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In Stroustrup's example, what does this colon mean in `return 1 : 2`? It's not a label or ternary operator

What does the explicit keyword mean?What does “static” mean in C?What does map(&:name) mean in Ruby?What does the question mark and the colon (?: ternary operator) mean in objective-c?What does the star operator mean?What does int argc, char *argv[] mean?What is the meaning of prepended double colon “::”?What does “dereferencing” a pointer mean?What does T&& (double ampersand) mean in C++11?C++11 introduced a standardized memory model. What does it mean? And how is it going to affect C++ programming?

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121

4

I don't understand one particular use of a colon.

I found it in the book "The C++ programming language" by Bjarne Stroustrup, 4th edition, section 11.4.4, page 297:

void g(double y)

 [&] f(y); // return type is void
 auto z1 = [=](int x) return x+y; // return type is double
 auto z2 = [=,y] if (y) return 1; else return 2; // error : body too complicated
 // for return type deduction
 auto z3 =[y]() return 1 : 2; // (Me: HERE!!!) return type is int
 auto z4 = [=,y]()−>int if (y) return 1; else return 2; // OK: explicit return type

All comments from Stroustrup, except the one inside the parentheses.

I have no idea what it could be.

It seems like a conditional ternary operator without the first member (and without the "?"), but in that case I don't understand how it could work without a condition.

c++ syntax

share|improve this question

edited yesterday

ᆼᆺᆼ

9,00243564

asked Apr 30 at 22:16

PiockñecPiockñec

666157

• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  
  It's a compile error on my end (gcc and clang). Plus all those lines need semicolons, but still an error.
  
  – Cruz Jean
  Apr 30 at 22:19
  
  

  
  
  
  


• 
  
  
  

  
  184
  

  
  
  
  
  
  

  
  
  Moderator Note: Please think very carefully before casting a vote to close this as a "typo" question. Yes, the problem is a typo, but it's not a typo that the asker made. Rather, it is one found in a published book. That means this question and its answers may well be useful to others in the future, which is a strong counter-indicator for closing it as a typo. (UPDATE: This topic is now being discussed on Meta; please feel free to weigh in there.)
  
  – Cody Gray♦
  May 1 at 1:11
  
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Perhaps the best answer would be: Try to compile the code; if it doesn't compile, that's a good indication that it's a typo.
  
  – John Wiersba
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I can think of a number of examples off the top of my head that fail to compile (or even cause an internal compiler error) on one compiler, but are accepted without issue on a different one
  
  – J. Antonio Perez
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @John I just tried some fold expressions with MSVC and they didn't compile. So clearly the whole chapter I just read must be a typo? ;) C++ compilers fail to compile valid C++ code all the time, comes from the language being absurdly complicated.
  
  – Voo
  23 hours ago
  
  

  
  
  
  

 | 
show 2 more comments

121

4

I don't understand one particular use of a colon.

I found it in the book "The C++ programming language" by Bjarne Stroustrup, 4th edition, section 11.4.4, page 297:

void g(double y)

 [&] f(y); // return type is void
 auto z1 = [=](int x) return x+y; // return type is double
 auto z2 = [=,y] if (y) return 1; else return 2; // error : body too complicated
 // for return type deduction
 auto z3 =[y]() return 1 : 2; // (Me: HERE!!!) return type is int
 auto z4 = [=,y]()−>int if (y) return 1; else return 2; // OK: explicit return type

All comments from Stroustrup, except the one inside the parentheses.

I have no idea what it could be.

It seems like a conditional ternary operator without the first member (and without the "?"), but in that case I don't understand how it could work without a condition.

c++ syntax

share|improve this question

edited yesterday

ᆼᆺᆼ

9,00243564

asked Apr 30 at 22:16

PiockñecPiockñec

666157

• 
  
  
  

  
  6
  

  
  
  
  
  
  

  
  
  It's a compile error on my end (gcc and clang). Plus all those lines need semicolons, but still an error.
  
