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9

In C++ there is an aliasing loophole which allows the object representation of any object to be read or written through some pointers of character type.

Does this apply only to char and unsigned char or also to signed char?

c++ language-lawyer

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edited 8 hours ago

StoryTeller

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9

In C++ there is an aliasing loophole which allows the object representation of any object to be read or written through some pointers of character type.

Does this apply only to char and unsigned char or also to signed char?

c++ language-lawyer

share|improve this question

edited 8 hours ago

StoryTeller

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asked 8 hours ago

BeeOnRopeBeeOnRope

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add a comment | 

In C++ there is an aliasing loophole which allows the object representation of any object to be read or written through some pointers of character type.

Does this apply only to char and unsigned char or also to signed char?

c++ language-lawyer

share|improve this question

edited 8 hours ago

StoryTeller

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asked 8 hours ago

BeeOnRopeBeeOnRope

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share|improve this question

edited 8 hours ago

StoryTeller

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asked 8 hours ago

BeeOnRopeBeeOnRope

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edited 8 hours ago

StoryTeller

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1 Answer
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12

No, the provision does not extend to signed char.

[basic.lval]

8 If a program attempts to access the stored value of an object
through a glvalue of other than one of the following types the
behavior is undefined:

• [...]


• a char, unsigned char, or std​::​byte type.

The quote above contains the very last bullet that pertains to aliasing with character types. signed char is excluded.

Nevertheless, this is also part of the subject CWG Issue 350 deals with, and so may change. Given the direction the issue has taken, the intent is for it to be (eventually, hopefully?) well-defined.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

StoryTellerStoryTeller

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• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  That's quite interesting. Why is char allowed, which is signed on my machine, but not signed char? But that's probably the reason why they want to add it in an upcoming standard.
  
  – mch
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @mch - Could be an editorial mistake. I can only guess, really.
  
  – StoryTeller
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @mch It's because in some cases, the standard does not allow char to be signed. For example, trap representations are allowed in all signed types, so a signed char may not be able to represent every byte, and char would have to be unsigned.
  
  – Artyer
  6 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Artyer - Can't believe I forgot that. Though it's worth noting that with C++20 moving towards "2's complement only", signed char no longer has place for trap values eel.is/c++draft/basic.fundamental#7.
  
  – StoryTeller
  6 hours ago
  

  
  
  
  

add a comment | 

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12

No, the provision does not extend to signed char.

[basic.lval]

8 If a program attempts to access the stored value of an object
through a glvalue of other than one of the following types the
behavior is undefined:

• [...]


• a char, unsigned char, or std​::​byte type.

The quote above contains the very last bullet that pertains to aliasing with character types. signed char is excluded.

Nevertheless, this is also part of the subject CWG Issue 350 deals with, and so may change. Given the direction the issue has taken, the intent is for it to be (eventually, hopefully?) well-defined.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

StoryTellerStoryTeller

118k1818 gold badges253253 silver badges320320 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  That's quite interesting. Why is char allowed, which is signed on my machine, but not signed char? But that's probably the reason why they want to add it in an upcoming standard.
  
  – mch
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @mch - Could be an editorial mistake. I can only guess, really.
  
  – StoryTeller
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @mch It's because in some cases, the standard does not allow char to be signed. For example, trap representations are allowed in all signed types, so a signed char may not be able to represent every byte, and char would have to be unsigned.
  
  – Artyer
  6 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Artyer - Can't believe I forgot that. Though it's worth noting that with C++20 moving towards "2's complement only", signed char no longer has place for trap values eel.is/c++draft/basic.fundamental#7.
  
  – StoryTeller
  6 hours ago
  

  
  
  
  

add a comment | 

12

No, the provision does not extend to signed char.

[basic.lval]

8 If a program attempts to access the stored value of an object
through a glvalue of other than one of the following types the
behavior is undefined:

• [...]


• a char, unsigned char, or std​::​byte type.

The quote above contains the very last bullet that pertains to aliasing with character types. signed char is excluded.

Nevertheless, this is also part of the subject CWG Issue 350 deals with, and so may change. Given the direction the issue has taken, the intent is for it to be (eventually, hopefully?) well-defined.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

StoryTellerStoryTeller

118k1818 gold badges253253 silver badges320320 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  That's quite interesting. Why is char allowed, which is signed on my machine, but not signed char? But that's probably the reason why they want to add it in an upcoming standard.
  
  – mch
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @mch - Could be an editorial mistake. I can only guess, really.
  
  – StoryTeller
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @mch It's because in some cases, the standard does not allow char to be signed. For example, trap representations are allowed in all signed types, so a signed char may not be able to represent every byte, and char would have to be unsigned.
  
  – Artyer
  6 hours ago
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Artyer - Can't believe I forgot that. Though it's worth noting that with C++20 moving towards "2's complement only", signed char no longer has place for trap values eel.is/c++draft/basic.fundamental#7.
  
  – StoryTeller
  6 hours ago
  

  
  
  
  

add a comment | 

No, the provision does not extend to signed char.

[basic.lval]

8 If a program attempts to access the stored value of an object
through a glvalue of other than one of the following types the
behavior is undefined:

• [...]


• a char, unsigned char, or std​::​byte type.

The quote above contains the very last bullet that pertains to aliasing with character types. signed char is excluded.

Nevertheless, this is also part of the subject CWG Issue 350 deals with, and so may change. Given the direction the issue has taken, the intent is for it to be (eventually, hopefully?) well-defined.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

StoryTellerStoryTeller

118k1818 gold badges253253 silver badges320320 bronze badges

share|improve this answer

edited 8 hours ago

answered 8 hours ago

StoryTellerStoryTeller

118k1818 gold badges253253 silver badges320320 bronze badges

answered 8 hours ago

StoryTellerStoryTeller

118k1818 gold badges253253 silver badges320320 bronze badges

answered 8 hours ago

StoryTellerStoryTeller

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