why comparing null function pointer results in "invalid operands to binary =="

Question

Can somebody please explain to me why the below code got the "invalid operands to binary ==" error?

typedef int (*func_t)(int);
#define NO_FUNC ((func_t) 0)
struct {
    const char *name;
    func_t func;
} table[] = { {"func1", NO_FUNC} };

if (table[0] == NO_FUNC) { // invalid operands to binary ==

}

Are you sure you didn't want table[0].func == NO_FUNC?

millimoose
– millimoose

2012-10-13 01:19:24 +00:00
Commented Oct 13, 2012 at 1:19 — millimoose
– millimoose, Commented Oct 13, 2012 at 1:19
@millimoose That's what I want. Thank you.

cody
– cody

2012-10-13 01:22:55 +00:00
Commented Oct 13, 2012 at 1:22 — cody
– cody, Commented Oct 13, 2012 at 1:22

MByD · Accepted Answer · 2012-10-13 01:18:47Z

3

And you should refer to the correct member in the struct:

if (table[0].func == NO_FUNC)

answered Oct 13, 2012 at 1:18

MByD

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1 Comment

MByD Over a year ago

Yes, thank you. I have noticed the mistake and edited it a minute ago.

caf · Accepted Answer · 2012-10-13 01:22:20Z

2

table[0] is of an unnamed struct type, and NO_FUNC has type int (*)(int). These two types can't be compared.

Instead, you could use:

if (table[0].func == NO_FUNC)

answered Oct 13, 2012 at 1:22

caf

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