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How to declare an array without specifying its size, but with an initializer inside a class in C++?

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8

It is allowed to declare an array without explicitly stating its size, if it has an initializer:

// very fine: decltype(nums) is deduced to be int[3]
int nums[] = 5, 4, 3 ; 

However the same doesn't work when the array is declared in a class:

class dummy_class

 // incomplete type is not allowed (VS 2019 c++17)
 int nums[] = 5, 4, 3 ; 
;

Why is this the case?

c++ arrays language-lawyer c++17

share|improve this question

edited Jul 21 at 13:37

songyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

asked Jul 21 at 11:30

Yan B.Yan B.

1,09044 silver badges2020 bronze badges

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Because it's simply not allowed by the C++ language specification? Perhaps use a std::vector<int> instead?
  
  – Some programmer dude
  Jul 21 at 11:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Because ISO said so.
  
  – Ayxan
  Jul 21 at 11:37
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Someprogrammerdude I tried, but my type is non-copyable, (I used int as an example), so I can't use initializer-list-construction at all.
  
  – Yan B.
  Jul 21 at 11:37
  

  
  
  
  

add a comment | 

8

It is allowed to declare an array without explicitly stating its size, if it has an initializer:

// very fine: decltype(nums) is deduced to be int[3]
int nums[] = 5, 4, 3 ; 

However the same doesn't work when the array is declared in a class:

class dummy_class

 // incomplete type is not allowed (VS 2019 c++17)
 int nums[] = 5, 4, 3 ; 
;

Why is this the case?

c++ arrays language-lawyer c++17

share|improve this question

edited Jul 21 at 13:37

songyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

asked Jul 21 at 11:30

Yan B.Yan B.

1,09044 silver badges2020 bronze badges

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Because it's simply not allowed by the C++ language specification? Perhaps use a std::vector<int> instead?
  
  – Some programmer dude
  Jul 21 at 11:36
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Because ISO said so.
  
  – Ayxan
  Jul 21 at 11:37
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Someprogrammerdude I tried, but my type is non-copyable, (I used int as an example), so I can't use initializer-list-construction at all.
  
  – Yan B.
  Jul 21 at 11:37
  

  
  
  
  

add a comment | 

It is allowed to declare an array without explicitly stating its size, if it has an initializer:

// very fine: decltype(nums) is deduced to be int[3]
int nums[] = 5, 4, 3 ; 

However the same doesn't work when the array is declared in a class:

class dummy_class

 // incomplete type is not allowed (VS 2019 c++17)
 int nums[] = 5, 4, 3 ; 
;

Why is this the case?

c++ arrays language-lawyer c++17

share|improve this question

edited Jul 21 at 13:37

songyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

asked Jul 21 at 11:30

Yan B.Yan B.

1,09044 silver badges2020 bronze badges

// very fine: decltype(nums) is deduced to be int[3]
int nums[] = 5, 4, 3 ; 

class dummy_class

 // incomplete type is not allowed (VS 2019 c++17)
 int nums[] = 5, 4, 3 ; 
;

share|improve this question

edited Jul 21 at 13:37

songyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

asked Jul 21 at 11:30

Yan B.Yan B.

1,09044 silver badges2020 bronze badges

share|improve this question

edited Jul 21 at 13:37

songyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

asked Jul 21 at 11:30

Yan B.Yan B.

1,09044 silver badges2020 bronze badges

edited Jul 21 at 13:37

songyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

edited Jul 21 at 13:37

songyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

asked Jul 21 at 11:30

Yan B.Yan B.

1,09044 silver badges2020 bronze badges

asked Jul 21 at 11:30

Yan B.Yan B.

1,09044 silver badges2020 bronze badges

2 Answers
2

active

oldest

votes

14

This is not allowed because non-static data members might be initialized in different ways (with different sizes), including member initializer list, default member initializer, aggregate initialization, ... But the size of array must be fixed and known at compile-time, which can't be postponed until the initialization. e.g.

class dummy_class

 int nums[] = 5, 4, 3 ; 
 dummy_class(...some_parameters) : nums 5, 4, 3, 2 ()
 dummy_class(...some_other_parameters) : nums 5, 4, 3, 2, 1 ()
;

share|improve this answer

edited Jul 22 at 1:01

answered Jul 21 at 11:39

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  Also aggregate initialization of the class itself :)
  
  – Rakete1111
  Jul 21 at 11:41
  
  

  
  
  
  

add a comment | 

1

Since it is not allowed, you can do one of these two things:

• Either use the constructors/methods for initialization along with vector type declaration.


