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Why did the range based 'for' loop specification change in C++17? [duplicate]

How the new range-based for loop in C++17 helps Ranges TS?What are the rules about using an underscore in a C++ identifier?What exactly do “IB” and “UB” mean?How to use range-based for() loop with std::map?C++11 reverse range-based for-loopWhy are elementwise additions much faster in separate loops than in a combined loop?Why does changing 0.1f to 0 slow down performance by 10x?Why is my program slow when looping over exactly 8192 elements?How the new range-based for loop in C++17 helps Ranges TS?Using Auto based Ranged for loop vs using pair based Ranged for looprange based for used for primitive typeRange based for in C++17 for custom container or general classes with different begin/end types

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margin-bottom:0;

60

2

I was looking over some ugly code (that was modifying the underlying sequence while iterating), and to explore the definition of the range-based for loop, I went to cppreference.

There I noticed something strange:

The range based for loop changed in C++17, but I do not see the reason for the change, and the code looks the same to me (just "refactored").
So the old one was:

 auto && __range = range_expression;
 for (auto __begin = begin_expr, __end = end_expr; __begin != __end; ++__begin) 
 range_declaration = *__begin;
 loop_statement
 
 

The new one is

 auto && __range = range_expression;
 auto __begin = begin_expr;
 auto __end = end_expr;
 for ( ; __begin != __end; ++__begin) 
 range_declaration = *__begin;
 loop_statement
 
 

Why was this change made, and does it make any legal C++14 programs exhibit undefined behavior (UB) in C++17?

c++ for-loop language-lawyer c++17

share|improve this question

edited Oct 4 at 17:18

Peter Mortensen

24.4k1919 gold badges8989 silver badges118118 bronze badges

asked Oct 2 at 19:50

NoSenseEtAlNoSenseEtAl

18.8k1818 gold badges8383 silver badges200200 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @r3musn0x I think this is probably a broader question than the one you’ve linked to.
  
  – Tim
  Oct 3 at 19:02
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Tim, I agree, however the accepted answer to that question is way more comprehensive and it covers both of them.
  
  – r3mus n0x
  Oct 3 at 19:11
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  As an aside, it's against the C++ standard to name your locals with double underscores. stackoverflow.com/a/228797
  
  – Max Barraclough
  Oct 5 at 14:59
  

  
  
  
  

add a comment
 | 

60

2

I was looking over some ugly code (that was modifying the underlying sequence while iterating), and to explore the definition of the range-based for loop, I went to cppreference.

There I noticed something strange:

The range based for loop changed in C++17, but I do not see the reason for the change, and the code looks the same to me (just "refactored").
So the old one was:

 auto && __range = range_expression;
 for (auto __begin = begin_expr, __end = end_expr; __begin != __end; ++__begin) 
 range_declaration = *__begin;
 loop_statement
 
 

The new one is

 auto && __range = range_expression;
 auto __begin = begin_expr;
 auto __end = end_expr;
 for ( ; __begin != __end; ++__begin) 
 range_declaration = *__begin;
 loop_statement
 
 

Why was this change made, and does it make any legal C++14 programs exhibit undefined behavior (UB) in C++17?

c++ for-loop language-lawyer c++17

share|improve this question

edited Oct 4 at 17:18

Peter Mortensen

24.4k1919 gold badges8989 silver badges118118 bronze badges

asked Oct 2 at 19:50

NoSenseEtAlNoSenseEtAl

18.8k1818 gold badges8383 silver badges200200 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @r3musn0x I think this is probably a broader question than the one you’ve linked to.
  
  – Tim
  Oct 3 at 19:02
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Tim, I agree, however the accepted answer to that question is way more comprehensive and it covers both of them.
  
  – r3mus n0x
  Oct 3 at 19:11
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  As an aside, it's against the C++ standard to name your locals with double underscores. stackoverflow.com/a/228797
  
  – Max Barraclough
  Oct 5 at 14:59
  

  
  
  
  

add a comment
 | 

I was looking over some ugly code (that was modifying the underlying sequence while iterating), and to explore the definition of the range-based for loop, I went to cppreference.

