Why does const auto &pnullptr work while auto *pnullptr doesn't in C++17?

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up vote
21
down vote

favorite

1

This definition works:

const auto &bnullptr;

while this fails:

auto *bnullptr;

I have tried to compile this in Visual C++, GCC, and Clang. They all complain "cannot deduce type".

In the second case, shouldn't b be deduced to have some type like std::nullptr_t?

c++ c++17

share|improve this question

edited yesterday

Peter Mortensen

12.9k1983111

asked yesterday

felix

929112

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  did you mean to use auto bnullptr; ?
  – Sander De Dycker
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  if you think the second bwould have type std::nullptr_t, what type do you think the first b has?
  – molbdnilo
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  Am I the only one that thinks std::nullptr_t b; is more readable?
  – rubenvb
  yesterday
  

  
  
  
  

add a comment | 

up vote
21
down vote

favorite

1

This definition works:

const auto &bnullptr;

while this fails:

auto *bnullptr;

I have tried to compile this in Visual C++, GCC, and Clang. They all complain "cannot deduce type".

In the second case, shouldn't b be deduced to have some type like std::nullptr_t?

c++ c++17

share|improve this question

edited yesterday

Peter Mortensen

12.9k1983111

asked yesterday

felix

929112

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  did you mean to use auto bnullptr; ?
  – Sander De Dycker
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  if you think the second bwould have type std::nullptr_t, what type do you think the first b has?
  – molbdnilo
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  Am I the only one that thinks std::nullptr_t b; is more readable?
  – rubenvb
  yesterday
  

  
  
  
  

add a comment | 

up vote
21
down vote

favorite

1

up vote
21
down vote

favorite

1

1

This definition works:

const auto &bnullptr;

while this fails:

auto *bnullptr;

I have tried to compile this in Visual C++, GCC, and Clang. They all complain "cannot deduce type".

In the second case, shouldn't b be deduced to have some type like std::nullptr_t?

c++ c++17

share|improve this question

edited yesterday

Peter Mortensen

12.9k1983111

asked yesterday

felix

929112

This definition works:

const auto &bnullptr;

while this fails:

auto *bnullptr;

I have tried to compile this in Visual C++, GCC, and Clang. They all complain "cannot deduce type".

In the second case, shouldn't b be deduced to have some type like std::nullptr_t?

c++ c++17

c++ c++17

share|improve this question

edited yesterday

Peter Mortensen

12.9k1983111

asked yesterday

felix

929112

share|improve this question

edited yesterday

Peter Mortensen

12.9k1983111

asked yesterday

felix

929112

share|improve this question

share|improve this question

edited yesterday

Peter Mortensen

12.9k1983111

edited yesterday

Peter Mortensen

12.9k1983111

edited yesterday

Peter Mortensen

12.9k1983111

12.9k1983111

asked yesterday

felix

929112

asked yesterday

felix

929112

asked yesterday

felix

929112

929112

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  did you mean to use auto bnullptr; ?
  – Sander De Dycker
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  if you think the second bwould have type std::nullptr_t, what type do you think the first b has?
  – molbdnilo
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  Am I the only one that thinks std::nullptr_t b; is more readable?
  – rubenvb
  yesterday
  

  
  
  
  

add a comment | 

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  did you mean to use auto bnullptr; ?
  – Sander De Dycker
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  if you think the second bwould have type std::nullptr_t, what type do you think the first b has?
  – molbdnilo
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  Am I the only one that thinks std::nullptr_t b; is more readable?
  – rubenvb
  yesterday
  

  
  
  
  

2

2

did you mean to use auto bnullptr; ?
– Sander De Dycker
yesterday

did you mean to use auto bnullptr; ?
– Sander De Dycker
yesterday

if you think the second bwould have type std::nullptr_t, what type do you think the first b has?
– molbdnilo
yesterday

if you think the second bwould have type std::nullptr_t, what type do you think the first b has?
– molbdnilo
yesterday

1

1

Am I the only one that thinks std::nullptr_t b; is more readable?
– rubenvb
yesterday

Am I the only one that thinks std::nullptr_t b; is more readable?
– rubenvb
yesterday

add a comment | 

3 Answers
3

active

oldest

votes

up vote
30
down vote



accepted

It's because you declare b to be a pointer, and initialize it to be a null pointer. But a null pointer to what type of data you don't say, so the compiler can't deduce the type.

