Unmatched parenthesis: missing ')' in #if directive - c++

Question

I wrote this simple program
#include <time.h>
int main()
{
#if ((clock_t)1000)
int x = 10;
#endif
return 0;
}
On compilation, I see the following error:
Error C1012 unmatched parenthesis: missing ')'
Why am I getting this error?
Changing the line from:
#if ((clock_t)1000)
to:
#if (clock_t)1000
resolves the compilation error.
But I can't do that, since ((clock_t)1000) is defined as a macro using the #define directive in the limits.h header file as :
#define CLOCKS_PER_SEC ((clock_t)1000)
and I need to use that directly.
EDIT:
Please pardon me for framing the question in such an unclear way.
Reframing my question now:
I have the following code:
#include <time.h>
#define DUMMY_CLOCKS_PER_SEC ((clock_t)1000)
int main()
{
#if CLOCKS_PER_SEC != DUMMY_CLOCKS_PER_SEC
#error "out of sync"
#endif
return 0;
}
But this gives the compilation error:
Error C1012 unmatched parenthesis: missing ')'

Answer 1

The preprocessor doesn't know anything about C++ datatypes, and doesn't understand cast expressions. It's used for simple text processing, and == and != can only compare single tokens.
Do the comparison in C++, not the preprocessor.
static_assert(CLOCKS_PER_SEC == DUMMY_CLOCKS_PER_SEC, "out of sync");
int main() {
return 0;
}
Don't worry about the runtime performance overhead. Since both macros expand to literals, the compiler will optimize it away.

Answer 2

You are confusing a preprocessor macro definition (CLOCKS_PER_SEC) with its expansion (that is implementation defined, and in your case seems to be ((clock_t)1000)).
It's not very clear what you want to do in your code.
If you want to check if this macro is defined, you can use the preprocessor #ifdef, e.g.:
#ifdef CLOCKS_PER_SEC
// your code
#endif
Anyway, this CLOCKS_PER_SEC macro is defined by the standard, so it should be always defined in a standard-compliant time.h library implementation.
If you have something different in your mind, please clarify your goal.
EDIT Based on your clarifying comment below, you may want to use an if to compare the values (expansions) of these two macros:
if (DUMMY_CLOCKS_PER_SEC != CLOCKS_PER_SEC) {
...
} else {
...
}

Answer 3

((clock_t)1000) is defined as a macro using the #define directive in the limits.h header file as :
#define CLOCKS_PER_SEC ((clock_t)1000)
The file does not define a macro named ((clock_t)1000). It defines a macro named CLOCKS_PER_SEC. ((clock_t)1000) is the value of the macro.
((clock_t)1000) is not a macro and is something that cannot be used in an #if directive.

Answer 4

Thanks for all the responses everyone.
Another solution I figured out for this problem is to use constexpr specifier which is a feature of c++11. ConstExpr allows us to evaluate the value of a variable or a function at compile time.
Changing the code from:
#if CLOCKS_PER_SEC != DUMMY_CLOCKS_PER_SEC
#error "out of sync"
#endif
to the following resolves the issue:
constexpr int DUMMY_CLOCK_NOT_EQUAL = (DUMMY_CLOCKS_PER_SEC != CLOCKS_PER_SEC) ? 1 : 0;
#if DUMMY_CLOCK_NOT_EQUAL
#error "out of sync"
#endif