c++ - 代码不会提前通过条件变量的等待函数

Question

因此，我一直试图更好地了解条件变量的工作原理，并且编写了以下代码来尝试从同一文本文件实现读取和写入:

    #include <iostream>
    #include <fstream>
    #include <cstdlib>
    #include <condition_variable>
    #include <mutex>
    #include <thread>

    using namespace std;

    mutex mtx;
    condition_variable conditionVariable;
    ifstream readFile;
    ofstream writeFile;
    bool doneReading=false;
    bool doneWriting=false;

    void readFromFile()
    {
        string line;

        readFile.open("./testFile.txt");

        if(readFile.is_open())
        {
            cout << "Successfully opened file!" << "\n";
        }
        else
        {
            cout << "Failed to open file..." << "\n";
        }

        cout << "The file contents are:" << "\n";
        while(getline(readFile,line))
        {
            unique_lock<mutex> lock(mtx);

            conditionVariable.wait(lock, [] () {return doneWriting;});

            cout << line << "\n";

            doneReading=true;

            lock.unlock();

            conditionVariable.notify_one();
        }
        readFile.close();
    }

    void writeToFile()
    {
        string input;

        writeFile.open("./testFile.txt");

        cout << "Enter something you want to write to the text file:" << "\n";
        cin >> input;
        cout << "Going to write " << input << " to the file" << "\n";

        if(writeFile.is_open())
        {
            cout << "Successfully opened file!" << "\n";

            unique_lock<mutex> lock2(mtx);

            /////////PROGRAM WON'T ADVANCE PAST THIS LINE/////////////
            conditionVariable.wait(lock2, [] () {return doneReading;}); 

            cout << "After calling the wait function for the condition variable" << "\n";

            writeFile << input;

            doneWriting=true;

            lock2.unlock();

            conditionVariable.notify_one();

            writeFile.close();
        }
        else
        {
            cout << "Failed to open file..." << "\n";
        }
    }


    int main()
    {

        int i;
        for(i=0;i<10;i++)
        {
            thread t1(readFromFile);
            thread t2(writeToFile);
            t1.join();
            t2.join();
        }
    }

并且，我在以下 example 之后模拟了我对 bool 变量的使用从cppreference.com(向下滚动到底部以查看示例代码)。但是，它与我传递给等待函数的谓词有关系，我不太确定它有什么问题。如果有人可以提供一些见解，那就太好了。谢谢!

Answer 1

初始状态是:

bool doneReading=false;
bool doneWriting=false;

readFromFile 对这些变量所做的第一件事就是等待 doneWriting 变为 true

conditionVariable.wait(lock, [] () {return doneWriting;});

writeFromFile 对这些变量所做的第一件事就是等待 doneReading 变为 true

conditionVariable.wait(lock2, [] () {return doneReading;}); 

这两个条件都不会成立。

请注意，cppreference 示例做了一些非常不同的事情:一个线程开始执行 cv.wait(lk, []{return ready;}); 而另一个线程开始执行 ready =真；