Passing std::promise object to a function via direct function call

Question

I am learning the std::promise and std::future in C++. I wrote one simple program to calculate the multiplication of two numbers.

void product(std::promise<int> intPromise, int a, int b)
{
    intPromise.set_value(a * b);
}

int main()
{
    int a = 20;
    int b = 10;
    std::promise<int> prodPromise;
    std::future<int> prodResult = prodPromise.get_future();
    // std::thread t{product, std::move(prodPromise), a, b};
    product(std::move(prodPromise), a, b);
    std::cout << "20*10= " << prodResult.get() << std::endl;
    // t.join();
}

In the above code if I invoke the product function using threads it's working fine. But if I invoke the function using direct function call I am getting the following error:

terminate called after throwing an instance of 'std::system_error'
  what():  Unknown error -1
Aborted (core dumped)

I added some logs to check the problem. I am getting the error while setting the value (set_value) in the function product. Is there anything I missed in the code?

Which compiler and platform? And how do you compile this code? — Evg, Jun 10 '20 at 16:03
I am running the code in Ubuntu machine.I tried in online compilers also but the result is the same — goodman, Jun 10 '20 at 16:04
Please be more specific. Ubuntu version, compiler version and compiler invocation command line. I can't reproduce this on my Kubuntu 19.04 machine. Do you add `-pthread` option? If not, add it and try again. — Evg, Jun 10 '20 at 16:06
the result of the command ```g++ --version``` is ```g++ (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0``` — goodman, Jun 10 '20 at 16:10
Do I need to use -pthread because I am not using threads(commented the code)? — goodman, Jun 10 '20 at 16:13
I added an explanation why it compiles but produces an exception. — Evg, Jun 11 '20 at 08:17

Evg · Accepted Answer · 2020-06-11T10:02:06.977

3

When you compile this code, even if don't use std::thread explicitly, you still have to add -pthread command line option, because internally std::promise and std::future depend on the pthread library.

Without -pthread on my machine I get:

terminate called after throwing an instance of 'std::system_error'
  what():  Unknown error -1

With -pthread:

20*10 = 200

My doubt is if std::promise using std::thread then it should throw some compilation or linkage error right?

Very good question. See my answer here.

edited Jun 11 '20 at 10:02

answered Jun 10 '20 at 16:13

Evg

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yes, It's working now. My doubt is if ```std::promise``` using ```std::thread``` then it should throw some compilation or linkage error right? – goodman Jun 10 '20 at 16:16

Passing std::promise object to a function via direct function call

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