ID Number: Q69934
5.00 5.10 6.00 6.00a 6.00ax 7.00 | 5.10 6.00 6.00a
MS-DOS | OS/2
Summary:
In Microsoft C versions 5.1, 6.0, 6.0a, 6.0ax, and C/C++ version 7.0,
when using the #define statement to define a constant, its value must
evaluate to an integer constant if it is to be used as the expression
in an #if, #elif, or #else compiler directive. In addition, for any
undefined constant that is used, the preprocessor assumes a value of
zero.
If you are not sure how the compiler is handling certain directives,
you can compile the code with one of the preprocessor options (/P /E
/EP). This will allow you to see the exact effect of the preprocessor
directives on the code.
More Information:
If a #define'd constant that does not evaluate to an integer constant
is used with the #if, #elif, or #else compiler directive, the
following error will result:
fatal error C1017: invalid integer constant expression
For example, this error will occur with the following code fragment:
#define CONSTANT_NAME "YES"
#if CONSTANT_NAME
...
#endif
Because CONSTANT_NAME evaluates to a string constant and not an
integer constant, the #if directive will generate a C1017 error and
halt the compiler.
The other notable situation is that any constant that is undefined is
evaluated by the preprocessor to be zero. If an undefined constant is
used inadvertently, unintended behavior may result. For example, in
the following code fragment, the result is the reverse of what is
actually intended:
#define CONSTANT_NAME YES
#if CONSTANT_NAME
Code to use on YES...
#elif CONSTANT_NAME==NO
Code to use on NO...
#endif
In this example, YES is undefined, so it evaluates to zero. Therefore,
CONSTANT_NAME is also zero. Thus, the #if expression "CONSTANT_NAME"
evaluates to false and the "Code to use on YES" is removed by the
preprocessor, which is clearly NOT the intended behavior. Furthermore,
since NO is also undefined, it also evaluates to zero, which means
that the #elif expression "CONSTANT_NAME==NO" evaluates to true
(0==0). This causes the preprocessor NOT to remove the "Code to do on
NO", which is also NOT the intended behavior.
By compiling with one of the preprocessor options (/P /E /EP), you can
observe the preprocessor output in order to see exactly how the
preprocessor is interpreting all directives. For more information on
these options, consult the documentation or online help that
accompanies your particular version of the compiler.
Additional reference words: 5.10 6.00 6.00a 6.00ax 7.00