  – Cruz Jean
  Apr 30 at 22:19
  
  

  
  
  
  


• 
  
  
  

  
  184
  

  
  
  
  
  
  

  
  
  Moderator Note: Please think very carefully before casting a vote to close this as a "typo" question. Yes, the problem is a typo, but it's not a typo that the asker made. Rather, it is one found in a published book. That means this question and its answers may well be useful to others in the future, which is a strong counter-indicator for closing it as a typo. (UPDATE: This topic is now being discussed on Meta; please feel free to weigh in there.)
  
  – Cody Gray♦
  May 1 at 1:11
  
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Perhaps the best answer would be: Try to compile the code; if it doesn't compile, that's a good indication that it's a typo.
  
  – John Wiersba
  yesterday
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I can think of a number of examples off the top of my head that fail to compile (or even cause an internal compiler error) on one compiler, but are accepted without issue on a different one
  
  – J. Antonio Perez
  yesterday
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @John I just tried some fold expressions with MSVC and they didn't compile. So clearly the whole chapter I just read must be a typo? ;) C++ compilers fail to compile valid C++ code all the time, comes from the language being absurdly complicated.
  
  – Voo
  23 hours ago
  
  

  
  
  
  

 | 
show 2 more comments

I don't understand one particular use of a colon.

I found it in the book "The C++ programming language" by Bjarne Stroustrup, 4th edition, section 11.4.4, page 297:

void g(double y)

 [&] f(y); // return type is void
 auto z1 = [=](int x) return x+y; // return type is double
 auto z2 = [=,y] if (y) return 1; else return 2; // error : body too complicated
 // for return type deduction
 auto z3 =[y]() return 1 : 2; // (Me: HERE!!!) return type is int
 auto z4 = [=,y]()−>int if (y) return 1; else return 2; // OK: explicit return type

All comments from Stroustrup, except the one inside the parentheses.

I have no idea what it could be.

It seems like a conditional ternary operator without the first member (and without the "?"), but in that case I don't understand how it could work without a condition.

c++ syntax

share|improve this question

edited yesterday

ᆼᆺᆼ

9,00243564

asked Apr 30 at 22:16

PiockñecPiockñec

666157

void g(double y)

 [&] f(y); // return type is void
 auto z1 = [=](int x) return x+y; // return type is double
 auto z2 = [=,y] if (y) return 1; else return 2; // error : body too complicated
 // for return type deduction
 auto z3 =[y]() return 1 : 2; // (Me: HERE!!!) return type is int
 auto z4 = [=,y]()−>int if (y) return 1; else return 2; // OK: explicit return type

share|improve this question

edited yesterday

ᆼᆺᆼ

9,00243564

asked Apr 30 at 22:16

PiockñecPiockñec

666157

share|improve this question

edited yesterday

ᆼᆺᆼ

9,00243564

asked Apr 30 at 22:16

PiockñecPiockñec

666157

edited yesterday

ᆼᆺᆼ

9,00243564

edited yesterday

ᆼᆺᆼ

9,00243564

asked Apr 30 at 22:16

PiockñecPiockñec

666157

asked Apr 30 at 22:16

PiockñecPiockñec

666157

3 Answers
3

active

oldest

votes

164

It's a typo. Look at Errata for 2nd and 3rd printings of The C++ Programming Language. The example must be like below:

auto z3 =[y]() return (y) ? 1 : 2; 

share|improve this answer

edited Apr 30 at 22:48

answered Apr 30 at 22:21

S.M.S.M.

7,11752231

• 
  
  
  

  
  9
  

  
  
  
  
  
  

  
  
  Why (y) and not just y?
  
  – Little Helper
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @LittleHelper Perhaps its a best practice or something, I always see it written like that. Maybe to avoid confusion with more complicated comparisons...
  
  – Redwolf Programs
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  22
  

  
  
  
  
  
  

  
  
  Personally, I often use (cond) ? a : b for clarity -- it helps me avoid misreading e.g. the statement foo = x > y ? a : b as foo = x ... when skimming through code.
  