• Or try making the variable static, but I am afraid that may not be helpful in your case.

share|improve this answer

answered Jul 21 at 11:46

Shiv KumarShiv Kumar

1933 bronze badges

add a comment | 

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14

This is not allowed because non-static data members might be initialized in different ways (with different sizes), including member initializer list, default member initializer, aggregate initialization, ... But the size of array must be fixed and known at compile-time, which can't be postponed until the initialization. e.g.

class dummy_class

 int nums[] = 5, 4, 3 ; 
 dummy_class(...some_parameters) : nums 5, 4, 3, 2 ()
 dummy_class(...some_other_parameters) : nums 5, 4, 3, 2, 1 ()
;

share|improve this answer

edited Jul 22 at 1:01

answered Jul 21 at 11:39

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  Also aggregate initialization of the class itself :)
  
  – Rakete1111
  Jul 21 at 11:41
  
  

  
  
  
  

add a comment | 

14

This is not allowed because non-static data members might be initialized in different ways (with different sizes), including member initializer list, default member initializer, aggregate initialization, ... But the size of array must be fixed and known at compile-time, which can't be postponed until the initialization. e.g.

class dummy_class

 int nums[] = 5, 4, 3 ; 
 dummy_class(...some_parameters) : nums 5, 4, 3, 2 ()
 dummy_class(...some_other_parameters) : nums 5, 4, 3, 2, 1 ()
;

share|improve this answer

edited Jul 22 at 1:01

answered Jul 21 at 11:39

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  Also aggregate initialization of the class itself :)
  
  – Rakete1111
  Jul 21 at 11:41
  
  

  
  
  
  

add a comment | 

This is not allowed because non-static data members might be initialized in different ways (with different sizes), including member initializer list, default member initializer, aggregate initialization, ... But the size of array must be fixed and known at compile-time, which can't be postponed until the initialization. e.g.

class dummy_class

 int nums[] = 5, 4, 3 ; 
 dummy_class(...some_parameters) : nums 5, 4, 3, 2 ()
 dummy_class(...some_other_parameters) : nums 5, 4, 3, 2, 1 ()
;

share|improve this answer

edited Jul 22 at 1:01

answered Jul 21 at 11:39

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

class dummy_class

 int nums[] = 5, 4, 3 ; 
 dummy_class(...some_parameters) : nums 5, 4, 3, 2 ()
 dummy_class(...some_other_parameters) : nums 5, 4, 3, 2, 1 ()
;

share|improve this answer

edited Jul 22 at 1:01

answered Jul 21 at 11:39

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

answered Jul 21 at 11:39

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

answered Jul 21 at 11:39

songyuanyaosongyuanyao

102k1111 gold badges199199 silver badges265265 bronze badges

1

Since it is not allowed, you can do one of these two things:

• Either use the constructors/methods for initialization along with vector type declaration.


• Or try making the variable static, but I am afraid that may not be helpful in your case.

share|improve this answer

answered Jul 21 at 11:46

Shiv KumarShiv Kumar

1933 bronze badges

add a comment | 

1

Since it is not allowed, you can do one of these two things:

• Either use the constructors/methods for initialization along with vector type declaration.


• Or try making the variable static, but I am afraid that may not be helpful in your case.

share|improve this answer

answered Jul 21 at 11:46

Shiv KumarShiv Kumar

1933 bronze badges

add a comment | 

Since it is not allowed, you can do one of these two things:

• Either use the constructors/methods for initialization along with vector type declaration.


• Or try making the variable static, but I am afraid that may not be helpful in your case.

share|improve this answer

answered Jul 21 at 11:46

Shiv KumarShiv Kumar

1933 bronze badges

share|improve this answer

answered Jul 21 at 11:46

Shiv KumarShiv Kumar

1933 bronze badges

answered Jul 21 at 11:46

Shiv KumarShiv Kumar

1933 bronze badges

answered Jul 21 at 11:46

Shiv KumarShiv Kumar

1933 bronze badges