There I noticed something strange:

The range based for loop changed in C++17, but I do not see the reason for the change, and the code looks the same to me (just "refactored").
So the old one was:

 auto && __range = range_expression;
 for (auto __begin = begin_expr, __end = end_expr; __begin != __end; ++__begin) 
 range_declaration = *__begin;
 loop_statement
 
 

The new one is

 auto && __range = range_expression;
 auto __begin = begin_expr;
 auto __end = end_expr;
 for ( ; __begin != __end; ++__begin) 
 range_declaration = *__begin;
 loop_statement
 
 

Why was this change made, and does it make any legal C++14 programs exhibit undefined behavior (UB) in C++17?

c++ for-loop language-lawyer c++17

share|improve this question

edited Oct 4 at 17:18

Peter Mortensen

24.4k1919 gold badges8989 silver badges118118 bronze badges

asked Oct 2 at 19:50

NoSenseEtAlNoSenseEtAl

18.8k1818 gold badges8383 silver badges200200 bronze badges

 auto && __range = range_expression;
 for (auto __begin = begin_expr, __end = end_expr; __begin != __end; ++__begin) 
 range_declaration = *__begin;
 loop_statement
 
 

 auto && __range = range_expression;
 auto __begin = begin_expr;
 auto __end = end_expr;
 for ( ; __begin != __end; ++__begin) 
 range_declaration = *__begin;
 loop_statement
 
 

share|improve this question

edited Oct 4 at 17:18

Peter Mortensen

24.4k1919 gold badges8989 silver badges118118 bronze badges

asked Oct 2 at 19:50

NoSenseEtAlNoSenseEtAl

18.8k1818 gold badges8383 silver badges200200 bronze badges

share|improve this question

edited Oct 4 at 17:18

Peter Mortensen

24.4k1919 gold badges8989 silver badges118118 bronze badges

asked Oct 2 at 19:50

NoSenseEtAlNoSenseEtAl

18.8k1818 gold badges8383 silver badges200200 bronze badges

edited Oct 4 at 17:18

Peter Mortensen

24.4k1919 gold badges8989 silver badges118118 bronze badges

edited Oct 4 at 17:18

Peter Mortensen

24.4k1919 gold badges8989 silver badges118118 bronze badges

asked Oct 2 at 19:50

NoSenseEtAlNoSenseEtAl

18.8k1818 gold badges8383 silver badges200200 bronze badges

asked Oct 2 at 19:50

NoSenseEtAlNoSenseEtAl

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2 Answers
2

active

oldest

votes

79

Using

auto __begin = begin_expr, __end = end_expr;

requires both begin_expr and end_expr to return the same type. This means you cannot have a sentinel iterator type that is different from the beginning type. Using

auto __begin = begin_expr ;
auto __end = end_expr ;

fixes that issue while proving full backwards compatibility with C++14.

share|improve this answer

edited Oct 2 at 20:23

answered Oct 2 at 19:57

NathanOliver- Reinstate MonicaNathanOliver- Reinstate Monica

124k1919 gold badges197197 silver badges280280 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It would be more logical to have this update together with Ranges, probably. Iterator-sentinel ranges are still very restricted in C++17.
  
  – L. F.
  Oct 3 at 5:33
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @L.F. IIRC ranges was originally slated to be in C++17, instead we got a TS since it wasn't quite ready. We still need this if we want to use the ranges TS, or ranges-v3.
  
  – NathanOliver- Reinstate Monica
  Oct 3 at 12:30
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @L.F. Ranges are a library feature. There is range-v3 lib on github that served as the basis for standardization and it benefited from this standardiese change.
  
  – Dan M.
  Oct 3 at 13:12
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Including the feature in C++17 proper also allows programmers to implement their own sentinels (or similar) if desired, and can potentially allow bugs to be discovered before the associated library additions are ready, @L.F.
  
  – Justin Time - Reinstate Monica
  Oct 3 at 18:30
  

  
  
  
  

add a comment
 | 

31

It is explained later in the "notes":

As of C++17, the types of the begin_expr and the end_expr do not have to be the same ...

and you can't have that with:

auto __begin = begin_expr, __end = end_expr;

share|improve this answer

answered Oct 2 at 20:02

danadamdanadam

2,0151515 silver badges1414 bronze badges

add a comment
 | 

79

Using

auto __begin = begin_expr, __end = end_expr;

requires both begin_expr and end_expr to return the same type. This means you cannot have a sentinel iterator type that is different from the beginning type. Using

auto __begin = begin_expr ;
auto __end = end_expr ;

fixes that issue while proving full backwards compatibility with C++14.

share|improve this answer

edited Oct 2 at 20:23

answered Oct 2 at 19:57

NathanOliver- Reinstate MonicaNathanOliver- Reinstate Monica

124k1919 gold badges197197 silver badges280280 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It would be more logical to have this update together with Ranges, probably. Iterator-sentinel ranges are still very restricted in C++17.
  
  – L. F.
  Oct 3 at 5:33
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @L.F. IIRC ranges was originally slated to be in C++17, instead we got a TS since it wasn't quite ready. We still need this if we want to use the ranges TS, or ranges-v3.
  
