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4

2

#define A;

#ifdef A
 (...)
#endif

I thought the predecessor would take this as false; however it would goes into this condition. Is A and A; taken as the same?

c macros

share|improve this question

edited Mar 27 at 3:44

Bhargav Rao♦

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asked Sep 5 '15 at 2:50

RenjieRenjie

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add a comment | 

4

2

#define A;

#ifdef A
 (...)
#endif

I thought the predecessor would take this as false; however it would goes into this condition. Is A and A; taken as the same?

c macros

share|improve this question

edited Mar 27 at 3:44

Bhargav Rao♦

32.3k2121 gold badges9494 silver badges115115 bronze badges

asked Sep 5 '15 at 2:50

RenjieRenjie

2122 bronze badges

add a comment | 

#define A;

#ifdef A
 (...)
#endif

I thought the predecessor would take this as false; however it would goes into this condition. Is A and A; taken as the same?

c macros

share|improve this question

edited Mar 27 at 3:44

Bhargav Rao♦

32.3k2121 gold badges9494 silver badges115115 bronze badges

asked Sep 5 '15 at 2:50

RenjieRenjie

2122 bronze badges

#define A;

#ifdef A
 (...)
#endif

share|improve this question

edited Mar 27 at 3:44

Bhargav Rao♦

32.3k2121 gold badges9494 silver badges115115 bronze badges

asked Sep 5 '15 at 2:50

RenjieRenjie

2122 bronze badges

share|improve this question

edited Mar 27 at 3:44

Bhargav Rao♦

32.3k2121 gold badges9494 silver badges115115 bronze badges

asked Sep 5 '15 at 2:50

RenjieRenjie

2122 bronze badges

edited Mar 27 at 3:44

Bhargav Rao♦

32.3k2121 gold badges9494 silver badges115115 bronze badges

edited Mar 27 at 3:44

Bhargav Rao♦

32.3k2121 gold badges9494 silver badges115115 bronze badges

asked Sep 5 '15 at 2:50

RenjieRenjie

2122 bronze badges

asked Sep 5 '15 at 2:50

RenjieRenjie

2122 bronze badges

1 Answer
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12

No, they're distinct.

In

#define A;

A and ; are two distinct tokens. A is the macro name, and ; is its definition. So you could, if you really wanted to, write:

printf("Hello, worldn")A

and it would be equivalent to

printf("Hello, worldn");

(But please don't do that.)

Since the only thing you do with A is refer to it in an #ifdef, all you're doing is testing whether it's been defined or not, regardless of how it's defined. The semicolon is irrelevant because you don't use it.

Just as a matter of style and clarity, you should always have a space between a macro name and its definition:

#define A ;

This is particularly important if the first token of the expansion is a ( character. If it immediately follows the macro name, you have a function-like macro definition (the macro takes arguments). If there's a space between the macro name and the (, the ( is just part of what the macro expands to.

Speaking of semicolons, a common error is including unnecessary semicolons in macro definitions:

#define THE_ANSWER 42;

...

printf("The answer is %dn", THE_ANSWER);

Since the semicolon is part of the macro definition, this expands to:

printf("The answer is %dn", 42;);

which is a syntax error.

share|improve this answer

edited Sep 5 '15 at 4:41

answered Sep 5 '15 at 2:53

Keith ThompsonKeith Thompson

200k2828 gold badges309309 silver badges500500 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @KeithThompson Why does A; won't be taken as a whole macro name?
  
  – Renjie
  Sep 6 '15 at 4:06
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Renjie: Because a macro name has to be an identifier.
  
  – Keith Thompson
  Sep 6 '15 at 4:48
  

  
  
  
  

add a comment | 

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12

No, they're distinct.

In

#define A;

A and ; are two distinct tokens. A is the macro name, and ; is its definition. So you could, if you really wanted to, write:

printf("Hello, worldn")A

and it would be equivalent to

printf("Hello, worldn");

(But please don't do that.)

Since the only thing you do with A is refer to it in an #ifdef, all you're doing is testing whether it's been defined or not, regardless of how it's defined. The semicolon is irrelevant because you don't use it.

Just as a matter of style and clarity, you should always have a space between a macro name and its definition:

#define A ;

This is particularly important if the first token of the expansion is a ( character. If it immediately follows the macro name, you have a function-like macro definition (the macro takes arguments). If there's a space between the macro name and the (, the ( is just part of what the macro expands to.

