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What's the difference between (size_t)-1 and ~0?

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6

I've seen both (size_t)-1 and ~0 used to represent large numbers, or numbers with all their bits flipped.

Is there any difference between the two? If so, what is it?

I found this question: What is the difference between -1 and ~0, however it did not answer my question because I'm dealing with unsigned integers (such as size_t), as opposed to signed integers (such as int).

c unsigned ones-complement

share|improve this question

asked Apr 14 at 18:50

JL2210JL2210

1,439726

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Note that ~0 is a signed quantity — you'd need ~0U to make it unsigned.
  
  – Jonathan Leffler
  Apr 14 at 18:56
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Should I edit that?
  
  – JL2210
  Apr 14 at 18:58
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Since you've got some answers which addresses ~0 rather than ~0U, best to leave it unchanged, I think, but note for the future that it is a good idea to be careful. You can change a question up until making the change would invalidate answers.
  
  – Jonathan Leffler
  Apr 14 at 19:05
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  related: Is static_cast<T>(-1) the right way to generate all-one-bits data without numeric_limits?, What is the purpose of "int mask = ~0;"?
  
  – phuclv
  Apr 15 at 1:47
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Differences: First is C++. Second is not unsigned.
  
  – JL2210
  Apr 15 at 11:03
  

  
  
  
  

add a comment | 

6

I've seen both (size_t)-1 and ~0 used to represent large numbers, or numbers with all their bits flipped.

Is there any difference between the two? If so, what is it?

I found this question: What is the difference between -1 and ~0, however it did not answer my question because I'm dealing with unsigned integers (such as size_t), as opposed to signed integers (such as int).

c unsigned ones-complement

share|improve this question

asked Apr 14 at 18:50

JL2210JL2210

1,439726

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Note that ~0 is a signed quantity — you'd need ~0U to make it unsigned.
  
  – Jonathan Leffler
  Apr 14 at 18:56
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Should I edit that?
  
  – JL2210
  Apr 14 at 18:58
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Since you've got some answers which addresses ~0 rather than ~0U, best to leave it unchanged, I think, but note for the future that it is a good idea to be careful. You can change a question up until making the change would invalidate answers.
  
  – Jonathan Leffler
  Apr 14 at 19:05
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  related: Is static_cast<T>(-1) the right way to generate all-one-bits data without numeric_limits?, What is the purpose of "int mask = ~0;"?
  
  – phuclv
  Apr 15 at 1:47
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Differences: First is C++. Second is not unsigned.
  
  – JL2210
  Apr 15 at 11:03
  

  
  
  
  

add a comment | 

I've seen both (size_t)-1 and ~0 used to represent large numbers, or numbers with all their bits flipped.

Is there any difference between the two? If so, what is it?

I found this question: What is the difference between -1 and ~0, however it did not answer my question because I'm dealing with unsigned integers (such as size_t), as opposed to signed integers (such as int).

c unsigned ones-complement

share|improve this question

asked Apr 14 at 18:50

JL2210JL2210

1,439726

share|improve this question

asked Apr 14 at 18:50

JL2210JL2210

1,439726

share|improve this question

asked Apr 14 at 18:50

JL2210JL2210

1,439726

asked Apr 14 at 18:50

JL2210JL2210

1,439726

asked Apr 14 at 18:50

JL2210JL2210

1,439726

3 Answers
3

active

oldest

votes

6

What's the difference between (size_t)-1 and ~0?

Type and value differ.

(size_t)-1 is the same value as SIZE_MAX and has a type of size_t.

~0 is often -1 and has the type of int.

---

Assigning both of those to a size_t will result in SIZE_MAX.

size_t a = (size_t)-1; 
size_t b = ~0;

In the 2nd case, -1 is assigned to a b and undergoes a conversion first, wrapping around the -1 to the maximum size_t value.

share|improve this answer

edited Apr 14 at 19:10

answered Apr 14 at 18:56

chuxchux

86.5k875160

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Comments are not for extended discussion; this conversation has been moved to chat.
  
