c++ - How could my code tell a compile-time constant versus a variable?

Question

Here's my problem. I have a BINARY_FLAG macro:

#define BINARY_FLAG( n ) ( static_cast<DWORD>( 1 << ( n ) ) )

Which can be used either like this ("constant" scenario):

static const SomeConstant = BINARY_FLAG( 5 );

or like this ("variable" scenario):

for( int i = 0; i < 10; i++ ) {
    DWORD flag = BINARY_FLAG( i );
    // do something with the value
}

This macro is not foolproof at all - one can pass -1 or 34 there and there will at most be a warning yet behavior will be undefined. I'd like to make it more foolproof.

For the constant scenario I could use a template:

template<int Shift> class BinaryFlag {
staticAssert( 0 <= Shift && Shift < sizeof( DWORD) * CHAR_BIT );
public:
static const DWORD FlagValue = static_cast<DWORD>( 1 << Shift );
};
#define BINARY_FLAG( n ) CBinaryFlag<n>::FlagValue

but this will not go for the "variable" scenario - I'd need a runtime assertion there:

inline DWORD ProduceBinaryFlag( int shift )
{
    assert( 0 <= shift && shift < sizeof( DWORD) * CHAR_BIT );
    return static_cast<DWORD>( 1 << shift );
}
#define BINARY_FLAG( n ) ProduceBinaryFlag(n)

The latter is good, but has no compile-time checks. Of course, I'd like a compile-time check where possible and a runtime check otherwise. At all times I want as little runtime overhead as possible so I don't want a function call (that maybe won't be inlined) when a compile-time check is possible.

I saw this question, but it doesn't look like it is about the same problem.

Is there some construct that would allow to alternate between the two depending on whether the expression passed as a flag number is a compile-time constant or a variable?

Answer 1

This is simpler than you think :)

Let's have a look:

#include <cassert>

static inline int FLAG(int n) {
    assert(n>=0 && n<32);
    return 1<<n;
}

int test1(int n) {
    return FLAG(n);
}
int test2() {
    return FLAG(5);
}

I don't use MSVC, but I compiled with Mingw GCC 4.5:

g++ -c -S -O3 08042.cpp

The resulting code for first method looks like:

__Z5test1i:
    pushl   %ebp
    movl    %esp, %ebp
    subl    $24, %esp
    movl    8(%ebp), %ecx
    cmpl    $31, %ecx
    ja  L4
    movl    $1, %eax
    sall    %cl, %eax
    leave
    ret
L4:
    movl    $4, 8(%esp)
    movl    $LC0, 4(%esp)
    movl    $LC1, (%esp)
    call    __assert
    .p2align 2,,3

And the second:

__Z5test2v:
    pushl   %ebp
    movl    %esp, %ebp
    movl    $32, %eax
    leave
    ret

See? The compiler is smart enough to do it for you. No need for macros, no need for metaprogramming, no need for C++0x. As simple as that.

Check if MSVC does the same... But look - it's really easy for the compiler to evaluate a constant expression and drop the unused conditional branch. Check it if you want to be sure... But generally - trust your tools.