CODEWITHSUNDEEP
c++multithreading
Sean
Sean

Reputation: 1021

C++ multithread program attempts to use deleted function

I am a novice in the field of C++ multithread program. I want to read block data from my disk and do some computation based on these data. In order to simplify the code, I write the following demo to test my idea. However, I found some questions. 

#include <iostream>
#include <thread>
#include <string>
#include <mutex>
#include <vector>
#include <condition_variable>

std::condition_variable cv;
std::mutex cv_m;

bool is_print;

void read_value(std::vector<int> &data)
{
    for (int j = 1; j < 5; j++)
    {
        std::cout << "read data iteration is " << j << std::endl;
        std::lock_guard<std::mutex> lk(cv_m);
        for (int i = 0; i < 5; i++)
            data.at(i) = i * j;
        is_print = true;
        cv.notify_one();
    }
}

void print_value(const std::vector<int> &data)
{
    std::cout << "output data" << std::endl;
    std::unique_lock<std::mutex> lk(cv_m);
    cv.wait(lk, []{return is_print;});
    for (int i = 0; i < data.size(); i++)
        std::cout << data[i] << std::endl;
    is_print = false;
}

int main()
{
    is_print = false;
    std::vector<int> data = {0, 0, 0, 0, 0};

    std::thread thread_read(read_value, data);
    std::thread thread_print(print_value, data);

    thread_print.join();
    thread_read.join();

    return 0;
}

In this demo, I use "read_value" to simulate read data from disk and I want to read data from disk several times. So, I add an outer loop in the function "read_value". After that, I use "print_value" function to output the data. However, in such case, I meet the error as the following, which tells me that I use the deleted function. I am not sure what the problem is and how to read data several times. Thank you so much!

In file included from 

/Users/zsk/Downloads/programming/C++/multi_thread/multi_thread/main.cpp:10:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/thread:342:5: error: attempt to use a deleted function
    __invoke(_VSTD::move(_VSTD::get<1>(__t)), _VSTD::move(_VSTD::get<_Indices>(__t))...);
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/thread:352:5: note: in instantiation of function template specialization 'std::__1::__thread_execute<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(std::__1::vector<int, std::__1::allocator<int> > &), std::__1::vector<int, std::__1::allocator<int> > , 2>' requested here
    __thread_execute(*__p, _Index());
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/thread:368:47: note: in instantiation of function template specialization 'std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(std::__1::vector<int, std::__1::allocator<int> > &), std::__1::vector<int, std::__1::allocator<int> > > >' requested here
    int __ec = __libcpp_thread_create(&__t_, &__thread_proxy<_Gp>, __p.get());
                                              ^
/Users/zsk/Downloads/programming/C++/multi_thread/multi_thread/main.cpp:51:17: note: in instantiation of function template specialization 'std::__1::thread::thread<void (&)(std::__1::vector<int, std::__1::allocator<int> > &), std::__1::vector<int, std::__1::allocator<int> > &, void>' requested here
    std::thread thread_read(read_value, data);

Upvotes: 0

Views: 929

Answers (2)

JVApen
JVApen

Reputation: 11317

Working with threads is surprising if you try to use references. The other answer is completely right, use std::ref to pass the arguments.

Personally, I prefer to use lambdas instead:

std::thread thread_read([&data](){ read_value(data); });
std::thread thread_print([&data = std::as_const(data)](){ print_value(data); });

It might not be as elegant as the std::ref though it uses the concepts you already know from all other places instead of requiring you to remember the unexpected copying.

Upvotes: 0

Some programmer dude
Some programmer dude

Reputation: 409136

The problem is that the std::thread object needs to be able to copy the arguments you pass to the threads. And references can't be copied.

To solve this you need to use std::ref or std::cref to wrap the arguments:

std::thread thread_read(read_value, std::ref(data));
std::thread thread_print(print_value, std::cref(data));

Upvotes: 5

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