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What's the purpose of this lambda?

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What's the purpose of this lambda?

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8

I see the following lambda in C++ code. What's the purpose of it?

static const auto faster = []()
 std::ios::sync_with_stdio(false);
 std::cin.tie(nullptr);
 return nullptr;
();

c++ c++14

share|improve this question

asked 8 hours ago

Saliya EkanayakeSaliya Ekanayake

12488 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Call those functions before main.
  
  – Jarod42
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Jarod42 I think having this at namespace scope is fraught with peril. You could not guarantee whether it runs before or after any code that is part of initialization of non-local static variables in any other translation unit. And presumably this is what you care about - otherwise you would just put this at the beginning of main.
  
  – Brian
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Brian: Unless of course, you're writing a library that has no control over main.
  
  – Nicol Bolas
  8 hours ago
  

  
  
  
  

add a comment | 

8

I see the following lambda in C++ code. What's the purpose of it?

static const auto faster = []()
 std::ios::sync_with_stdio(false);
 std::cin.tie(nullptr);
 return nullptr;
();

c++ c++14

share|improve this question

asked 8 hours ago

Saliya EkanayakeSaliya Ekanayake

12488 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Call those functions before main.
  
  – Jarod42
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Jarod42 I think having this at namespace scope is fraught with peril. You could not guarantee whether it runs before or after any code that is part of initialization of non-local static variables in any other translation unit. And presumably this is what you care about - otherwise you would just put this at the beginning of main.
  
  – Brian
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Brian: Unless of course, you're writing a library that has no control over main.
  
  – Nicol Bolas
  8 hours ago
  

  
  
  
  

add a comment | 

I see the following lambda in C++ code. What's the purpose of it?

static const auto faster = []()
 std::ios::sync_with_stdio(false);
 std::cin.tie(nullptr);
 return nullptr;
();

c++ c++14

share|improve this question

asked 8 hours ago

Saliya EkanayakeSaliya Ekanayake

12488 bronze badges

static const auto faster = []()
 std::ios::sync_with_stdio(false);
 std::cin.tie(nullptr);
 return nullptr;
();

share|improve this question

asked 8 hours ago

Saliya EkanayakeSaliya Ekanayake

12488 bronze badges

share|improve this question

asked 8 hours ago

Saliya EkanayakeSaliya Ekanayake

12488 bronze badges

asked 8 hours ago

Saliya EkanayakeSaliya Ekanayake

12488 bronze badges

asked 8 hours ago

Saliya EkanayakeSaliya Ekanayake

12488 bronze badges

2 Answers
2

active

oldest

votes

16

A local static variable is initialized at most once, by the first thread that executes its declaration. By using a lambda, we can take advantage of this fact to run arbitrary code at most once. The first time the declaration is reached, the thread that reaches it will execute the code in the lambda as part of initializing the variable. The variable's value is presumably not actually used, but the program will remember that the variable has been initialized, so the lambda will not be run a second time.

share|improve this answer

answered 8 hours ago

BrianBrian

72k77 gold badges104104 silver badges203203 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  But the lambda isn't really what allows us to run arbitrary code right? We could use a normal function too? It seems to me the lambda is mainly there to make it a simple, single declaration?
  
  – Bas in het Veld
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @BasinhetVeld Yes, I agree.
  
  – Brian
  8 hours ago
  

  
  
  
  

add a comment | 

6

You typically see this code in competitive programming contest submissions for online judges that use C++ I/O behind the scenes. In order to boost the clock runtime of your submissions, you can untie the streams and unsychronize C++ IOStreams from the standard C streams. The use of the lambda is a slick way to execute the code immediately where the variable is declared. In order to get the full effect of this optimization it should be placed before any other code is executed.

share|improve this answer

edited 6 hours ago

answered 7 hours ago

0x499602D20x499602D2

71.5k2929 gold badges125125 silver badges213213 bronze badges

add a comment | 

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16

A local static variable is initialized at most once, by the first thread that executes its declaration. By using a lambda, we can take advantage of this fact to run arbitrary code at most once. The first time the declaration is reached, the thread that reaches it will execute the code in the lambda as part of initializing the variable. The variable's value is presumably not actually used, but the program will remember that the variable has been initialized, so the lambda will not be run a second time.

share|improve this answer

answered 8 hours ago

BrianBrian

72k77 gold badges104104 silver badges203203 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  But the lambda isn't really what allows us to run arbitrary code right? We could use a normal function too? It seems to me the lambda is mainly there to make it a simple, single declaration?
  