  – NathanOliver- Reinstate Monica
  Oct 3 at 12:30
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @L.F. Ranges are a library feature. There is range-v3 lib on github that served as the basis for standardization and it benefited from this standardiese change.
  
  – Dan M.
  Oct 3 at 13:12
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Including the feature in C++17 proper also allows programmers to implement their own sentinels (or similar) if desired, and can potentially allow bugs to be discovered before the associated library additions are ready, @L.F.
  
  – Justin Time - Reinstate Monica
  Oct 3 at 18:30
  

  
  
  
  

add a comment
 | 

79

Using

auto __begin = begin_expr, __end = end_expr;

requires both begin_expr and end_expr to return the same type. This means you cannot have a sentinel iterator type that is different from the beginning type. Using

auto __begin = begin_expr ;
auto __end = end_expr ;

fixes that issue while proving full backwards compatibility with C++14.

share|improve this answer

edited Oct 2 at 20:23

answered Oct 2 at 19:57

NathanOliver- Reinstate MonicaNathanOliver- Reinstate Monica

124k1919 gold badges197197 silver badges280280 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  It would be more logical to have this update together with Ranges, probably. Iterator-sentinel ranges are still very restricted in C++17.
  
  – L. F.
  Oct 3 at 5:33
  
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @L.F. IIRC ranges was originally slated to be in C++17, instead we got a TS since it wasn't quite ready. We still need this if we want to use the ranges TS, or ranges-v3.
  
  – NathanOliver- Reinstate Monica
  Oct 3 at 12:30
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @L.F. Ranges are a library feature. There is range-v3 lib on github that served as the basis for standardization and it benefited from this standardiese change.
  
  – Dan M.
  Oct 3 at 13:12
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Including the feature in C++17 proper also allows programmers to implement their own sentinels (or similar) if desired, and can potentially allow bugs to be discovered before the associated library additions are ready, @L.F.
  
  – Justin Time - Reinstate Monica
  Oct 3 at 18:30
  

  
  
  
  

add a comment
 | 

Using

auto __begin = begin_expr, __end = end_expr;

requires both begin_expr and end_expr to return the same type. This means you cannot have a sentinel iterator type that is different from the beginning type. Using

auto __begin = begin_expr ;
auto __end = end_expr ;

fixes that issue while proving full backwards compatibility with C++14.

share|improve this answer

edited Oct 2 at 20:23

answered Oct 2 at 19:57

NathanOliver- Reinstate MonicaNathanOliver- Reinstate Monica

124k1919 gold badges197197 silver badges280280 bronze badges

auto __begin = begin_expr, __end = end_expr;

auto __begin = begin_expr ;
auto __end = end_expr ;

share|improve this answer

edited Oct 2 at 20:23

answered Oct 2 at 19:57

NathanOliver- Reinstate MonicaNathanOliver- Reinstate Monica

124k1919 gold badges197197 silver badges280280 bronze badges

answered Oct 2 at 19:57

NathanOliver- Reinstate MonicaNathanOliver- Reinstate Monica

124k1919 gold badges197197 silver badges280280 bronze badges

answered Oct 2 at 19:57

NathanOliver- Reinstate MonicaNathanOliver- Reinstate Monica

124k1919 gold badges197197 silver badges280280 bronze badges

31

It is explained later in the "notes":

As of C++17, the types of the begin_expr and the end_expr do not have to be the same ...

and you can't have that with:

auto __begin = begin_expr, __end = end_expr;

share|improve this answer

answered Oct 2 at 20:02

danadamdanadam

2,0151515 silver badges1414 bronze badges

add a comment
 | 

31

It is explained later in the "notes":

As of C++17, the types of the begin_expr and the end_expr do not have to be the same ...

and you can't have that with:

auto __begin = begin_expr, __end = end_expr;

share|improve this answer

answered Oct 2 at 20:02

danadamdanadam

2,0151515 silver badges1414 bronze badges

add a comment
 | 

It is explained later in the "notes":

As of C++17, the types of the begin_expr and the end_expr do not have to be the same ...

and you can't have that with:

auto __begin = begin_expr, __end = end_expr;

share|improve this answer

answered Oct 2 at 20:02

danadamdanadam

2,0151515 silver badges1414 bronze badges

auto __begin = begin_expr, __end = end_expr;

share|improve this answer

answered Oct 2 at 20:02

danadamdanadam

2,0151515 silver badges1414 bronze badges

answered Oct 2 at 20:02

danadamdanadam

2,0151515 silver badges1414 bronze badges

answered Oct 2 at 20:02

danadamdanadam

2,0151515 silver badges1414 bronze badges