  – Bhargav Rao♦
  Apr 14 at 19:51
  

  
  
  
  

add a comment | 

3

(size_t)-1 is of type size_t. It typically has a value of 2³²-1 or 2⁶⁴-1 (4294967295 or 18446744073709551615).

~0 is of type int, and has the value -1 on a 2's-complement system (i.e., just about everywhere).

Both are likely to have the same bit pattern -- if int and size_t are the same size, which they very commonly are not.

If you want the maximum value of type size_t, you can use the SIZE_MAX macro, defined in <stdint.h>. If you're using an older implementation (pre-C99) that doesn't provide SIZE_MAX, (size_t)-1 will work. I'm not sure why you'd want to write ~0 rather than -1 -- unless perhaps you're considering non-two's-complement systems.

share|improve this answer

edited Apr 14 at 19:01

Jonathan Leffler

582k966961052

answered Apr 14 at 19:00

Keith ThompsonKeith Thompson

197k27292490

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks for letting me know. I'll remember to define SIZE_MAX in my stdint.h implementation.
  
  – JL2210
  Apr 14 at 19:02
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Re; ~0 versus -1 - some compilers warn on implicit wrapping in constant expressions.
  
  – TLW
  Apr 14 at 21:43
  

  
  
  
  

add a comment | 

2

Note that the previous answers assume a 2's complement machine (very likely to be the case these days, but not guaranteed).

If you had a sign-magnitude machine then -1 would have a sign bit and least significant bit set with all others clear, if you had a 1's complement machine then -1 would have all bits but the LSB set.

In all of these cases (including the common 2's complement machine), ~0 has all bits set.

share|improve this answer

edited Apr 15 at 0:22

JL2210

1,439726

answered Apr 14 at 20:39

SoronelHaetirSoronelHaetir

7,4911514

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Note that in the question I'm asking about unsigned integers, so there's no such thing as a "sign bit".
  
  – JL2210
  Apr 14 at 20:56
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @JL2210 - note that the intermediate values of both ~0 and -1 are signed, not unsigned.
  
  – TLW
  Apr 14 at 21:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah. I understand now. I'll upvote as soon as this is edited.
  
  – JL2210
  Apr 14 at 22:02
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Concerning non-2s-complement: The most recent non-2's complement machine I have heard of was from somewhere in 2000-2008 and I have not used one since pre-1990. Do you know of any in use in the last 10 years?
  
  – chux
  Apr 15 at 0:54
  

  
  
  
  

add a comment | 

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6

What's the difference between (size_t)-1 and ~0?

Type and value differ.

(size_t)-1 is the same value as SIZE_MAX and has a type of size_t.

~0 is often -1 and has the type of int.

---

Assigning both of those to a size_t will result in SIZE_MAX.

size_t a = (size_t)-1; 
size_t b = ~0;

In the 2nd case, -1 is assigned to a b and undergoes a conversion first, wrapping around the -1 to the maximum size_t value.

share|improve this answer

edited Apr 14 at 19:10

answered Apr 14 at 18:56

chuxchux

86.5k875160

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Comments are not for extended discussion; this conversation has been moved to chat.
  
  – Bhargav Rao♦
  Apr 14 at 19:51
  

  
  
  
  

add a comment | 

6

What's the difference between (size_t)-1 and ~0?

Type and value differ.

(size_t)-1 is the same value as SIZE_MAX and has a type of size_t.

~0 is often -1 and has the type of int.

---

Assigning both of those to a size_t will result in SIZE_MAX.

size_t a = (size_t)-1; 
size_t b = ~0;

In the 2nd case, -1 is assigned to a b and undergoes a conversion first, wrapping around the -1 to the maximum size_t value.

share|improve this answer

edited Apr 14 at 19:10

answered Apr 14 at 18:56

chuxchux

86.5k875160

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Comments are not for extended discussion; this conversation has been moved to chat.
  
  – Bhargav Rao♦
  Apr 14 at 19:51
  

  
  
  
  

add a comment | 

What's the difference between (size_t)-1 and ~0?

