need help on a syntax in conditional operator

[Bill Davidson]

Hello there,

I need help on understanding part of the macro below:

#define MY_MACRO( OP, RESULT ) \
( (RESULT = (OP)) != 0 ? (errno = RESULT, -1) : 0 )

What I do not understand is how is the part (errno = RESULT, -1)
get evaluated.

Bill

 

[Rob Williscroft]

Bill Davidson wrote in
in comp.lang.c++:

Hello there,

I need help on understanding part of the macro below:

#define MY_MACRO( OP, RESULT ) \
( (RESULT = (OP)) != 0 ? (errno = RESULT, -1) : 0 )

What I do not understand is how is the part (errno = RESULT, -1)
get evaluated.

The default behaviour of the , (comma) operator is to evaluate
the left expression and dicard the result and then to evaluate
the right expression:

value = ( evalute_and_dicard, result );

value = ( evalute_and_dicard, evalute_and_dicard_2, result );

Both of the above would assign result to value.

Assuming OP and RESULT in your example above are int's we could
rewrite it as:

inline int my_function( int op, int &result )
{
int return_value;

result = op;
if ( result != 0 )
{
errno = result;
return_value = -1;
}
else
{
return_value = 0;
}

return return_value;
}

HTH

Rob.

 

[Claudio Jolowicz]

#define MY_MACRO( OP, RESULT ) \
( (RESULT = (OP)) != 0 ? (errno = RESULT, -1) : 0 )

What I do not understand is how is the part (errno = RESULT, -1)
get evaluated.

The comma in between the assignment and -1 is the sequencing operator.
The expressions are evaluated from left to right: RESULT is assigned to
errno, and the overall value will be -1.

BTW you are more likely to find this sort of code in C than in C++.