  – grawity
  2 days ago
  
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  @LittleHelper It's not really needed there. However in a function-like macro it's best practise to put parentheses round the arguments where they are used, because otherwise expansion of the arguments can give unexpected behaviour. Consider a function-like macro to double a value "foo(x) x * 2" where you call it with "foo(2+3)". The result will be 2+(3*2) because the argument gets expanded as-is and precedence rules take over. If your macro is "foo(x) (x)*2" then you will correctly get (2+3)*2. It may be that Stroustrup has a habit of using that style everywhere for coding safety.
  
  – Graham
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Graham Very unlikely. Stroustrup essentially doesn't write function macros (C++ inline functions are better). Much more likely is that the ternary operator has somewhat complicated precedence rules, so it is good to habitually clarify the precedence with parens.
  
  – Martin Bonner
  22 hours ago
  

  
  
  
  

add a comment | 

15

Looks to me like a simple typo. Should probably be:

auto z3 =[y]() return y ? 1 : 2; 

Note that since the lambda doesn't take any parameters, the parens are optional. You could use this instead, if you preferred:

auto z3 =[y] return y ? 1 : 2; 

share|improve this answer

answered Apr 30 at 22:21

Jerry CoffinJerry Coffin

391k52477925

add a comment | 

10

return 1 : 2; is a syntax error, it is not valid code.

A correct statement would be more like return (y) ? 1 : 2; instead.

share|improve this answer

answered Apr 30 at 22:21

Remy LebeauRemy Lebeau

347k19273469

add a comment | 

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164

It's a typo. Look at Errata for 2nd and 3rd printings of The C++ Programming Language. The example must be like below:

auto z3 =[y]() return (y) ? 1 : 2; 

share|improve this answer

edited Apr 30 at 22:48

answered Apr 30 at 22:21

S.M.S.M.

7,11752231

• 
  
  
  

  
  9
  

  
  
  
  
  
  

  
  
  Why (y) and not just y?
  
  – Little Helper
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @LittleHelper Perhaps its a best practice or something, I always see it written like that. Maybe to avoid confusion with more complicated comparisons...
  
  – Redwolf Programs
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  22
  

  
  
  
  
  
  

  
  
  Personally, I often use (cond) ? a : b for clarity -- it helps me avoid misreading e.g. the statement foo = x > y ? a : b as foo = x ... when skimming through code.
  
  – grawity
  2 days ago
  
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  @LittleHelper It's not really needed there. However in a function-like macro it's best practise to put parentheses round the arguments where they are used, because otherwise expansion of the arguments can give unexpected behaviour. Consider a function-like macro to double a value "foo(x) x * 2" where you call it with "foo(2+3)". The result will be 2+(3*2) because the argument gets expanded as-is and precedence rules take over. If your macro is "foo(x) (x)*2" then you will correctly get (2+3)*2. It may be that Stroustrup has a habit of using that style everywhere for coding safety.
  
  – Graham
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Graham Very unlikely. Stroustrup essentially doesn't write function macros (C++ inline functions are better). Much more likely is that the ternary operator has somewhat complicated precedence rules, so it is good to habitually clarify the precedence with parens.
  
  – Martin Bonner
  22 hours ago
  

  
  
  
  

add a comment | 

164

It's a typo. Look at Errata for 2nd and 3rd printings of The C++ Programming Language. The example must be like below:

auto z3 =[y]() return (y) ? 1 : 2; 

share|improve this answer

edited Apr 30 at 22:48

answered Apr 30 at 22:21

S.M.S.M.

7,11752231

• 
  
  
  

  
  9
  

  
  
  
  
  
  

  
  
  Why (y) and not just y?
  
  – Little Helper
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @LittleHelper Perhaps its a best practice or something, I always see it written like that. Maybe to avoid confusion with more complicated comparisons...
  
  – Redwolf Programs
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  22
  

  
  
  
  
  
  

  
  
  Personally, I often use (cond) ? a : b for clarity -- it helps me avoid misreading e.g. the statement foo = x > y ? a : b as foo = x ... when skimming through code.
  