Type and value differ.

(size_t)-1 is the same value as SIZE_MAX and has a type of size_t.

~0 is often -1 and has the type of int.

---

Assigning both of those to a size_t will result in SIZE_MAX.

size_t a = (size_t)-1; 
size_t b = ~0;

In the 2nd case, -1 is assigned to a b and undergoes a conversion first, wrapping around the -1 to the maximum size_t value.

share|improve this answer

edited Apr 14 at 19:10

answered Apr 14 at 18:56

chuxchux

86.5k875160

size_t a = (size_t)-1; 
size_t b = ~0;

share|improve this answer

edited Apr 14 at 19:10

answered Apr 14 at 18:56

chuxchux

86.5k875160

answered Apr 14 at 18:56

chuxchux

86.5k875160

answered Apr 14 at 18:56

chuxchux

86.5k875160

3

(size_t)-1 is of type size_t. It typically has a value of 2³²-1 or 2⁶⁴-1 (4294967295 or 18446744073709551615).

~0 is of type int, and has the value -1 on a 2's-complement system (i.e., just about everywhere).

Both are likely to have the same bit pattern -- if int and size_t are the same size, which they very commonly are not.

If you want the maximum value of type size_t, you can use the SIZE_MAX macro, defined in <stdint.h>. If you're using an older implementation (pre-C99) that doesn't provide SIZE_MAX, (size_t)-1 will work. I'm not sure why you'd want to write ~0 rather than -1 -- unless perhaps you're considering non-two's-complement systems.

share|improve this answer

edited Apr 14 at 19:01

Jonathan Leffler

582k966961052

answered Apr 14 at 19:00

Keith ThompsonKeith Thompson

197k27292490

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks for letting me know. I'll remember to define SIZE_MAX in my stdint.h implementation.
  
  – JL2210
  Apr 14 at 19:02
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Re; ~0 versus -1 - some compilers warn on implicit wrapping in constant expressions.
  
  – TLW
  Apr 14 at 21:43
  

  
  
  
  

add a comment | 

3

(size_t)-1 is of type size_t. It typically has a value of 2³²-1 or 2⁶⁴-1 (4294967295 or 18446744073709551615).

~0 is of type int, and has the value -1 on a 2's-complement system (i.e., just about everywhere).

Both are likely to have the same bit pattern -- if int and size_t are the same size, which they very commonly are not.

If you want the maximum value of type size_t, you can use the SIZE_MAX macro, defined in <stdint.h>. If you're using an older implementation (pre-C99) that doesn't provide SIZE_MAX, (size_t)-1 will work. I'm not sure why you'd want to write ~0 rather than -1 -- unless perhaps you're considering non-two's-complement systems.

share|improve this answer

edited Apr 14 at 19:01

Jonathan Leffler

582k966961052

answered Apr 14 at 19:00

Keith ThompsonKeith Thompson

197k27292490

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks for letting me know. I'll remember to define SIZE_MAX in my stdint.h implementation.
  
  – JL2210
  Apr 14 at 19:02
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Re; ~0 versus -1 - some compilers warn on implicit wrapping in constant expressions.
  
  – TLW
  Apr 14 at 21:43
  

  
  
  
  

add a comment | 

(size_t)-1 is of type size_t. It typically has a value of 2³²-1 or 2⁶⁴-1 (4294967295 or 18446744073709551615).

~0 is of type int, and has the value -1 on a 2's-complement system (i.e., just about everywhere).

Both are likely to have the same bit pattern -- if int and size_t are the same size, which they very commonly are not.

