多執行緒訪問

原子型別可用於安全地讀取和寫入兩個執行緒之間共享的記憶體位置。

一個可能導致資料競爭的錯誤示例：

#include <thread>
#include <iostream>

//function will add all values including and between 'a' and 'b' to 'result'
void add(int a, int b, int * result) {
    for (int i = a; i <= b; i++) {
        *result += i;
    }
}

int main() {
    //a primitive data type has no thread safety
    int shared = 0;

    //create a thread that may run parallel to the 'main' thread
    //the thread will run the function 'add' defined above with paramters a = 1, b = 100, result = &shared
    //analogous to 'add(1,100, &shared);'
    std::thread addingThread(add, 1, 100, &shared);

    //attempt to print the value of 'shared' to console
    //main will keep repeating this until the addingThread becomes joinable
    while (!addingThread.joinable()) {
        //this may cause undefined behavior or print a corrupted value
        //if the addingThread tries to write to 'shared' while the main thread is reading it
        std::cout << shared << std::endl;  
    }

    //rejoin the thread at the end of execution for cleaning purposes
    addingThread.join();
    
    return 0;
}

上面的示例可能會導致讀取損壞，並可能導致未定義的行為。

執行緒安全的一個例子：

#include <atomic>
#include <thread>
#include <iostream>

    //function will add all values including and between 'a' and 'b' to 'result'
void add(int a, int b, std::atomic<int> * result) {
    for (int i = a; i <= b; i++) {
        //atomically add 'i' to result
        result->fetch_add(i);
    }
}

int main() {
    //atomic template used to store non-atomic objects
    std::atomic<int> shared = 0;

    //create a thread that may run parallel to the 'main' thread
    //the thread will run the function 'add' defined above with paramters a = 1, b = 100, result = &shared
    //analogous to 'add(1,100, &shared);'
    std::thread addingThread(add, 1, 10000, &shared);

    //print the value of 'shared' to console
    //main will keep repeating this until the addingThread becomes joinable
    while (!addingThread.joinable()) {
        //safe way to read the value of shared atomically for thread safe read
        std::cout << shared.load() << std::endl;  
    }

    //rejoin the thread at the end of execution for cleaning purposes
    addingThread.join();
    
    return 0;
}

上面的例子是安全的，因為 atomic 資料型別的所有 store() 和 load() 操作都保護封裝的 int 免於同時訪問。