C++0x |为什么 std::atomic 使用 volatile-qualifier 重载每个方法？

Question

当前草案的以下摘录说明了我的意思:

namespace std {
    typedef struct atomic_bool {
        bool is_lock_free() const volatile;
        bool is_lock_free() const;
        void store(bool, memory_order = memory_order_seq_cst) volatile;
        void store(bool, memory_order = memory_order_seq_cst);
        bool load(memory_order = memory_order_seq_cst) const volatile;
        bool load(memory_order = memory_order_seq_cst) const;
        operator bool() const volatile;
        operator bool() const;
        bool exchange(bool, memory_order = memory_order_seq_cst) volatile;
        bool exchange(bool, memory_order = memory_order_seq_cst);
        bool compare_exchange_weak(bool&, bool, memory_order, memory_order) volatile;
        bool compare_exchange_weak(bool&, bool, memory_order, memory_order);
        bool compare_exchange_strong(bool&, bool, memory_order, memory_order) volatile;
        bool compare_exchange_strong(bool&, bool, memory_order, memory_order);
        bool compare_exchange_weak(bool&, bool, memory_order = memory_order_seq_cst) volatile;
        bool compare_exchange_weak(bool&, bool, memory_order = memory_order_seq_cst);
        bool compare_exchange_strong(bool&, bool, memory_order = memory_order_seq_cst) volatile;
        bool compare_exchange_strong(bool&, bool, memory_order = memory_order_seq_cst);
        atomic_bool() = default;
        constexpr atomic_bool(bool);
        atomic_bool(const atomic_bool&) = delete;
        atomic_bool& operator=(const atomic_bool&) = delete;
        atomic_bool& operator=(const atomic_bool&) volatile = delete;
        bool operator=(bool) volatile;
    } atomic_bool;
}

Volatile 具有传递性。因此，您不能从 volatile 对象调用非 volatile 成员函数。另一方面，允许从非 volatile 对象调用 volatile 成员函数。

那么，原子类中的volatile和non-volatile成员函数在实现上有什么区别吗？换句话说，是否需要非 volatile 重载？

Answer 1

我认为 volatile 重载的存在是出于效率原因。在 C++0x 中， volatile 读取和写入本质上比非 volatile 读取和写入更昂贵，因为内存模型提出了一些严格的要求，以防止缓存 volatile 变量的值。如果所有函数都只标记为 volatile，那么代码不一定会进行某些优化以提高性能。有了这种区别，编译器就可以在可能的情况下优化非 volatile 读取和写入，同时在需要 volatile 读取和写入时优雅地降级。