An instance of [*RatePolicy] is associated with a [link beast.ref.boost__beast__basic_stream `basic_stream`], and controls the rate at which bytes may be independently sent and received. This may be used to achieve fine-grained bandwidth management and flow control.
* `P` denotes a type that meets the requirements of [*RatePolicy]. * `x` denotes an xvalue of type `P` * `a` denotes a value of type `P`. * `n` denotes a value of type `std::size_t`
The member functions required in `P` should be private. [link beast.ref.boost__beast__rate_policy_access `rate_policy_access`] must be a friend of `P` for the implementation to gain access to the required member functions.
This function is called by the implementation to determine the maximum number of allowed bytes to be transferred in the next read operation. The actual number of bytes subsequently transferred may be less than this number. If the policy returns a value of zero, the read operation will asynchronously wait until the next timer interval before retrying. When the retry occurs, this function will be called again.