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Class variable with scope restricted to function

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-6

A member function will be called repeatedly.

I suppose it is time consuming to allocate memory to local variables every time.

A solution would be to declare these variables in the class declaration.

But I do not want them to be visible to other functions of the same class.

Is there a way to do that?

Tried to use namespaces, but it didn't work.

class A

 int n; // only function 'One' should see 'n'

 public:
 A(void); // default constructor
 int One(void); // member function
 int Two(void); // member function
;


A::A(void) // default constructor

 n = 7;



int A::One(void) // function 'One'

 return n; // no time lost declaring 'n'


int A::Two(void) // function 'Two'

 int n = 7; // obligatory, no other 'n' available
 return n;

c++

share|improve this question

asked Mar 27 at 17:38

user11132556user11132556

111 bronze badge

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  " I suppose it is time consuming to allocate memory to local variables every time." Is this actually true? Have you tested that?
  
  – Jodocus
  Mar 27 at 17:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Sounds like a XY-problem for me. What's the reason for this requirement: "But I do not want them to be visible to other functions of the same class."? However such could be probably achieved with inheritance constructs.
  
  – πάντα ῥεῖ
  Mar 27 at 17:47
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @user11132556 " I suppose it is time consuming to allocate memory to local variables every time." It's usually not. Please don't do premature that kindo micro optimization, before you actually hit a performance bottleneck with your application when local variables are allocated. If so it might be rather a problem with those constructor functions, but not really the allocation of the variable representation itself.
  
  – πάντα ῥεῖ
  Mar 27 at 18:01
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Good question. I had not tested, but now I did. In fact, it really takes time to allocate memory to local variables. But the subsequent function operations probably will take much more time. Surprisingly (to me), in some tests, repeated local variable declarations run faster than once outside function ones. Maybe reading values stored in local variables takes less time, I don't know.
  
  – user11132556
  Mar 27 at 19:05
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @user11132556 I supposed you have not tested that in a release build with optimisation enabled and stuff? It's almost useless to test debug builds.
  
  – Guillaume Racicot
  Mar 27 at 20:49
  

  
  
  
  

add a comment | 

-6

A member function will be called repeatedly.

I suppose it is time consuming to allocate memory to local variables every time.

A solution would be to declare these variables in the class declaration.

But I do not want them to be visible to other functions of the same class.

Is there a way to do that?

Tried to use namespaces, but it didn't work.

class A

 int n; // only function 'One' should see 'n'

 public:
 A(void); // default constructor
 int One(void); // member function
 int Two(void); // member function
;


A::A(void) // default constructor

 n = 7;



int A::One(void) // function 'One'

 return n; // no time lost declaring 'n'


int A::Two(void) // function 'Two'

 int n = 7; // obligatory, no other 'n' available
 return n;

c++

share|improve this question

asked Mar 27 at 17:38

user11132556user11132556

111 bronze badge

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  " I suppose it is time consuming to allocate memory to local variables every time." Is this actually true? Have you tested that?
  
  – Jodocus
  Mar 27 at 17:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Sounds like a XY-problem for me. What's the reason for this requirement: "But I do not want them to be visible to other functions of the same class."? However such could be probably achieved with inheritance constructs.
  
  – πάντα ῥεῖ
  Mar 27 at 17:47
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @user11132556 " I suppose it is time consuming to allocate memory to local variables every time." It's usually not. Please don't do premature that kindo micro optimization, before you actually hit a performance bottleneck with your application when local variables are allocated. If so it might be rather a problem with those constructor functions, but not really the allocation of the variable representation itself.
  
  – πάντα ῥεῖ
  Mar 27 at 18:01
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Good question. I had not tested, but now I did. In fact, it really takes time to allocate memory to local variables. But the subsequent function operations probably will take much more time. Surprisingly (to me), in some tests, repeated local variable declarations run faster than once outside function ones. Maybe reading values stored in local variables takes less time, I don't know.
  
  – user11132556
  Mar 27 at 19:05
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @user11132556 I supposed you have not tested that in a release build with optimisation enabled and stuff? It's almost useless to test debug builds.
  
  – Guillaume Racicot
  Mar 27 at 20:49
  

  
  
  
  

add a comment | 

A member function will be called repeatedly.

I suppose it is time consuming to allocate memory to local variables every time.

A solution would be to declare these variables in the class declaration.

But I do not want them to be visible to other functions of the same class.

Is there a way to do that?