If you want the maximum value of type size_t, you can use the SIZE_MAX macro, defined in <stdint.h>. If you're using an older implementation (pre-C99) that doesn't provide SIZE_MAX, (size_t)-1 will work. I'm not sure why you'd want to write ~0 rather than -1 -- unless perhaps you're considering non-two's-complement systems.

share|improve this answer

edited Apr 14 at 19:01

Jonathan Leffler

582k966961052

answered Apr 14 at 19:00

Keith ThompsonKeith Thompson

197k27292490

share|improve this answer

edited Apr 14 at 19:01

Jonathan Leffler

582k966961052

answered Apr 14 at 19:00

Keith ThompsonKeith Thompson

197k27292490

edited Apr 14 at 19:01

Jonathan Leffler

582k966961052

edited Apr 14 at 19:01

Jonathan Leffler

582k966961052

answered Apr 14 at 19:00

Keith ThompsonKeith Thompson

197k27292490

answered Apr 14 at 19:00

Keith ThompsonKeith Thompson

197k27292490

2

Note that the previous answers assume a 2's complement machine (very likely to be the case these days, but not guaranteed).

If you had a sign-magnitude machine then -1 would have a sign bit and least significant bit set with all others clear, if you had a 1's complement machine then -1 would have all bits but the LSB set.

In all of these cases (including the common 2's complement machine), ~0 has all bits set.

share|improve this answer

edited Apr 15 at 0:22

JL2210

1,439726

answered Apr 14 at 20:39

SoronelHaetirSoronelHaetir

7,4911514

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Note that in the question I'm asking about unsigned integers, so there's no such thing as a "sign bit".
  
  – JL2210
  Apr 14 at 20:56
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @JL2210 - note that the intermediate values of both ~0 and -1 are signed, not unsigned.
  
  – TLW
  Apr 14 at 21:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah. I understand now. I'll upvote as soon as this is edited.
  
  – JL2210
  Apr 14 at 22:02
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Concerning non-2s-complement: The most recent non-2's complement machine I have heard of was from somewhere in 2000-2008 and I have not used one since pre-1990. Do you know of any in use in the last 10 years?
  
  – chux
  Apr 15 at 0:54
  

  
  
  
  

add a comment | 

2

Note that the previous answers assume a 2's complement machine (very likely to be the case these days, but not guaranteed).

If you had a sign-magnitude machine then -1 would have a sign bit and least significant bit set with all others clear, if you had a 1's complement machine then -1 would have all bits but the LSB set.

In all of these cases (including the common 2's complement machine), ~0 has all bits set.

share|improve this answer

edited Apr 15 at 0:22

JL2210

1,439726

answered Apr 14 at 20:39

SoronelHaetirSoronelHaetir

7,4911514

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Note that in the question I'm asking about unsigned integers, so there's no such thing as a "sign bit".
  
  – JL2210
  Apr 14 at 20:56
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @JL2210 - note that the intermediate values of both ~0 and -1 are signed, not unsigned.
  
  – TLW
  Apr 14 at 21:41
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ah. I understand now. I'll upvote as soon as this is edited.
  
  – JL2210
  Apr 14 at 22:02
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Concerning non-2s-complement: The most recent non-2's complement machine I have heard of was from somewhere in 2000-2008 and I have not used one since pre-1990. Do you know of any in use in the last 10 years?
  
  – chux
  Apr 15 at 0:54
  

  
  
  
  

add a comment | 

Note that the previous answers assume a 2's complement machine (very likely to be the case these days, but not guaranteed).

If you had a sign-magnitude machine then -1 would have a sign bit and least significant bit set with all others clear, if you had a 1's complement machine then -1 would have all bits but the LSB set.

In all of these cases (including the common 2's complement machine), ~0 has all bits set.

share|improve this answer

edited Apr 15 at 0:22

JL2210

1,439726

answered Apr 14 at 20:39

SoronelHaetirSoronelHaetir

7,4911514

share|improve this answer

edited Apr 15 at 0:22

JL2210

1,439726

answered Apr 14 at 20:39

SoronelHaetirSoronelHaetir

7,4911514

edited Apr 15 at 0:22

JL2210

1,439726

edited Apr 15 at 0:22

JL2210

1,439726

answered Apr 14 at 20:39

SoronelHaetirSoronelHaetir

7,4911514

answered Apr 14 at 20:39

SoronelHaetirSoronelHaetir

7,4911514