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Q&A When should I use wait() instead of get() when using C++ threads

I'm trying to understand the purpose of the wait() function in Class future<> for C++. At the moment I don't see any purpose for calling wait() instead of get(). What I have tried in code: i...

1 answer · posted 4y ago by hamsasimon‭ · last activity 4y ago by Derek Elkins‭

Question c++ multithreading

#4: Post edited by

hamsasimon‭ · 2020-12-20T18:06:59Z (about 4 years ago)
Clearer wording

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~~I'm trying to understand the purpose of the `wait()` function in Class future<> for C++. I'm trying to know when would I use `wait()` instead of `get()`. What I have tried in my code:~~
int main()
{
std::future<int>result1(std::async(std::launch::deferred,func1));
// start synchronously.
//result1.get();
result1.wait(); // gives the same output as calling get()
//print character L
for (int i = 0; i < 20; ++i)
{
std::cout.put('L').flush();
}
}
here is what's getting called:
int doSomething(char c)
{
// random-number generator (use c as seed to get different sequences)
std::default_random_engine dre(c);
std::uniform_int_distribution<int> id(10,1000);
//loop to print character after a random period of time
for (int i=0; i<10; ++i)
{
std::cout.put(c).flush();
}
return c;
}
int func1 ()
{
return doSomething('.');
}
~~But the output I get is the same ! i.e func1 gets called prints some characters to the screen and then character is `L` is printed 20 times.~~
~~So, I'm trying to figure out if there is any use of calling `wait()`.~~
In the book C++ standard library by Nicolai M. Josuttis, it says that wait() function call allows a background operation to finish without processing its outcome. So, when would we need to do that ?

I'm trying to understand the purpose of the `wait()` function in Class future<> for C++. At the moment I don't see any purpose for calling `wait()` instead of `get()`. What I have tried in code:
int main()
{
std::future<int>result1(std::async(std::launch::deferred,func1));
// start synchronously.
//result1.get();
result1.wait(); // gives the same output as calling get()
//print character L
for (int i = 0; i < 20; ++i)
{
std::cout.put('L').flush();
}
}
here is what's getting called:
int doSomething(char c)
{
// random-number generator (use c as seed to get different sequences)
std::default_random_engine dre(c);
std::uniform_int_distribution<int> id(10,1000);
//loop to print character after a random period of time
for (int i=0; i<10; ++i)
{
std::cout.put(c).flush();
}
return c;
}
int func1 ()
{
return doSomething('.');
}
But the output I get is the same as I would have had if I called `get()` instead. That is, func1 gets called prints some characters to the screen and then character is `L` is printed 20 times.
So, I'm trying to figure out in which situations `wait()` would be better suited than `get()`.
In the book C++ standard library by Nicolai M. Josuttis, it says that wait() function call allows a background operation to finish without processing its outcome. So, when would we need to do that ?

#3: Post edited by

hamsasimon‭ · 2020-12-20T17:59:34Z (about 4 years ago)
Added more code for context.

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I'm trying to understand the purpose of the `wait()` function in Class future<> for C++. I'm trying to know when would I use `wait()` instead of `get()`. What I have tried in my code:
int main()
{
std::future<int>result1(std::async(std::launch::deferred,func1));
// start synchronously.
//result1.get();
result1.wait(); // gives the same output as calling get()
//print character L
for (int i = 0; i < 20; ++i)
{
std::cout.put('L').flush();
}
}
But the output I get is the same ! i.e func1 gets called prints some characters to the screen and then character is `L` is printed 20 times.
So, I'm trying to figure out if there is any use of calling `wait()`.
In the book C++ standard library by Nicolai M. Josuttis, it says that wait() function call allows a background operation to finish without processing its outcome. So, when would we need to do that ?

I'm trying to understand the purpose of the `wait()` function in Class future<> for C++. I'm trying to know when would I use `wait()` instead of `get()`. What I have tried in my code:
int main()
{
std::future<int>result1(std::async(std::launch::deferred,func1));
// start synchronously.
//result1.get();
result1.wait(); // gives the same output as calling get()
//print character L
for (int i = 0; i < 20; ++i)
{
std::cout.put('L').flush();
}
}
here is what's getting called:
int doSomething(char c)
{
// random-number generator (use c as seed to get different sequences)
std::default_random_engine dre(c);
std::uniform_int_distribution<int> id(10,1000);
//loop to print character after a random period of time
for (int i=0; i<10; ++i)
{
std::cout.put(c).flush();
}
return c;
}
int func1 ()
{
return doSomething('.');
}
But the output I get is the same ! i.e func1 gets called prints some characters to the screen and then character is `L` is printed 20 times.
So, I'm trying to figure out if there is any use of calling `wait()`.
In the book C++ standard library by Nicolai M. Josuttis, it says that wait() function call allows a background operation to finish without processing its outcome. So, when would we need to do that ?

#2: Post edited by

hamsasimon‭ · 2020-12-20T17:56:04Z (about 4 years ago)

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I'm trying to understand the purpose of the `wait()` function in Class future<> for C++. I'm trying to know when would I use `wait()` instead of `get()`. What I have tried in my code:
int main()
{
std::future<int>result1(std::async(std::launch::deferred,func1));
// start synchronously.
//result1.get();
result1.wait(); // gives the same output as calling get()
//print character L
for (int i = 0; i < 20; ++i)
{
std::cout.put('L').flush();
}
}
But the output I get is the same ! i.e func1 gets called prints some characters to the screen and then character is `L` is printed 20 times.
~~So, I'm trying to figure out if there is any use of calling `wait()`.~~

I'm trying to understand the purpose of the `wait()` function in Class future<> for C++. I'm trying to know when would I use `wait()` instead of `get()`. What I have tried in my code:
int main()
{
std::future<int>result1(std::async(std::launch::deferred,func1));
// start synchronously.
//result1.get();
result1.wait(); // gives the same output as calling get()
//print character L
for (int i = 0; i < 20; ++i)
{
std::cout.put('L').flush();
}
}
But the output I get is the same ! i.e func1 gets called prints some characters to the screen and then character is `L` is printed 20 times.
So, I'm trying to figure out if there is any use of calling `wait()`.
In the book C++ standard library by Nicolai M. Josuttis, it says that wait() function call allows a background operation to finish without processing its outcome. So, when would we need to do that ?

#1: Initial revision by

hamsasimon‭ · 2020-12-20T17:52:21Z (about 4 years ago)

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When should I use wait() instead of get() when using C++ threads

I'm trying to understand the purpose of the `wait()` function in Class future<> for C++. I'm trying to know when would I use `wait()` instead of `get()`. What I have tried in my code:

    int main()
    {
        
        std::future<int>result1(std::async(std::launch::deferred,func1));
    
        // start synchronously.
        //result1.get();
        result1.wait(); // gives the same output as calling get()

        //print character L
       for (int i = 0; i < 20; ++i)
       {
            std::cout.put('L').flush();
       }

    }

But the output I get is the same ! i.e func1 gets called prints some characters to the screen and then character is `L` is printed 20 times.

So, I'm trying to figure out if there is any use of calling `wait()`.

c++ multithreading

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