Tried to use namespaces, but it didn't work.

class A

 int n; // only function 'One' should see 'n'

 public:
 A(void); // default constructor
 int One(void); // member function
 int Two(void); // member function
;


A::A(void) // default constructor

 n = 7;



int A::One(void) // function 'One'

 return n; // no time lost declaring 'n'


int A::Two(void) // function 'Two'

 int n = 7; // obligatory, no other 'n' available
 return n;

c++

share|improve this question

asked Mar 27 at 17:38

user11132556user11132556

111 bronze badge

class A

 int n; // only function 'One' should see 'n'

 public:
 A(void); // default constructor
 int One(void); // member function
 int Two(void); // member function
;


A::A(void) // default constructor

 n = 7;



int A::One(void) // function 'One'

 return n; // no time lost declaring 'n'


int A::Two(void) // function 'Two'

 int n = 7; // obligatory, no other 'n' available
 return n;

share|improve this question

asked Mar 27 at 17:38

user11132556user11132556

111 bronze badge

share|improve this question

asked Mar 27 at 17:38

user11132556user11132556

111 bronze badge

asked Mar 27 at 17:38

user11132556user11132556

111 bronze badge

asked Mar 27 at 17:38

user11132556user11132556

111 bronze badge

2 Answers
2

active

oldest

votes

2

you look at a wrong question/problem

• if you always want a given method returns a given value whatever the instance and the historic of course do not use an attribute for that


• if the value of the attribute depends on the instance use a (non static) attribute


• if the value does not depend on the instance but can change during the execution use a class instance (e.g. static)

But I do not want them to be visible to other functions of the same class.

This is not possible just using the visibility, all the methods of the class have (fortunately) access to the attributes of the class, this cannot be a problem because the class knows what it does. Of course the attribute must be at least protected.

Anyway

• if the value do not depends on the instance you can may be use a static var defined in the method.


• else you can use a map defined static in the method where the key are the instances and the value the expected value, but that complicate the way to do because you at least need to take into account the deletion of instances. I encourage you to not do that kind of gaz factory
  

share|improve this answer

edited Mar 27 at 17:54

answered Mar 27 at 17:48

brunobruno

19.3k33 gold badges1616 silver badges2828 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  "this is not possible" Well, introducing inheritance level indirections maybe.
  
  – πάντα ῥεῖ
  Mar 27 at 17:49
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @πάνταῥεῖ yes I said badly, I edited my answer
  
  – bruno
  Mar 27 at 17:51
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  "because the class knows what it does" At least its developer/maintainer should do ;-D ...
  
  – πάντα ῥεῖ
  Mar 27 at 17:53
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @πάνταῥεῖ the existence of the visibility says the developers don't know what they do (joke) ^^
  
  – bruno
  Mar 27 at 17:55
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  "I encourage you to not do that kind of gas factory" You made my day now, that deserves an upvote :-D :-D :-D
  
  – πάντα ῥεῖ
  Mar 27 at 17:56
  

  
  
  
  

add a comment | 

1

It is not time-consuming to allocate local variables, they are part of a call stack that is allocated in large blocks. What you are intending to do is the opposite of what is good practice. Good practice is to declare a variable in the smallest scope you can.
Also, you cannot hide fields from member functions in the same class.

share|improve this answer

answered Mar 27 at 17:43

Sean FSean F

2,18666 silver badges1414 bronze badges

add a comment | 

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2

you look at a wrong question/problem

• if you always want a given method returns a given value whatever the instance and the historic of course do not use an attribute for that


• if the value of the attribute depends on the instance use a (non static) attribute


• if the value does not depend on the instance but can change during the execution use a class instance (e.g. static)

But I do not want them to be visible to other functions of the same class.

This is not possible just using the visibility, all the methods of the class have (fortunately) access to the attributes of the class, this cannot be a problem because the class knows what it does. Of course the attribute must be at least protected.

Anyway

• if the value do not depends on the instance you can may be use a static var defined in the method.


• else you can use a map defined static in the method where the key are the instances and the value the expected value, but that complicate the way to do because you at least need to take into account the deletion of instances. I encourage you to not do that kind of gaz factory
  

share|improve this answer

edited Mar 27 at 17:54

answered Mar 27 at 17:48

brunobruno

19.3k33 gold badges1616 silver badges2828 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  "this is not possible" Well, introducing inheritance level indirections maybe.
  
  – πάντα ῥεῖ
  Mar 27 at 17:49
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @πάνταῥεῖ yes I said badly, I edited my answer
  
  – bruno
  Mar 27 at 17:51
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  "because the class knows what it does" At least its developer/maintainer should do ;-D ...
  
