CODEWITHSUNDEEP
c++cc-preprocessorpreprocessorpreprocessor-directive
Nethrenial
Nethrenial

Reputation: 728

How do c/c++ preprocessors work when encountering unknown directives?

Do c/c++ preprocessors process all lines that begin with #? Does is errors out when encountering unknown macros or will it just ignore them?

for an example,

#include <stdio.h>

#hello
int main(){
    printf("Hello World!");
    return 0;
}

what happens in this situation?will it produce an error or will it work (ignoring #hello line)?

Upvotes: 1

Views: 2030

Answers (3)

eerorika
eerorika

Reputation: 238341

The language grammar specifies all pre-processor directives that exist in the language. If you use any other name for the directive, then that is a "conditionally-supported-directive". If the conditionally supported directive isn't supported, then the the language implementation is required to issue a diagnostic message and is free to refuse to proceed.

Upvotes: 6

Ian Abbott
Ian Abbott

Reputation: 17403

C

Syntactically, #hello is a "non-directive" preprocessing directive.

C17/C18 section 6.10 paragraph 9 (newly added in C17/C18) says:

The execution of a non-directive preprocessing directive results in undefined behavior.

"Undefined behavior" does not necessarily mean that the compiler will fail to translate the code or issue a diagnostic. (EDIT: As pointed out by Eric Postpischil in the comments, execution of a non-directive preprocessing directive does not violate a constraint so a diagnostic is not required.) It could behave in a documented manner, for example if the directive is part of an extension to the C language.

C++

Syntactically, #hello is a "conditionally-supported-directive" preprocessing directive.

C++20 section 15.1 paragraph 2 says:

A conditionally-supported-directive is conditionally-supported with implementation-defined semantics.

"Conditionally-supported" means that an implementation is not required to support it. Implementations need to document all conditionally-supported constructs that they do not support. (In the case of conditionally-supported-directives, I guess that would amount to documenting that none of them are supported, or documenting the semantics of those that are supported.)

Upvotes: 5

Amr Wael
Amr Wael

Reputation: 49

An unrecognized preprocessing directive error will rise, and your code won't compile

Upvotes: 0

Related Questions