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Is it legal to call shared_future::get() multiple times on the same instance in the same thread?

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8

2

I can find neither direct confirmation nor refutation on the matter. All answers seem to address the "access from multiple threads" aspect, rather than repetitive access itself.

Does the standard define the behavior for std::shared_future? What about boost::shared_future?

c++ boost future

share|improve this question

asked Jun 25 at 14:15

vinesvines

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8

2

I can find neither direct confirmation nor refutation on the matter. All answers seem to address the "access from multiple threads" aspect, rather than repetitive access itself.

Does the standard define the behavior for std::shared_future? What about boost::shared_future?

c++ boost future

share|improve this question

asked Jun 25 at 14:15

vinesvines

4,46811 gold badge2020 silver badges4444 bronze badges

add a comment | 

I can find neither direct confirmation nor refutation on the matter. All answers seem to address the "access from multiple threads" aspect, rather than repetitive access itself.

Does the standard define the behavior for std::shared_future? What about boost::shared_future?

c++ boost future

share|improve this question

asked Jun 25 at 14:15

vinesvines

4,46811 gold badge2020 silver badges4444 bronze badges

share|improve this question

asked Jun 25 at 14:15

vinesvines

4,46811 gold badge2020 silver badges4444 bronze badges

share|improve this question

asked Jun 25 at 14:15

vinesvines

4,46811 gold badge2020 silver badges4444 bronze badges

asked Jun 25 at 14:15

vinesvines

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asked Jun 25 at 14:15

vinesvines

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2 Answers
2

active

oldest

votes

12

Per cppreference in std::shared_future<T>::valid

Unlike std::future, std::shared_future's shared state is not invalidated when get() is called.

Which makes sense. If it wasn't the case then you couldn't have multiple threads be able to call get. We can further back this up by looking at the standard. In [futures.unique.future]/15 they explicitly state get only works once with

releases any shared state ([futures.state]).

while in [futures.shared.future]/18 it states no such thing so the state is still valid after get is called.

---

boost::shared_future has the same behavior. Per the reference get has no text stating it invalidates the shared state on a call to get so you can call it multiple times.

share|improve this answer

edited Jun 25 at 15:16

answered Jun 25 at 14:22

NathanOliverNathanOliver

107k1919 gold badges159159 silver badges236236 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  By the way, a curious observation on boost::futures. The link in the answer leads to docs on boost::fibers::shared_future, which (again!) seem to differ in behavior from boost::shared_future (which is defined in 'boost::thread').
  
  – vines
  Jun 25 at 15:11
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @vines Oh wow. Didn't notice that. I'll fix the answer.
  
  – NathanOliver
  Jun 25 at 15:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah, found! It is stated in the first example on shared_future: use3( ftr.get() ); // second use is defined
  
  – vines
  Jun 25 at 16:23
  

  
  
  
  

add a comment | 

8

AFAIK this is legal. std::shared_future<T>::get() says:

The behavior is undefined if valid() is false before the call to this
function.

Going to std::shared_future<T>::valid() it says:

Checks if the future refers to a shared state.

...

Unlike std::future, std::shared_future's shared state is not
invalidated when get() is called.

Which would make multiple get() calls from the same thread and on the same instance valid.

share|improve this answer

edited Jun 25 at 14:28

answered Jun 25 at 14:23

Sombrero ChickenSombrero Chicken

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12

Per cppreference in std::shared_future<T>::valid

Unlike std::future, std::shared_future's shared state is not invalidated when get() is called.

Which makes sense. If it wasn't the case then you couldn't have multiple threads be able to call get. We can further back this up by looking at the standard. In [futures.unique.future]/15 they explicitly state get only works once with

releases any shared state ([futures.state]).

while in [futures.shared.future]/18 it states no such thing so the state is still valid after get is called.

---

boost::shared_future has the same behavior. Per the reference get has no text stating it invalidates the shared state on a call to get so you can call it multiple times.

share|improve this answer

edited Jun 25 at 15:16

answered Jun 25 at 14:22

NathanOliverNathanOliver

107k1919 gold badges159159 silver badges236236 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  By the way, a curious observation on boost::futures. The link in the answer leads to docs on boost::fibers::shared_future, which (again!) seem to differ in behavior from boost::shared_future (which is defined in 'boost::thread').
  
  – vines
  Jun 25 at 15:11
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @vines Oh wow. Didn't notice that. I'll fix the answer.
  
  – NathanOliver
  Jun 25 at 15:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah, found! It is stated in the first example on shared_future: use3( ftr.get() ); // second use is defined
  
  – vines
  Jun 25 at 16:23
  

  
  
  
  

add a comment | 

12

Per cppreference in std::shared_future<T>::valid

Unlike std::future, std::shared_future's shared state is not invalidated when get() is called.

