# 标准库头文件 <semaphore>

< cpp‎ | header

此头文件是线程支持库的一部分。

### 类
counting_semaphore (C++20) | 实现非负资源计数的信号量 (类模板)
binary_semaphore (C++20) | 仅拥有二个状态的信号量 (typedef)

### 概要

    namespace std {
      template<ptrdiff_t LeastMaxValue = /* 实现定义 */>
        class counting_semaphore;

      using binary_semaphore = counting_semaphore<1>;
    }

#### 类模板 std::counting_semaphore

    namespace std {
      template<ptrdiff_t LeastMaxValue = /* 实现定义 */>
      class counting_semaphore {
      public:
        static constexpr ptrdiff_t max() noexcept;

        constexpr explicit counting_semaphore(ptrdiff_t desired);
        ~counting_semaphore();

        counting_semaphore(const counting_semaphore&) = delete;
        counting_semaphore& operator=(const counting_semaphore&) = delete;

        void release(ptrdiff_t update = 1);
        void acquire();
        bool try_acquire() noexcept;
        template<class Rep, class Period>
          bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time);
        template<class Clock, class Duration>
          bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time);

      private:
        ptrdiff_t counter;          // 仅用于阐释
      };
    }