  – Bas in het Veld
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @BasinhetVeld Yes, I agree.
  
  – Brian
  8 hours ago
  

  
  
  
  

add a comment | 

16

A local static variable is initialized at most once, by the first thread that executes its declaration. By using a lambda, we can take advantage of this fact to run arbitrary code at most once. The first time the declaration is reached, the thread that reaches it will execute the code in the lambda as part of initializing the variable. The variable's value is presumably not actually used, but the program will remember that the variable has been initialized, so the lambda will not be run a second time.

share|improve this answer

answered 8 hours ago

BrianBrian

72k77 gold badges104104 silver badges203203 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  But the lambda isn't really what allows us to run arbitrary code right? We could use a normal function too? It seems to me the lambda is mainly there to make it a simple, single declaration?
  
  – Bas in het Veld
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @BasinhetVeld Yes, I agree.
  
  – Brian
  8 hours ago
  

  
  
  
  

add a comment | 

A local static variable is initialized at most once, by the first thread that executes its declaration. By using a lambda, we can take advantage of this fact to run arbitrary code at most once. The first time the declaration is reached, the thread that reaches it will execute the code in the lambda as part of initializing the variable. The variable's value is presumably not actually used, but the program will remember that the variable has been initialized, so the lambda will not be run a second time.

share|improve this answer

answered 8 hours ago

BrianBrian

72k77 gold badges104104 silver badges203203 bronze badges

share|improve this answer

answered 8 hours ago

BrianBrian

72k77 gold badges104104 silver badges203203 bronze badges

answered 8 hours ago

BrianBrian

72k77 gold badges104104 silver badges203203 bronze badges

answered 8 hours ago

BrianBrian

72k77 gold badges104104 silver badges203203 bronze badges

6

You typically see this code in competitive programming contest submissions for online judges that use C++ I/O behind the scenes. In order to boost the clock runtime of your submissions, you can untie the streams and unsychronize C++ IOStreams from the standard C streams. The use of the lambda is a slick way to execute the code immediately where the variable is declared. In order to get the full effect of this optimization it should be placed before any other code is executed.

share|improve this answer

edited 6 hours ago

answered 7 hours ago

0x499602D20x499602D2

71.5k2929 gold badges125125 silver badges213213 bronze badges

add a comment | 

6

You typically see this code in competitive programming contest submissions for online judges that use C++ I/O behind the scenes. In order to boost the clock runtime of your submissions, you can untie the streams and unsychronize C++ IOStreams from the standard C streams. The use of the lambda is a slick way to execute the code immediately where the variable is declared. In order to get the full effect of this optimization it should be placed before any other code is executed.

share|improve this answer

edited 6 hours ago

answered 7 hours ago

0x499602D20x499602D2

71.5k2929 gold badges125125 silver badges213213 bronze badges

add a comment | 

You typically see this code in competitive programming contest submissions for online judges that use C++ I/O behind the scenes. In order to boost the clock runtime of your submissions, you can untie the streams and unsychronize C++ IOStreams from the standard C streams. The use of the lambda is a slick way to execute the code immediately where the variable is declared. In order to get the full effect of this optimization it should be placed before any other code is executed.

share|improve this answer

edited 6 hours ago

answered 7 hours ago

0x499602D20x499602D2

71.5k2929 gold badges125125 silver badges213213 bronze badges

share|improve this answer

edited 6 hours ago

answered 7 hours ago

0x499602D20x499602D2

71.5k2929 gold badges125125 silver badges213213 bronze badges

answered 7 hours ago

0x499602D20x499602D2

71.5k2929 gold badges125125 silver badges213213 bronze badges

answered 7 hours ago

0x499602D20x499602D2

71.5k2929 gold badges125125 silver badges213213 bronze badges