C++
原子タイプ

構文

• std :: atomic <T>
• std :: atomic_flag

備考

マルチスレッドアクセス

アトミック型を使用して、2つのスレッド間で共有されるメモリ位置を安全に読み書きすることができます。

データ競合を引き起こす可能性のある悪い例：

#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;
}

上記の例は、破損した読み取りを引き起こし、未定義の動作につながる可能性があります。

スレッドセーフな例：

#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;
}

上記の例は、 atomicデータ型のすべてのstore()およびload()オペレーションが、カプセル化されたintを同時アクセスから保護するので安全です。