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Function call after constructor declaration [duplicate]

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This question already has an answer here:

• 
  What is this weird colon-member (“ : ”) syntax in the constructor?
  
  12 answers
  
  

I'm currently reading some C++ code, and can't find an explanation for the following syntax.

The function rng_fn is just a random number generator that I want to seed with the same value every time the constructor is called. What does the rng_fn(nullptr) do after the colon, and does this override what happens inside the constructor?

#include Object.h

Object::Object() : rng_fn(nullptr)
 
 unsigned int seed = 1;
 rng_fn(seed);

(I have removed everything from the constructor except for the lines I don't understand.)

c++ constructor

share|improve this question

asked Mar 27 at 8:37

OrcaOrca

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Mar 27 at 8:53

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

add a comment | 

0

This question already has an answer here:

• 
  What is this weird colon-member (“ : ”) syntax in the constructor?
  
  12 answers
  
  

I'm currently reading some C++ code, and can't find an explanation for the following syntax.

The function rng_fn is just a random number generator that I want to seed with the same value every time the constructor is called. What does the rng_fn(nullptr) do after the colon, and does this override what happens inside the constructor?

#include Object.h

Object::Object() : rng_fn(nullptr)
 
 unsigned int seed = 1;
 rng_fn(seed);

(I have removed everything from the constructor except for the lines I don't understand.)

c++ constructor

share|improve this question

asked Mar 27 at 8:37

OrcaOrca

11555 bronze badges

marked as duplicate by StoryTeller c++
Users with the  c++ badge can single-handedly close c++ questions as duplicates and reopen them as needed.

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Mar 27 at 8:53

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

add a comment | 

This question already has an answer here:

• 
  What is this weird colon-member (“ : ”) syntax in the constructor?
  
  12 answers
  
  

I'm currently reading some C++ code, and can't find an explanation for the following syntax.

The function rng_fn is just a random number generator that I want to seed with the same value every time the constructor is called. What does the rng_fn(nullptr) do after the colon, and does this override what happens inside the constructor?

#include Object.h

Object::Object() : rng_fn(nullptr)
 
 unsigned int seed = 1;
 rng_fn(seed);

(I have removed everything from the constructor except for the lines I don't understand.)

c++ constructor

share|improve this question

asked Mar 27 at 8:37

OrcaOrca

11555 bronze badges

#include Object.h

Object::Object() : rng_fn(nullptr)
 
 unsigned int seed = 1;
 rng_fn(seed);

share|improve this question

asked Mar 27 at 8:37

OrcaOrca

11555 bronze badges

share|improve this question

asked Mar 27 at 8:37

OrcaOrca

11555 bronze badges

asked Mar 27 at 8:37

OrcaOrca

11555 bronze badges

asked Mar 27 at 8:37

OrcaOrca

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1 Answer
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What you are looking at is an initialization list. The members of the class are listed in order of declaration and constructed in place before the body of the constructor is even run.

share|improve this answer

answered Mar 27 at 8:39

BugSquasherBugSquasher

1101010 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I see, so any time the random number generated is used after the first two lines of the constructor, it will be seeded by 'seed'?
  
  – Orca
  Mar 27 at 8:43
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well this case is particular. If rng_fn is a function pointer, it will be initialized by a nullptr and then it attempts to dereference the function pointer and pass the seed to it.
  
  – BugSquasher
  Mar 27 at 14:38
  

  
  
  
  

add a comment | 

0

What you are looking at is an initialization list. The members of the class are listed in order of declaration and constructed in place before the body of the constructor is even run.

share|improve this answer

answered Mar 27 at 8:39

BugSquasherBugSquasher

1101010 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I see, so any time the random number generated is used after the first two lines of the constructor, it will be seeded by 'seed'?
  
  – Orca
  Mar 27 at 8:43
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well this case is particular. If rng_fn is a function pointer, it will be initialized by a nullptr and then it attempts to dereference the function pointer and pass the seed to it.
  
  – BugSquasher
  Mar 27 at 14:38
  

  
  
  
  

add a comment | 

0

What you are looking at is an initialization list. The members of the class are listed in order of declaration and constructed in place before the body of the constructor is even run.

share|improve this answer

answered Mar 27 at 8:39

BugSquasherBugSquasher

1101010 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I see, so any time the random number generated is used after the first two lines of the constructor, it will be seeded by 'seed'?
  
  – Orca
  Mar 27 at 8:43
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well this case is particular. If rng_fn is a function pointer, it will be initialized by a nullptr and then it attempts to dereference the function pointer and pass the seed to it.
  
  – BugSquasher
  Mar 27 at 14:38
  

  
  
  
  

add a comment | 

What you are looking at is an initialization list. The members of the class are listed in order of declaration and constructed in place before the body of the constructor is even run.

share|improve this answer

answered Mar 27 at 8:39

BugSquasherBugSquasher

1101010 bronze badges

share|improve this answer

answered Mar 27 at 8:39

BugSquasherBugSquasher

1101010 bronze badges

answered Mar 27 at 8:39

BugSquasherBugSquasher

1101010 bronze badges

answered Mar 27 at 8:39

BugSquasherBugSquasher

1101010 bronze badges