为什么我用不同的线程生成相同的数字？ [复制]

Question

我的错误可能很明显，但我似乎无法弄清楚...... 运行它时，Add 和 Subtract 函数在启动新线程时生成相同的值。如果我每次调用函数时都声明一个新变量，为什么我无法在每次调用时生成不同的值？

我有以下代码，

#include <cstdlib>
#include <iostream>
#include <string>
#include <vector>
#include <ctime>
#include <numeric>
#include <cmath>
#include <sstream>
#include <thread>
#include <chrono>
#include <ctime>
#include <mutex>

int GetRandom(int max){
    //generate a pseudo-random number with time as the seed
    //time as seed will always return a different starting point
    srand(time(NULL));
    //mod max to ensure we return a number below max
    return rand() % max;
}

std::string GetTime(){
    auto nowTime = std::chrono::system_clock::now();
    std::time_t sleepTime =
            std::chrono::system_clock::to_time_t(nowTime);
    return std::ctime(&sleepTime);
}

double acctBalance = 10;

// Protects shared data from being accessed at the
// same time
std::mutex acctLock;

void Add(int id,
        double adding, int sleep){
    int rand = GetRandom(10);

        std::cout <<"Add: "<<rand<< "\n";
    }
    // acctLock.unlock();


void Subtract(int id, double subtract, int sleep){
    int rand = GetRandom(10);
    std::cout <<"Subtract: "<<rand<< "\n";
    // acctLock.unlock();
}

int main()
{
    int rand = 3;


int count = 10;
std::vector<std::thread> threads;
threads.reserve(count);
for (size_t i = 0; i < count; i=i+2)
{
  threads.emplace_back(Add, i, 15,rand);
  threads.emplace_back(Subtract, i+1, 15,rand);
}
for (auto& thread : threads)
{
  thread.join();
}


    return 0;
}

Answer 1

srand() 和 rand() 不保证是线程安全的。

使用#include <random>中可用的东西