  – πάντα ῥεῖ
  Mar 27 at 17:53
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @πάνταῥεῖ the existence of the visibility says the developers don't know what they do (joke) ^^
  
  – bruno
  Mar 27 at 17:55
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  "I encourage you to not do that kind of gas factory" You made my day now, that deserves an upvote :-D :-D :-D
  
  – πάντα ῥεῖ
  Mar 27 at 17:56
  

  
  
  
  

add a comment | 

2

you look at a wrong question/problem

• if you always want a given method returns a given value whatever the instance and the historic of course do not use an attribute for that


• if the value of the attribute depends on the instance use a (non static) attribute


• if the value does not depend on the instance but can change during the execution use a class instance (e.g. static)

But I do not want them to be visible to other functions of the same class.

This is not possible just using the visibility, all the methods of the class have (fortunately) access to the attributes of the class, this cannot be a problem because the class knows what it does. Of course the attribute must be at least protected.

Anyway

• if the value do not depends on the instance you can may be use a static var defined in the method.


• else you can use a map defined static in the method where the key are the instances and the value the expected value, but that complicate the way to do because you at least need to take into account the deletion of instances. I encourage you to not do that kind of gaz factory
  

share|improve this answer

edited Mar 27 at 17:54

answered Mar 27 at 17:48

brunobruno

19.3k33 gold badges1616 silver badges2828 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  "this is not possible" Well, introducing inheritance level indirections maybe.
  
  – πάντα ῥεῖ
  Mar 27 at 17:49
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @πάνταῥεῖ yes I said badly, I edited my answer
  
  – bruno
  Mar 27 at 17:51
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  "because the class knows what it does" At least its developer/maintainer should do ;-D ...
  
  – πάντα ῥεῖ
  Mar 27 at 17:53
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @πάνταῥεῖ the existence of the visibility says the developers don't know what they do (joke) ^^
  
  – bruno
  Mar 27 at 17:55
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  "I encourage you to not do that kind of gas factory" You made my day now, that deserves an upvote :-D :-D :-D
  
  – πάντα ῥεῖ
  Mar 27 at 17:56
  

  
  
  
  

add a comment | 

you look at a wrong question/problem

• if you always want a given method returns a given value whatever the instance and the historic of course do not use an attribute for that


• if the value of the attribute depends on the instance use a (non static) attribute


• if the value does not depend on the instance but can change during the execution use a class instance (e.g. static)

But I do not want them to be visible to other functions of the same class.

This is not possible just using the visibility, all the methods of the class have (fortunately) access to the attributes of the class, this cannot be a problem because the class knows what it does. Of course the attribute must be at least protected.

Anyway

• if the value do not depends on the instance you can may be use a static var defined in the method.


• else you can use a map defined static in the method where the key are the instances and the value the expected value, but that complicate the way to do because you at least need to take into account the deletion of instances. I encourage you to not do that kind of gaz factory
  

share|improve this answer

edited Mar 27 at 17:54

answered Mar 27 at 17:48

brunobruno

19.3k33 gold badges1616 silver badges2828 bronze badges

share|improve this answer

edited Mar 27 at 17:54

answered Mar 27 at 17:48

brunobruno

19.3k33 gold badges1616 silver badges2828 bronze badges

answered Mar 27 at 17:48

brunobruno

19.3k33 gold badges1616 silver badges2828 bronze badges

answered Mar 27 at 17:48

brunobruno

19.3k33 gold badges1616 silver badges2828 bronze badges

1

It is not time-consuming to allocate local variables, they are part of a call stack that is allocated in large blocks. What you are intending to do is the opposite of what is good practice. Good practice is to declare a variable in the smallest scope you can.
Also, you cannot hide fields from member functions in the same class.

share|improve this answer

answered Mar 27 at 17:43

Sean FSean F

2,18666 silver badges1414 bronze badges

add a comment | 

1

It is not time-consuming to allocate local variables, they are part of a call stack that is allocated in large blocks. What you are intending to do is the opposite of what is good practice. Good practice is to declare a variable in the smallest scope you can.
Also, you cannot hide fields from member functions in the same class.

share|improve this answer

answered Mar 27 at 17:43

Sean FSean F

2,18666 silver badges1414 bronze badges

add a comment | 

It is not time-consuming to allocate local variables, they are part of a call stack that is allocated in large blocks. What you are intending to do is the opposite of what is good practice. Good practice is to declare a variable in the smallest scope you can.
Also, you cannot hide fields from member functions in the same class.

share|improve this answer

answered Mar 27 at 17:43

Sean FSean F

2,18666 silver badges1414 bronze badges

share|improve this answer

answered Mar 27 at 17:43

Sean FSean F

2,18666 silver badges1414 bronze badges

answered Mar 27 at 17:43

Sean FSean F

2,18666 silver badges1414 bronze badges

answered Mar 27 at 17:43

Sean FSean F

2,18666 silver badges1414 bronze badges