C++11 std::promise 从线程返回 std::string，数据指针看起来已复制未移动

Question

我开始使用 C++11 标准和内置线程。根据我在获得 future 值时收集到的信息，它是使用移动运算符完成的，将所有权从原始对象中移走(就像旧的 auto_ptr 曾经在赋值时所做的那样)。我通过在线程期间打印出 std::string 对象内的 char 数组的指针并在 main 中接收到它后打印指针来测试这一点。但是，指针不同。如果有人能告诉我为什么他们在这个简单的代码中不同以及代码必须是什么样子才能使它们相等，我将不胜感激:

#include <cstdlib>
#include <iostream>
#include <vector>
#include <algorithm>
#include <chrono>
#include <string>
#include <thread>
#include <future>
using namespace std;
void thrfut(promise<string>&& promReceived)
{
    string strObj("Hello from future");
    cout << "Address of char array inside string inside of thread " << (void*)strObj.data() << endl;
    promReceived.set_value(strObj);
}
int main(int argc, char** argv) 
{
    promise<string> promiseOfText;
    future<string> futureText = promiseOfText.get_future(); // has to be before creating thread if the promise is passed rvalue reference, should be moved on calling get or not ? 
    thread threadHandlingPromise(&thrfut, std::move(promiseOfText));
    string stringReceived = futureText.get();
    cout << "Received from promise through thread: " << stringReceived  << endl;
    cout << "Address of of char array inside string received from promise in main " << (void*)stringReceived.data() << endl;
    threadHandlingPromise.join(); 
    return 0;
}

这是一个示例输出

Address of char array inside string inside of thread 0x10ebc9be1
Received from promise through thread: Hello from future
Address of of char array inside string received from promise in main 0x7fff510f68c9

仅供引用:OS X 10.9.1 w/Xcode 5 clang++ in Netbeans 8.0 其他人在 Ubuntu 和 Windows 上运行代码并返回相同的地址。

编辑(引用答案中的评论)* * * * *

我试过这个:

struct MYC 
{   MYC() = default;
    ~MYC() { delete _pInt; };
    MYC(const MYC & myc) { puts("MYC copy"); _pInt = nullptr; if(myc._pInt != nullptr) {   _pInt = new int{*myc._pInt}; } }  
    MYC(MYC && myc) { puts("MYC move"); delete _pInt; _pInt = myc._pInt; myc._pInt = nullptr; }
    void setMe(int value) { delete _pInt; _pInt = new int{value} ; }
    int * _pInt = nullptr;
};
void thrfut(promise<MYC>&& promReceived)
{
    MYC obj;
    obj.setMe(5);
    cout << "Address of int inside MYC inside thread " << (void*)obj._pInt << endl;
    promReceived.set_value(std::move(obj));    
}
int main(int argc, char** argv) 
{
    promise<MYC> promiseOfMYC;
    future<MYC> futureMYC = promiseOfMYC.get_future();  
    thread threadHandlingPromise(&thrfut, std::move(promiseOfMYC));
    auto mycReceived = futureMYC.get();
    cout << "Address of int inside MYC received from promise in main " << (void*)mycReceived._pInt << endl;
    cout << "Value of int inside MYC received from promise in main " << *(mycReceived._pInt) << endl;
    threadHandlingPromise.join(); 
    return 0;
}

得到:

Address of int inside MYC inside thread 0x7fd1b9c00110
MYC move
MYC move
Address of int inside MYC received from promise in main 0x7fd1b9c00110
Value of int inside MYC received from promise in main 5

这证实了非字符串类的移动动态。

Answer 1

编辑

终于想起GNU libstdc++ std::string使用引用计数。也许这解释了什么？？ (虽然不是全部)

• Is std::string refcounted in GCC 4.x / C++11?

原始答案

(1)确实是系统依赖

$ g++48 promiseStr.cpp -o promiseStr -Wall -Wextra -std=c++0x -O0 -g3 -pthread && echo OK
OK
$ ./promiseStr
Address of char array inside string inside of thread 0x7f4b400008d8
Received from promise through thread: Hello from future
Address of of char array inside string received from promise in main 0x7f4b400008d8

$ lsb_release -a
LSB Version: (snip)
Distributor ID: Ubuntu
Description:    Ubuntu 12.04.2 LTS
Release:        12.04
Codename:       precise
$ g++48 --version
g++48 (GCC) 4.8.1
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

(2) future<T>::get()返回 T不是&&T , 所以 stringReceived可能是复制构造的。

• http://www.cplusplus.com/reference/future/future/get/

(3) 你不妨试试ltrace在 Linux 上查看引擎盖下发生了什么(抱歉，我没有 OS X 机器。)

$ ltrace -n2 -f -C ./promiseStr 2>&1 | grep basic_string
[pid 6899]       std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&)(0x7f486c17fde0, 0x406ea0, 0x7f486c17fdef, 0x7f486c180700, 0x7f486c180700) = 0x7f48640008d8
[pid 6899]             std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::string const&)(0x244e0b0, 0x7f486c17fde0, 0x7f4864000900, 0x7f486c17fd90, 0x7f486c17fd20) = 0x7f48640008d8
[pid 6899]         std::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()(0x7f486c17fde0, 0xffffffff, 0, 0x7f4864000028, 0x244e038 <unfinished ...>
[pid 6899]         <... std::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string() resumed> ) = 1
[pid 6898]   std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::string&&)(0x7fff5c17ece0, 0x244e0b0, 0x244e0b0, -1, 0x244e038) = 0x7f486cf723d8
[pid 6898]   std::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()(0x244e0b0, 1, 0x244e0a0, -1, 0x244e060) = 0x244e0b0
[pid 6898]   std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)(0x60bd40, 0x7fff5c17ece0, 0x7fff5c17ece0, 0x203a6461, 0x7f486c53cab0) = 0x60bd40
[pid 6898]       std::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()(0x7fff5c17ece0, 0x7f486c759250, 0x7f486c180700, 0x7f486c759250, 0) = 0