithDouble:TO_DOUBLE(value)];
  } else {
    return nullptr;
  }
}


- (NJSValue * _Nullable __strong)get:(NSString * _Nonnull)key  CF_RETURNS_RETAINED
{
  if (m._ref == nullptr) return nullptr;
  v8::HandleScope scope(JS_ISOLATE());
  v8::EscapableHandleScope handle_scope(JS_ISOLATE());
  Local<Value> value(Nan::New(*m._ref));
  if (!value->IsObject()) return nullptr;
  Local<Object> obj(JS_OBJ(value));
  Local<Value> jsKey(JS_STR([key UTF8String]));
  if (!obj->Has(JS_CONTEXT(), jsKey).FromJust()) return nullptr;
  Local<Value> result(obj->Get(jsKey));
  v8::String::Utf8Value str(JS_ISOLATE(), result);
  const char* cstr = ToCString(str);
  printf("got %s\n", cstr);
  NJSValue* ret = [[NJSValue alloc] initWithValue:handle_scope.Escape(result)];
//  [self associateValue:ret withKey:key];
  return ret;
}
@end

@implementation JSApplication

- (instancetype)init
{
   self = [super init];
   if (self) {
   }
   return self;
}

- (instancetype)initWithFrame:(CGRect)frame
{
   self = [super init];
   if (self) {
     self.frame = frame;
   }
   return self;
}

// Executes a string within the current v8 context.
v8::Local<v8::Value>
ExecuteString(v8::Isolate* isolate, v8::Local<v8::String> source,
                   v8::Local<v8::Value> name, bool print_result,
                   bool report_exceptions) {
  v8::EscapableHandleScope handle_scope(isolate);
  v8::TryCatch try_catch(isolate);
  v8::ScriptOrigin origin(name);
  v8::Local<v8::Context> context(isolate->GetCurrentContext());
  v8::Local<v8::Script> script;
  if (!v8::Script::Compile(context, source, &origin).ToLocal(&script)) {
    // Print errors that happened during compilation.
    if (report_exceptions)
      ReportException(isolate, &try_catch);
    return handle_scope.Escape(v8::Undefined(isolate));
  } else {
    v8::Local<v8::Value> result;
    if (!script->Run(context).ToLocal(&result)) {
      assert(try_catch.HasCaught());
      // Print errors that happened during execution.
      if (report_exceptions)
        ReportException(isolate, &try_catch);
      return handle_scope.Escape(v8::Undefined(isolate));
    } else {
      assert(!try_catch.HasCaught());
      if (print_result && !result->IsUndefined()) {
        // If all went well and the result wasn't undefined then print
        // the returned value.
        v8::String::Utf8Value str(isolate, result);
        const char* cstr = ToCString(str);
        printf("eval result: %s\n", cstr);
      }
      return handle_scope.Escape(result);
    }
  }
}


void ReportException(v8::Isolate* isolate, v8::TryCatch* try_catch) {
  v8::HandleScope handle_scope(isolate);
  v8::String::Utf8Value exception(isolate, try_catch->Exception());
  const char* exception_string = ToCString(exception);
  v8::Local<v8::Message> message = try_catch->Message();
  if (message.IsEmpty()) {
    // V8 didn't provide any extra information about this error; just
    // print the exception.
    fprintf(stderr, "%s\n", exception_string);
  } else {
    // Print (filename):(line number): (message).
    v8::String::Utf8Value filename(isolate,
                                   message->GetScriptOrigin().ResourceName());
    v8::Local<v8::Context> context(isolate->GetCurrentContext());
    const char* filename_string = ToCString(filename);
    int linenum = message->GetLineNumber(context).FromJust();
    fprintf(stderr, "%s:%i: %s\n", filename_string, linenum, exception_string);
    // Print line of source code.
    v8::String::Utf8Value sourceline(
        isolate, message->GetSourceLine(context).ToLocalChecked());
    const char* sourceline_string = ToCString(sourceline);
    fprintf(stderr, "%s\n", sourceline_string);
    // Print wavy underline (GetUnderline is deprecated).
    int start = message->GetStartColumn(context).FromJust();
    for (int i = 0; i < start; i++) {
      fprintf(stderr, " ");
    }
    int end = message->GetEndColumn(context).FromJust();
    for (int i = start; i < end; i++) {
      fprintf(stderr, "^");
    }
    fprintf(stderr, "\n");
    v8::Local<v8::Value> stack_trace_string;
    if (try_catch->StackTrace(context).ToLocal(&stack_trace_string) &&
        stack_trace_string->IsString() &&
        v8::Local<v8::String>::Cast(stack_trace_string)->Length() > 0) {
      v8::String::Utf8Value stack_trace(isolate, stack_trace_string);
      const char* stack_trace_string = ToCString(stack_trace);
      fprintf(stderr, "%s\n", stack_trace_string);
    }
  }
}


+ (NJSValue *)Eval:(NSString *)string  __attribute((ns_returns_retained))
{
  Isolate* isolate = Isolate::GetCurrent();
  v8::HandleScope handle_scope(isolate);
  Local<Context> context = isolate->GetCurrentContext();
  v8::Context::Scope context_scope(context);
  const char* str = [string UTF8String];
  Local<Value> result = ExecuteString(
      context->GetIsolate(),
      v8::String::NewFromUtf8(context->GetIsolate(), str,
                              v8::NewStringType::kNormal).ToLocalChecked(),
      JS_STR("JSApplication.Eval"), false, true);

  return [[NJSValue alloc] initWithValue:result];
}

@end

这是我在这个问题顶部的 Swift 代码得到的输出：

TKTK
Alloc 0x81d34230
Dealloc 0x81d34230
Alloc 0x81d34270
Dealloc 0x81d34270
TKTK
Alloc 0x81d38230
Dealloc 0x81d38230
[object Object]
Alloc 0x81d38250
got 42
Alloc 0x81d38240
Dealloc 0x81d38240
Dealloc 0x81d38240
42
Alloc 0x81d38250
Dealloc 0x81d38250
3

Answer 1

你的问题的很大一部分是你的 NJSRef m; 声明没有声明实例变量，即使它在 @implementation 中。它只是一个 file-scope 全局。只有一个，它被 NJSValue 的所有实例共享（和破坏）。您必须将它括在大括号 {...} 中才能使其成为实例变量。

这至少解释了为什么它从未被摧毁过。可能还有很多其他症状，但鉴于您使用的我不熟悉的外部类型，很难说清楚。

Objective-C++ 中的 C++ 对象何时销毁？

When are C++ objects destroyed in Objective-C++?

objective-c

objective-c++

swift