Current File : /home/vacivi36/vittasync.vacivitta.com.br/vittasync/node/deps/v8/src/base/contextual.h
// Copyright 2018 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef V8_BASE_CONTEXTUAL_H_
#define V8_BASE_CONTEXTUAL_H_
#include <type_traits>
#include "src/base/export-template.h"
#include "src/base/macros.h"
#include "src/base/platform/platform.h"
namespace v8::base {
// {ContextualVariable} provides a clean alternative to a global variable.
// The contextual variable is mutable, and supports managing the value of
// a variable in a well-nested fashion via the {Scope} class.
// {ContextualVariable} only stores a pointer to the current value, which
// is stored in a {Scope} object. The most recent value can be retrieved
// via Get(). Because only {Scope} has actual storage, there must be at
// least one active {Scope} (i.e. in a surrounding C++ scope), whenever Get()
// is called.
// Note that contextual variables must only be used from the same thread,
// i.e. {Scope} and Get() have to be in the same thread.
template <class Derived, class VarType>
class V8_EXPORT_PRIVATE ContextualVariable {
public:
using VarT = VarType;
// A {Scope} contains a new object of type {VarType} and gives
// ContextualVariable::Get() access to it. Upon destruction, the contextual
// variable is restored to the state before the {Scope} was created. Scopes
// have to follow a stack discipline: A {Scope} has to be destructed before
// any older scope is destructed.
class V8_NODISCARD Scope {
public:
template <class... Args>
explicit Scope(Args&&... args)
: value_(std::forward<Args>(args)...), previous_(Top()) {
Top() = this;
}
~Scope() {
// Ensure stack discipline.
DCHECK_EQ(this, Top());
Top() = previous_;
}
Scope(const Scope&) = delete;
Scope& operator=(const Scope&) = delete;
VarType& Value() { return value_; }
private:
VarType value_;
Scope* previous_;
static_assert(std::is_base_of<ContextualVariable, Derived>::value,
"Curiously Recurring Template Pattern");
DISALLOW_NEW_AND_DELETE()
};
static VarType& Get() {
DCHECK(HasScope());
return Top()->Value();
}
static bool HasScope() { return Top() != nullptr; }
private:
inline static thread_local Scope* top_ = nullptr;
#if defined(USING_V8_SHARED)
// Hide the access to `top_` from other DLLs/libraries, since access to
// thread_local variables from other DLLs/libraries does not work correctly.
static Scope*& Top() { return ExportedTop(); }
#else
static Scope*& Top() { return top_; }
#endif
// Same as `Top()`, but non-inline and exported to DLLs/libraries.
// If there is a linking error for `ExportedTop()`, then the contextual
// variable probably needs to be exported using EXPORT_CONTEXTUAL_VARIABLE.
static Scope*& ExportedTop();
};
// Usage: DECLARE_CONTEXTUAL_VARIABLE(VarName, VarType)
#define DECLARE_CONTEXTUAL_VARIABLE(VarName, ...) \
struct VarName : ::v8::base::ContextualVariable<VarName, __VA_ARGS__> {}
// Contextual variables that are accessed in tests need to be
// exported. For this, place the following macro in the global namespace inside
// of a .cc file.
#define EXPORT_CONTEXTUAL_VARIABLE(VarName) \
namespace v8::base { \
template <> \
V8_EXPORT_PRIVATE typename VarName::Scope*& \
ContextualVariable<VarName, typename VarName::VarT>::ExportedTop() { \
return top_; \
} \
}
// By inheriting from {ContextualClass} a class can become a contextual variable
// of itself, which is very similar to a singleton.
template <class T>
using ContextualClass = ContextualVariable<T, T>;
} // namespace v8::base
#endif // V8_BASE_CONTEXTUAL_H_
Mini Shell