深入学习c++--多线程编程（二）【当线程间需要共享非const资源】

1. 遇到的问题

#include <iostream>
#include <thread>
#include <chrono>
#include <future>
#include <cmath> 
#include <vector>
#include <cstdlib>
using namespace std; 

class Counter 
{
public:
    void addCount() {
        m_count++;
    }
    int count() const { return m_count; }
    Counter() : m_count(0) {    }
private:
    int m_count;
};

int work(int a)
{
    return a + a;
}

template<class Iter>
void realWork(Counter& c, double &totalValue, Iter b, Iter e)
{
    for (; b != e; ++b)
    {
        totalValue += work(*b);
        c.addCount();
    }
}

int main()
{
    unsigned n = std::thread::hardware_concurrency();
    cout << n << " concurrent threads are support.\n";
    
    vector<int> vec;
    double totalValue = 0;
    for (int i = 0; i < 10000000; i++)
    {
        vec.push_back(rand() % 100);
    }
    Counter counter;
    realWork(counter, totalValue, vec.begin(), vec.end());
    cout << "total times: " << counter.count() << " " << totalValue << endl;


    totalValue = 0;
    Counter counter2;
    auto iter = vec.begin() + (vec.size() / 3);
    auto iter2 = vec.begin() + (vec.size() / 3 * 2);
    thread b([&counter2, &totalValue, iter, iter2](){
        realWork(counter2, totalValue, iter, iter2);
    });
    auto end = vec.end();
    thread c([&counter2, &totalValue, iter2, end](){
        realWork(counter2, totalValue, iter2, end);
    });

    realWork(counter2, totalValue, vec.begin(), iter);

    b.join();
    c.join();
    cout << "total times use multithread: " << counter2.count() << " " << totalValue << endl; 

    return 0;
}

计算结果不一致！三个线程共享一份资源，有的加了有的没加。

2. 解决

2.1 法一：不共享变量

#include <iostream>
#include <thread>
#include <chrono>
#include <future>
#include <cmath> 
#include <vector>
#include <cstdlib>
using namespace std; 

class Counter 
{
public:
    void addCount() {
        m_count++;
    }
    int count() const { return m_count; }
    Counter() : m_count(0) {    }
private:
    int m_count;
};

int work(int a)
{
    return a + a;
}

template<class Iter>
void realWork(Counter& c, double &totalValue, Iter b, Iter e)
{
    for (; b != e; ++b)
    {
        totalValue += work(*b);
        c.addCount();
    }
}

int main()
{
    unsigned n = std::thread::hardware_concurrency();
    cout << n << " concurrent threads are support.\n";
    
    vector<int> vec;
    double totalValue = 0;
    for (int i = 0; i < 10000000; i++)
    {
        vec.push_back(rand() % 100);
    }
    Counter counter;
    realWork(counter, totalValue, vec.begin(), vec.end());
    cout << "total times: " << counter.count() << " " << totalValue << endl;


    totalValue = 0;
    Counter counter2;
    auto iter = vec.begin() + (vec.size() / 3);
    auto iter2 = vec.begin() + (vec.size() / 3 * 2);
    double totalC = 0;
    thread b([&counter2, &totalValue, iter, iter2](){
        realWork(counter2, totalValue, iter, iter2);
    });
    auto end = vec.end();
    thread c([&counter2, &totalC, iter2, end](){
        realWork(counter2, totalC, iter2, end);
    });

    double totalD = 0;
    realWork(counter2, totalD, vec.begin(), iter);

    b.join();
    c.join();
    cout << "total times use multithread: " << counter2.count() << " " << totalValue+totalC+totalD << endl; 

    return 0;
}

2.2 法二：