Which makes sense. If it wasn't the case then you couldn't have multiple threads be able to call get. We can further back this up by looking at the standard. In [futures.unique.future]/15 they explicitly state get only works once with

releases any shared state ([futures.state]).

while in [futures.shared.future]/18 it states no such thing so the state is still valid after get is called.

---

boost::shared_future has the same behavior. Per the reference get has no text stating it invalidates the shared state on a call to get so you can call it multiple times.

share|improve this answer

edited Jun 25 at 15:16

answered Jun 25 at 14:22

NathanOliverNathanOliver

107k1919 gold badges159159 silver badges236236 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  By the way, a curious observation on boost::futures. The link in the answer leads to docs on boost::fibers::shared_future, which (again!) seem to differ in behavior from boost::shared_future (which is defined in 'boost::thread').
  
  – vines
  Jun 25 at 15:11
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @vines Oh wow. Didn't notice that. I'll fix the answer.
  
  – NathanOliver
  Jun 25 at 15:15
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah, found! It is stated in the first example on shared_future: use3( ftr.get() ); // second use is defined
  
  – vines
  Jun 25 at 16:23
  

  
  
  
  

add a comment | 

Per cppreference in std::shared_future<T>::valid

Unlike std::future, std::shared_future's shared state is not invalidated when get() is called.

Which makes sense. If it wasn't the case then you couldn't have multiple threads be able to call get. We can further back this up by looking at the standard. In [futures.unique.future]/15 they explicitly state get only works once with

releases any shared state ([futures.state]).

while in [futures.shared.future]/18 it states no such thing so the state is still valid after get is called.

---

boost::shared_future has the same behavior. Per the reference get has no text stating it invalidates the shared state on a call to get so you can call it multiple times.

share|improve this answer

edited Jun 25 at 15:16

answered Jun 25 at 14:22

NathanOliverNathanOliver

107k1919 gold badges159159 silver badges236236 bronze badges

share|improve this answer

edited Jun 25 at 15:16

answered Jun 25 at 14:22

NathanOliverNathanOliver

107k1919 gold badges159159 silver badges236236 bronze badges

answered Jun 25 at 14:22

NathanOliverNathanOliver

107k1919 gold badges159159 silver badges236236 bronze badges

answered Jun 25 at 14:22

NathanOliverNathanOliver

107k1919 gold badges159159 silver badges236236 bronze badges

8

AFAIK this is legal. std::shared_future<T>::get() says:

The behavior is undefined if valid() is false before the call to this
function.

Going to std::shared_future<T>::valid() it says:

Checks if the future refers to a shared state.

...

Unlike std::future, std::shared_future's shared state is not
invalidated when get() is called.

Which would make multiple get() calls from the same thread and on the same instance valid.

share|improve this answer

edited Jun 25 at 14:28

answered Jun 25 at 14:23

Sombrero ChickenSombrero Chicken

26.8k33 gold badges3535 silver badges8484 bronze badges

add a comment | 

8

AFAIK this is legal. std::shared_future<T>::get() says:

The behavior is undefined if valid() is false before the call to this
function.

Going to std::shared_future<T>::valid() it says:

Checks if the future refers to a shared state.

...

Unlike std::future, std::shared_future's shared state is not
invalidated when get() is called.

Which would make multiple get() calls from the same thread and on the same instance valid.

share|improve this answer

edited Jun 25 at 14:28

answered Jun 25 at 14:23

Sombrero ChickenSombrero Chicken

26.8k33 gold badges3535 silver badges8484 bronze badges

add a comment | 

AFAIK this is legal. std::shared_future<T>::get() says:

The behavior is undefined if valid() is false before the call to this
function.

Going to std::shared_future<T>::valid() it says:

Checks if the future refers to a shared state.

...

Unlike std::future, std::shared_future's shared state is not
invalidated when get() is called.

Which would make multiple get() calls from the same thread and on the same instance valid.

share|improve this answer

edited Jun 25 at 14:28

answered Jun 25 at 14:23

Sombrero ChickenSombrero Chicken

26.8k33 gold badges3535 silver badges8484 bronze badges

share|improve this answer

edited Jun 25 at 14:28

answered Jun 25 at 14:23

Sombrero ChickenSombrero Chicken

26.8k33 gold badges3535 silver badges8484 bronze badges

answered Jun 25 at 14:23

Sombrero ChickenSombrero Chicken

26.8k33 gold badges3535 silver badges8484 bronze badges

answered Jun 25 at 14:23

Sombrero ChickenSombrero Chicken

26.8k33 gold badges3535 silver badges8484 bronze badges