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Does the aliasing loophole apply to signed characters?
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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
edited 8 hours ago
StoryTeller
118k18 gold badges253 silver badges320 bronze badges
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
edited 8 hours ago
StoryTeller
118k18 gold badges253 silver badges320 bronze badges
asked 8 hours ago
BeeOnRopeBeeOnRope
29.5k9 gold badges97 silver badges199 bronze badges
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
edited 8 hours ago
StoryTeller
118k18 gold badges253 silver badges320 bronze badges
asked 8 hours ago
BeeOnRopeBeeOnRope
29.5k9 gold badges97 silver badges199 bronze badges
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
c++ language-lawyer
edited 8 hours ago
StoryTeller
118k18 gold badges253 silver badges320 bronze badges
asked 8 hours ago
BeeOnRopeBeeOnRope
29.5k9 gold badges97 silver badges199 bronze badges
edited 8 hours ago
StoryTeller
118k18 gold badges253 silver badges320 bronze badges
asked 8 hours ago
BeeOnRopeBeeOnRope
29.5k9 gold badges97 silver badges199 bronze badges
edited 8 hours ago
StoryTeller
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edited 8 hours ago
StoryTeller
118k18 gold badges253 silver badges320 bronze badges
edited 8 hours ago
StoryTeller
118k18 gold badges253 silver badges320 bronze badges
118k18 gold badges253 silver badges320 bronze badges
asked 8 hours ago
BeeOnRopeBeeOnRope
29.5k9 gold badges97 silver badges199 bronze badges
asked 8 hours ago
BeeOnRopeBeeOnRope
29.5k9 gold badges97 silver badges199 bronze badges
asked 8 hours ago
BeeOnRopeBeeOnRope
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1 Answer
1
active
oldest
votes
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, orstd::bytetype.
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.
answered 8 hours ago
StoryTellerStoryTeller
118k18 gold badges253 silver badges320 bronze badges
That's quite interesting. Why ischarallowed, which is signed on my machine, but notsigned 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 allowcharto be signed. For example, trap representations are allowed in all signed types, so asigned charmay not be able to represent every byte, andcharwould 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 charno longer has place for trap values eel.is/c++draft/basic.fundamental#7.
– StoryTeller
6 hours ago
add a comment |
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1 Answer
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active
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votes
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, orstd::bytetype.
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.
answered 8 hours ago
StoryTellerStoryTeller
118k18 gold badges253 silver badges320 bronze badges
That's quite interesting. Why ischarallowed, which is signed on my machine, but notsigned 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 allowcharto be signed. For example, trap representations are allowed in all signed types, so asigned charmay not be able to represent every byte, andcharwould 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 charno 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, orstd::bytetype.
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.
answered 8 hours ago
StoryTellerStoryTeller
118k18 gold badges253 silver badges320 bronze badges
That's quite interesting. Why ischarallowed, which is signed on my machine, but notsigned 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 allowcharto be signed. For example, trap representations are allowed in all signed types, so asigned charmay not be able to represent every byte, andcharwould 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 charno 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, orstd::bytetype.
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.
answered 8 hours ago
StoryTellerStoryTeller
118k18 gold badges253 silver badges320 bronze badges
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, orstd::bytetype.
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.
answered 8 hours ago
StoryTellerStoryTeller
118k18 gold badges253 silver badges320 bronze badges
edited 8 hours ago
answered 8 hours ago
StoryTellerStoryTeller
118k18 gold badges253 silver badges320 bronze badges
answered 8 hours ago
StoryTellerStoryTeller
118k18 gold badges253 silver badges320 bronze badges
answered 8 hours ago
StoryTellerStoryTeller
118k18 gold badges253 silver badges320 bronze badges
118k18 gold badges253 silver badges320 bronze badges
That's quite interesting. Why ischarallowed, which is signed on my machine, but notsigned 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 allowcharto be signed. For example, trap representations are allowed in all signed types, so asigned charmay not be able to represent every byte, andcharwould 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 charno longer has place for trap values eel.is/c++draft/basic.fundamental#7.
– StoryTeller
6 hours ago
add a comment |
That's quite interesting. Why ischarallowed, which is signed on my machine, but notsigned 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 allowcharto be signed. For example, trap representations are allowed in all signed types, so asigned charmay not be able to represent every byte, andcharwould 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 charno longer has place for trap values eel.is/c++draft/basic.fundamental#7.
– StoryTeller
6 hours ago
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
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
@mch - Could be an editorial mistake. I can only guess, really.
– StoryTeller
8 hours ago
3
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
@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
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
@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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