Speaking of semicolons, a common error is including unnecessary semicolons in macro definitions:

#define THE_ANSWER 42;

...

printf("The answer is %dn", THE_ANSWER);

Since the semicolon is part of the macro definition, this expands to:

printf("The answer is %dn", 42;);

which is a syntax error.

share|improve this answer

edited Sep 5 '15 at 4:41

answered Sep 5 '15 at 2:53

Keith ThompsonKeith Thompson

200k2828 gold badges309309 silver badges500500 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @KeithThompson Why does A; won't be taken as a whole macro name?
  
  – Renjie
  Sep 6 '15 at 4:06
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Renjie: Because a macro name has to be an identifier.
  
  – Keith Thompson
  Sep 6 '15 at 4:48
  

  
  
  
  

add a comment | 

12

No, they're distinct.

In

#define A;

A and ; are two distinct tokens. A is the macro name, and ; is its definition. So you could, if you really wanted to, write:

printf("Hello, worldn")A

and it would be equivalent to

printf("Hello, worldn");

(But please don't do that.)

Since the only thing you do with A is refer to it in an #ifdef, all you're doing is testing whether it's been defined or not, regardless of how it's defined. The semicolon is irrelevant because you don't use it.

Just as a matter of style and clarity, you should always have a space between a macro name and its definition:

#define A ;

This is particularly important if the first token of the expansion is a ( character. If it immediately follows the macro name, you have a function-like macro definition (the macro takes arguments). If there's a space between the macro name and the (, the ( is just part of what the macro expands to.

Speaking of semicolons, a common error is including unnecessary semicolons in macro definitions:

#define THE_ANSWER 42;

...

printf("The answer is %dn", THE_ANSWER);

Since the semicolon is part of the macro definition, this expands to:

printf("The answer is %dn", 42;);

which is a syntax error.

share|improve this answer

edited Sep 5 '15 at 4:41

answered Sep 5 '15 at 2:53

Keith ThompsonKeith Thompson

200k2828 gold badges309309 silver badges500500 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @KeithThompson Why does A; won't be taken as a whole macro name?
  
  – Renjie
  Sep 6 '15 at 4:06
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @Renjie: Because a macro name has to be an identifier.
  
  – Keith Thompson
  Sep 6 '15 at 4:48
  

  
  
  
  

add a comment | 

No, they're distinct.

In

#define A;

A and ; are two distinct tokens. A is the macro name, and ; is its definition. So you could, if you really wanted to, write:

printf("Hello, worldn")A

and it would be equivalent to

printf("Hello, worldn");

(But please don't do that.)

Since the only thing you do with A is refer to it in an #ifdef, all you're doing is testing whether it's been defined or not, regardless of how it's defined. The semicolon is irrelevant because you don't use it.

Just as a matter of style and clarity, you should always have a space between a macro name and its definition:

#define A ;

This is particularly important if the first token of the expansion is a ( character. If it immediately follows the macro name, you have a function-like macro definition (the macro takes arguments). If there's a space between the macro name and the (, the ( is just part of what the macro expands to.

Speaking of semicolons, a common error is including unnecessary semicolons in macro definitions:

#define THE_ANSWER 42;

...

printf("The answer is %dn", THE_ANSWER);

Since the semicolon is part of the macro definition, this expands to:

printf("The answer is %dn", 42;);

which is a syntax error.

share|improve this answer

edited Sep 5 '15 at 4:41

answered Sep 5 '15 at 2:53

Keith ThompsonKeith Thompson

200k2828 gold badges309309 silver badges500500 bronze badges

#define A;

printf("Hello, worldn")A

printf("Hello, worldn");

#define A ;

#define THE_ANSWER 42;

...

printf("The answer is %dn", THE_ANSWER);

printf("The answer is %dn", 42;);

share|improve this answer

edited Sep 5 '15 at 4:41

answered Sep 5 '15 at 2:53

Keith ThompsonKeith Thompson

200k2828 gold badges309309 silver badges500500 bronze badges

answered Sep 5 '15 at 2:53

Keith ThompsonKeith Thompson

200k2828 gold badges309309 silver badges500500 bronze badges

answered Sep 5 '15 at 2:53

Keith ThompsonKeith Thompson

200k2828 gold badges309309 silver badges500500 bronze badges