  – grawity
  2 days ago
  
  

  
  
  
  


• 
  
  
  

  
  5
  

  
  
  
  
  
  

  
  
  @LittleHelper It's not really needed there. However in a function-like macro it's best practise to put parentheses round the arguments where they are used, because otherwise expansion of the arguments can give unexpected behaviour. Consider a function-like macro to double a value "foo(x) x * 2" where you call it with "foo(2+3)". The result will be 2+(3*2) because the argument gets expanded as-is and precedence rules take over. If your macro is "foo(x) (x)*2" then you will correctly get (2+3)*2. It may be that Stroustrup has a habit of using that style everywhere for coding safety.
  
  – Graham
  yesterday
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @Graham Very unlikely. Stroustrup essentially doesn't write function macros (C++ inline functions are better). Much more likely is that the ternary operator has somewhat complicated precedence rules, so it is good to habitually clarify the precedence with parens.
  
  – Martin Bonner
  22 hours ago
  

  
  
  
  

add a comment | 

It's a typo. Look at Errata for 2nd and 3rd printings of The C++ Programming Language. The example must be like below:

auto z3 =[y]() return (y) ? 1 : 2; 

share|improve this answer

edited Apr 30 at 22:48

answered Apr 30 at 22:21

S.M.S.M.

7,11752231

auto z3 =[y]() return (y) ? 1 : 2; 

share|improve this answer

edited Apr 30 at 22:48

answered Apr 30 at 22:21

S.M.S.M.

7,11752231

answered Apr 30 at 22:21

S.M.S.M.

7,11752231

answered Apr 30 at 22:21

S.M.S.M.

7,11752231

15

Looks to me like a simple typo. Should probably be:

auto z3 =[y]() return y ? 1 : 2; 

Note that since the lambda doesn't take any parameters, the parens are optional. You could use this instead, if you preferred:

auto z3 =[y] return y ? 1 : 2; 

share|improve this answer

answered Apr 30 at 22:21

Jerry CoffinJerry Coffin

391k52477925

add a comment | 

15

Looks to me like a simple typo. Should probably be:

auto z3 =[y]() return y ? 1 : 2; 

Note that since the lambda doesn't take any parameters, the parens are optional. You could use this instead, if you preferred:

auto z3 =[y] return y ? 1 : 2; 

share|improve this answer

answered Apr 30 at 22:21

Jerry CoffinJerry Coffin

391k52477925

add a comment | 

Looks to me like a simple typo. Should probably be:

auto z3 =[y]() return y ? 1 : 2; 

Note that since the lambda doesn't take any parameters, the parens are optional. You could use this instead, if you preferred:

auto z3 =[y] return y ? 1 : 2; 

share|improve this answer

answered Apr 30 at 22:21

Jerry CoffinJerry Coffin

391k52477925

auto z3 =[y]() return y ? 1 : 2; 

auto z3 =[y] return y ? 1 : 2; 

share|improve this answer

answered Apr 30 at 22:21

Jerry CoffinJerry Coffin

391k52477925

answered Apr 30 at 22:21

Jerry CoffinJerry Coffin

391k52477925

answered Apr 30 at 22:21

Jerry CoffinJerry Coffin

391k52477925

10

return 1 : 2; is a syntax error, it is not valid code.

A correct statement would be more like return (y) ? 1 : 2; instead.

share|improve this answer

answered Apr 30 at 22:21

Remy LebeauRemy Lebeau

347k19273469

add a comment | 

10

return 1 : 2; is a syntax error, it is not valid code.

A correct statement would be more like return (y) ? 1 : 2; instead.

share|improve this answer

answered Apr 30 at 22:21

Remy LebeauRemy Lebeau

347k19273469

add a comment | 

return 1 : 2; is a syntax error, it is not valid code.

A correct statement would be more like return (y) ? 1 : 2; instead.

share|improve this answer

answered Apr 30 at 22:21

Remy LebeauRemy Lebeau

347k19273469

share|improve this answer

answered Apr 30 at 22:21

Remy LebeauRemy Lebeau

347k19273469

answered Apr 30 at 22:21

Remy LebeauRemy Lebeau

347k19273469

answered Apr 30 at 22:21

Remy LebeauRemy Lebeau

347k19273469