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Return void type in C and C++

Posted by: admin November 30, 2017 Leave a comment

Questions:

This compiles without any warnings.

Is this legal in C and C++ or does it just work in gcc and clang?

If it is legal, is it some new thing after C99?

void f(){

}

void f2(){
    return f();
}

Update

as “Rad Lexus” suggested I tried this:

$ gcc -Wall -Wpedantic -c x.c 
x.c: In function ‘f2’:
x.c:7:9: warning: ISO C forbids ‘return’ with expression, in function returning void [-Wpedantic]
  return f();

$ clang -Wall -Wpedantic -c x.c 
x.c:7:2: warning: void function 'f2' should not return void expression [-Wpedantic]
        return f();
        ^      ~~~~~
1 warning generated.

$ gcc -Wall -Wpedantic -c x.cc
(no errors)

$ clang -Wall -Wpedantic -c x.cc
(no errors)

Update

Someone asked how this construction is helping. Well is more or less syntactic sugar. Here is one good example:

void error_report(const char *s){
    printf("Error %s\n", s);
    exit(0);
}

void process(){
   if (step1() == 0)
      return error_report("Step 1");

   switch(step2()){
   case 0: return error_report("Step 2 - No Memory");
   case 1: return error_report("Step 2 - Internal Error");
   }

   printf("Processing Done!\n");
}
Answers:

C11, 6.8.6.4 “The return statement”:

A return statement with an expression shall not appear in a function whose return type is void.

No, you may not use an expression, even if it is of void type.

From the foreword of the same document:

Major changes in the second edition included:

[…]
  • return without expression not permitted in function that returns a value (and vice versa)

So this was a change from C89 -> C99 (the second edition of the language standard), and has been that way ever since.


C++14, 6.6.3 “The return statement”:

A return statement with an expression of non-void type can be used only in functions returning a value […] A return statement with an expression of type void can be used only in functions with a return type of cv
void; the expression is evaluated just before the function returns to its caller.

Yes, you may use an expression if it is of void type (that’s been valid since C++98).

Questions:
Answers:

This code is allowed in C++ but not allowed in C

From Return statement @ cppreference

In a function returning void, the return statement with expression can
be used, if the expression type is void.


OTOH in C11 specs draft n1570:

Major changes in the second edition included:

return without expression not permitted in function that returns a
value (and vice versa)

(return with expression not permitted in function that returns a void)

and 6.8.6.4 return

A return statement with an expression shall not appear in a function
whose return type is void. A return statement without an expression
shall only appear in a function whose return type is void.

(even if the expression evaluates to void)

Questions:
Answers:

C++ allows something like that:

void f() 
{
    return void();
}

While C does not. That’s why a warning is issued if you compile it a ISO C rather than ISO C++. This is formally described as:

A return statement with an expression of type void can be used only in
functions with a return type of cv void

Questions:
Answers:

ISO/IEC 9899:201x Committee draft says the following:

6.8.6.4 The return statement

Constraints

  1. return statement with an expression shall not appear in a function whose return type is void.

    A return statement without an expression shall only appear in a
    function whose return type is void.

So, it is forbidden in C.


You need to use -pedantic switch to gcc for it to complain about standard violations:

test.c: In function ‘f2’:
test.c:6:12: warning: ISO C forbids ‘return’ with expression, in function returning void 
            [-Wpedantic]
     return f();

Questions:
Answers:

Standard C does not support this construction:

C11 6.8.6.4: The return statement

Constraints

1 A return statement with an expression shall not appear in a function whose return type is void. A return statement without an expression shall only appear in a function whose return type is void.

No special provisions are added for the special case in the question. Some C compilers do support this as an extension (gcc does, unless instructed to conform to one of the C Standards), but C11 and previous versions consider it a constraint violation.