If you want b to be a std::nullptr_t object, you should drop the asterisk:

auto bnullptr;

share|improve this answer

answered yesterday

Some programmer dude

282k23229384

• 
  
  
  

  
  21
  

  
  
  
  
  
  

  
  Ah, nullptr, the not-a-pointer pointer literal
  – Caleth
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  Thanks! I have just realized that nullptr has type std::nullptr_t which is not a pointer.... It is strange but make sense in this case.
  – felix
  yesterday
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  @felix It ceases to be strange when you take into account that nullptr is convertible to both all pointer types and also all member pointer types.
  – Angew
  yesterday
  

  
  
  
  

add a comment | 

up vote
12
down vote

decltype(nullptr) is std::nullptr_t.

so with

const auto &bnullptr; // auto is std::nullptr_t
// b is a reference to a temporary (with lifetime extension)

but nullptr is NOT a pointer (even if it is convertible to).

so auto *bnullptr; is invalid.

You might use instead

auto bnullptr; // auto is std::nullptr_t

share|improve this answer

edited yesterday

answered yesterday

Jarod42

106k1296168

add a comment | 

up vote
9
down vote

nullptr is of type std::nullptr_t. As a nullptr does not point to anything, there is no corresponding pointee type for std::nullptr_t (you are not allowed to dereference a nullptr), hence

auto *b nullptr;

requests a type that does not exist. If you want a to be of type nullptr_t simply write

auto b nullptr;

share|improve this answer

edited yesterday

answered yesterday

user463035818

14k22256

add a comment | 

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3 Answers
3

active

oldest

votes

3 Answers
3

active

oldest

votes

active

oldest

votes

active

oldest

votes

up vote
30
down vote



accepted

It's because you declare b to be a pointer, and initialize it to be a null pointer. But a null pointer to what type of data you don't say, so the compiler can't deduce the type.

If you want b to be a std::nullptr_t object, you should drop the asterisk:

auto bnullptr;

share|improve this answer

answered yesterday

Some programmer dude

282k23229384

• 
  
  
  

  
  21
  

  
  
  
  
  
  

  
  Ah, nullptr, the not-a-pointer pointer literal
  – Caleth
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  Thanks! I have just realized that nullptr has type std::nullptr_t which is not a pointer.... It is strange but make sense in this case.
  – felix
  yesterday
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  @felix It ceases to be strange when you take into account that nullptr is convertible to both all pointer types and also all member pointer types.
  – Angew
  yesterday
  

  
  
  
  

add a comment | 

up vote
30
down vote



accepted

It's because you declare b to be a pointer, and initialize it to be a null pointer. But a null pointer to what type of data you don't say, so the compiler can't deduce the type.

If you want b to be a std::nullptr_t object, you should drop the asterisk:

auto bnullptr;

share|improve this answer

answered yesterday

Some programmer dude

282k23229384

• 
  
  
  

  
  21
  

  
  
  
  
  
  

  
  Ah, nullptr, the not-a-pointer pointer literal
  – Caleth
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  Thanks! I have just realized that nullptr has type std::nullptr_t which is not a pointer.... It is strange but make sense in this case.
  – felix
  yesterday
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  @felix It ceases to be strange when you take into account that nullptr is convertible to both all pointer types and also all member pointer types.
  – Angew
  yesterday
  

  
  
  
  

add a comment | 

up vote
30
down vote



accepted

up vote
30
down vote



accepted

It's because you declare b to be a pointer, and initialize it to be a null pointer. But a null pointer to what type of data you don't say, so the compiler can't deduce the type.

If you want b to be a std::nullptr_t object, you should drop the asterisk:

auto bnullptr;

share|improve this answer

answered yesterday

Some programmer dude

282k23229384

It's because you declare b to be a pointer, and initialize it to be a null pointer. But a null pointer to what type of data you don't say, so the compiler can't deduce the type.

If you want b to be a std::nullptr_t object, you should drop the asterisk:

auto bnullptr;

share|improve this answer

answered yesterday

Some programmer dude

282k23229384

share|improve this answer

share|improve this answer

answered yesterday

Some programmer dude

282k23229384

answered yesterday

Some programmer dude

282k23229384

answered yesterday

Some programmer dude

282k23229384

282k23229384

• 
  
  
  

  
  21
  

  
  
  
  
  
  

  
  Ah, nullptr, the not-a-pointer pointer literal
  – Caleth
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  Thanks! I have just realized that nullptr has type std::nullptr_t which is not a pointer.... It is strange but make sense in this case.
  – felix
  yesterday
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  @felix It ceases to be strange when you take into account that nullptr is convertible to both all pointer types and also all member pointer types.
  – Angew
  yesterday
  

  
  
  
  

add a comment | 

• 
  
  
  

  
  21
  

  
  
  
  
  
  

  
  Ah, nullptr, the not-a-pointer pointer literal
  – Caleth
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  Thanks! I have just realized that nullptr has type std::nullptr_t which is not a pointer.... It is strange but make sense in this case.
  – felix
  yesterday
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  @felix It ceases to be strange when you take into account that nullptr is convertible to both all pointer types and also all member pointer types.
  – Angew
  yesterday
  

  
  
  
  

