c++ - 传递字符串作为线程启动例程参数 : C++

Question

引用http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html#SCHEDULING

我试图在 C++ 中创建两个线程，并试图将字符串作为参数传递给 Thread Start Routine。 Thread Start Routine 参数只能是 (void *) 定义的类型:

int pthread_create(pthread_t * thread, 
                       const pthread_attr_t * attr,
                       void * (*start_routine)(void *), 
                       void *arg);

但我得到以下错误:

$ make
g++ -g -Wall Trial.cpp -o Trial
Trial.cpp: In function `int main()':
Trial.cpp:22: error: cannot convert `message1' from type `std::string' to type `void*'
Trial.cpp:23: error: cannot convert `message2' from type `std::string' to type `void*'
Makefile:2: recipe for target `Trial' failed
make: *** [Trial] Error 1

代码是

#include <iostream>
#include <pthread.h>
#include <string>



using namespace std;

void *print_message_function( void *ptr );


int main()
{
    pthread_t thread1, thread2;

    string message1 = "Thread 1";
    string message2 = "Thread 2";

    int  iret1, iret2;


     iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);
     iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);



     pthread_join( thread1, NULL);
     pthread_join( thread2, NULL);



     cout << "Thread 1 returns: " <<  iret1 << endl;
     cout << "Thread 2 returns: " << iret2 << endl;


    return 0;
    }
void *print_message_function( void *ptr )

{
     cout << endl <<  ptr << endl;
     //return 0;

}

有什么方法可以将 string 作为 (void *) 参数传递吗？或者只有 C 样式字符串 可以用作多线程参数 - 如链接中的引用代码所示。

Answer 1

参数需要是一个指针，你尝试传递一个对象给它。

您有两个选择，要么传递指向 std::string 对象的指针，要么传递指向底层字符串的指针。我推荐第一个:

 iret1 = pthread_create(&thread1, NULL, print_message_function, &message1);

然后你必须修改线程函数，否则它会打印指针而不是它指向的字符串:

void* print_message_function(void* ptr)
{
    std::string str = *reinterpret_cast<std::string*>(ptr);

    std::cout << str << std::endl;

    return nullptr;
}

---

除非需要使用 POSIX 线程，否则我实际上更愿意推荐 C++ standard library 中的线程功能。 :

#include <iostream>
#include <string>
#include <thread>

void print_message_function(const std::string& msg);

int main()
{
    std::string message1 = "Thread 1";
    std::string message2 = "Thread 2";

    std::thread thread1(print_message_function, std::cref(message1));
    std::thread thread2(print_message_function, std::cref(message2));

    thread1.join();
    thread2.join();
}

void print_message_function(const std:string& msg)
{
    std::cout << msg << std::endl;
}