c-resources/CPlusPlus20ForProgrammers-m.../examples/ch17/fig17_09-10/CircularBuffer.h

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// Fig. 17.9: CircularBuffer.h
// Synchronizing access to a shared three-element circular buffer.
#pragma once
#include <array>
#include <condition_variable>
#include <fmt/format.h>
#include <mutex>
#include <iostream>
#include <string>
#include <string_view>
using namespace std::string_literals;
class CircularBuffer {
public:
// place value into m_buffer
void put(int value) {
// critical section that requires a lock to modify shared data
{
// lock on m_mutex to be able to write to m_buffer
std::unique_lock dataLock{m_mutex};
// if no empty locations, wait on condition variable m_cv
if (m_occupiedCells == m_buffer.size()) {
std::cout << "Buffer is full. Producer waits.\n\n";
// wait on the condition variable; the lambda argument
// ensures that if the thread gets the processor before
// the m_buffer has open cells, the thread continues waiting
m_cv.wait(dataLock,
[&] {return m_occupiedCells < m_buffer.size(); });
}
m_buffer[m_writeIndex] = value; // write to m_buffer
++m_occupiedCells; // one more m_buffer cell is occupied
m_writeIndex = (m_writeIndex + 1) % m_buffer.size();
displayState(fmt::format("Producer writes {}", value));
} // dataLock's destructor releases the lock on m_mutex here
m_cv.notify_one(); // notify threads waiting to read from m_buffer
}
// return value from m_buffer
int get() {
int readValue; // will temporarily hold the next value read
// critical section that requires a lock to modify shared data
{
// lock on m_mutex to be able to write to m_buffer
std::unique_lock dataLock{m_mutex};
// if no data to read, place thread in waiting state
if (m_occupiedCells == 0) {
std::cout << "Buffer is empty. Consumer waits.\n\n";
// wait on the condition variable; the lambda argument
// ensures that if the thread gets the processor before
// there is data in the m_buffer, the thread continues waiting
m_cv.wait(dataLock, [&]() {return m_occupiedCells > 0; });
}
readValue = m_buffer[m_readIndex]; // read value from m_buffer
m_readIndex = (m_readIndex + 1) % m_buffer.size();
--m_occupiedCells; // one fewer m_buffer cells is occupied
displayState(fmt::format("Consumer reads {}", readValue));
} // dataLock's destructor releases the lock on m_mutex here
m_cv.notify_one(); // notify threads waiting to write to m_buffer
return readValue;
}
// display current operation and m_buffer state
void displayState(std::string_view operation) const {
std::string s;
// add operation argument and number of occupied m_buffer cells
s += fmt::format("{} (buffer cells occupied: {})\n{:<15}",
operation, m_occupiedCells, "buffer cells:");
// add values in m_buffer
for (int value : m_buffer) {
s += fmt::format(" {:2d} ", value);
}
s += fmt::format("\n{:<15}", ""); // padding
// add underlines
for (int i{0}; i < m_buffer.size(); ++i) {
s += "---- "s;
}
s += fmt::format("\n{:<15}", ""); // padding
for (int i{0}; i < m_buffer.size(); ++i) {
s += fmt::format(" {}{} ",
(i == m_writeIndex ? 'W' : ' '),
(i == m_readIndex ? 'R' : ' '));
}
s += "\n\n";
std::cout << s; // display the state string
}
private:
std::condition_variable m_cv;
std::mutex m_mutex;
std::array<int, 3> m_buffer{-1, -1, -1}; // shared m_buffer
int m_occupiedCells{0}; // count number of buffers used
int m_writeIndex{0}; // index of next element to write to
int m_readIndex{0}; // index of next element to read
};
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* *
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