21

21

Ah, nullptr, the not-a-pointer pointer literal
– Caleth
yesterday

Ah, nullptr, the not-a-pointer pointer literal
– Caleth
yesterday

2

2

Thanks! I have just realized that nullptr has type std::nullptr_t which is not a pointer.... It is strange but make sense in this case.
– felix
yesterday

Thanks! I have just realized that nullptr has type std::nullptr_t which is not a pointer.... It is strange but make sense in this case.
– felix
yesterday

4

4

@felix It ceases to be strange when you take into account that nullptr is convertible to both all pointer types and also all member pointer types.
– Angew
yesterday

@felix It ceases to be strange when you take into account that nullptr is convertible to both all pointer types and also all member pointer types.
– Angew
yesterday

add a comment | 

up vote
12
down vote

decltype(nullptr) is std::nullptr_t.

so with

const auto &bnullptr; // auto is std::nullptr_t
// b is a reference to a temporary (with lifetime extension)

but nullptr is NOT a pointer (even if it is convertible to).

so auto *bnullptr; is invalid.

You might use instead

auto bnullptr; // auto is std::nullptr_t

share|improve this answer

edited yesterday

answered yesterday

Jarod42

106k1296168

add a comment | 

up vote
12
down vote

decltype(nullptr) is std::nullptr_t.

so with

const auto &bnullptr; // auto is std::nullptr_t
// b is a reference to a temporary (with lifetime extension)

but nullptr is NOT a pointer (even if it is convertible to).

so auto *bnullptr; is invalid.

You might use instead

auto bnullptr; // auto is std::nullptr_t

share|improve this answer

edited yesterday

answered yesterday

Jarod42

106k1296168

add a comment | 

up vote
12
down vote

up vote
12
down vote

decltype(nullptr) is std::nullptr_t.

so with

const auto &bnullptr; // auto is std::nullptr_t
// b is a reference to a temporary (with lifetime extension)

but nullptr is NOT a pointer (even if it is convertible to).

so auto *bnullptr; is invalid.

You might use instead

auto bnullptr; // auto is std::nullptr_t

share|improve this answer

edited yesterday

answered yesterday

Jarod42

106k1296168

decltype(nullptr) is std::nullptr_t.

so with

const auto &bnullptr; // auto is std::nullptr_t
// b is a reference to a temporary (with lifetime extension)

but nullptr is NOT a pointer (even if it is convertible to).

so auto *bnullptr; is invalid.

You might use instead

auto bnullptr; // auto is std::nullptr_t

share|improve this answer

edited yesterday

answered yesterday

Jarod42

106k1296168

share|improve this answer

share|improve this answer

edited yesterday

edited yesterday

edited yesterday

answered yesterday

Jarod42

106k1296168

answered yesterday

Jarod42

106k1296168

answered yesterday

Jarod42

106k1296168

106k1296168

add a comment | 

add a comment | 

up vote
9
down vote

nullptr is of type std::nullptr_t. As a nullptr does not point to anything, there is no corresponding pointee type for std::nullptr_t (you are not allowed to dereference a nullptr), hence

auto *b nullptr;

requests a type that does not exist. If you want a to be of type nullptr_t simply write

auto b nullptr;

share|improve this answer

edited yesterday

answered yesterday

user463035818

14k22256

add a comment | 

up vote
9
down vote

nullptr is of type std::nullptr_t. As a nullptr does not point to anything, there is no corresponding pointee type for std::nullptr_t (you are not allowed to dereference a nullptr), hence

auto *b nullptr;

requests a type that does not exist. If you want a to be of type nullptr_t simply write

auto b nullptr;

share|improve this answer

edited yesterday

answered yesterday

user463035818

14k22256

add a comment | 

up vote
9
down vote

up vote
9
down vote

nullptr is of type std::nullptr_t. As a nullptr does not point to anything, there is no corresponding pointee type for std::nullptr_t (you are not allowed to dereference a nullptr), hence

auto *b nullptr;

requests a type that does not exist. If you want a to be of type nullptr_t simply write

auto b nullptr;

share|improve this answer

edited yesterday

answered yesterday

user463035818

14k22256

nullptr is of type std::nullptr_t. As a nullptr does not point to anything, there is no corresponding pointee type for std::nullptr_t (you are not allowed to dereference a nullptr), hence

auto *b nullptr;

requests a type that does not exist. If you want a to be of type nullptr_t simply write

auto b nullptr;

share|improve this answer

edited yesterday

answered yesterday

user463035818

14k22256

share|improve this answer

share|improve this answer

edited yesterday

edited yesterday

edited yesterday

answered yesterday

user463035818

14k22256

answered yesterday

user463035818

14k22256

answered yesterday

user463035818

14k22256

14k22256

add a comment | 

add a comment | 

 

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