something about compile time assertions in Modern C++ Design

Hi,All
the following codes are from chapter 2 of Modern C++ Design,

template <boo> struct CompileTimeChecker
{
CompileTimeChecker(...);
};

template<> struct CompileTimeChecker<false> {};

#define STATIC_CHECK(expr,msg) \
{\
class ERROR_##msg {} ; \
(void)sizeof(CompileTimeChecker<(expr) != 0>(ERROR_##msg())));\
}


why there use sizeof(...)
not just

#define STATIC_CHECK(expr,msg) \
{\
class ERROR_##msg {} ; \
CompileTimeChecker<(expr) != 0>(ERROR_##msg()));\
}

thanks all in advance.


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On 4 Nov 2005 06:07:36 -0500, "lfeiman888@gmail.com"
<lfeiman888@gmail.com> wrote:

>Hi,All
> the following codes are from chapter 2 of Modern C++ Design,
>
>template <boo> struct CompileTimeChecker
>{
> CompileTimeChecker(...);
>};
>
>template<> struct CompileTimeChecker<false> {};
>
>#define STATIC_CHECK(expr,msg) \
>{\
> class ERROR_##msg {} ; \
> (void)sizeof(CompileTimeChecker<(expr) != 0>(ERROR_##msg())));\
>}
>
>
>why there use sizeof(...)
>not just
>
>#define STATIC_CHECK(expr,msg) \
>{\
> class ERROR_##msg {} ; \
> CompileTimeChecker<(expr) != 0>(ERROR_##msg()));\
>}
>
>thanks all in advance.
>
>

The second creates a local variable, with all code and overhead
implied.
The first checks the expression, but is optimized away not generating
any code at all, thus limiting its effects only to compile time.

Regards,

-- Zara

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Hi,

> why there use sizeof(...)
> not just
>
> #define STATIC_CHECK(expr,msg) \
> {\
> class ERROR_##msg {} ; \
> CompileTimeChecker<(expr) != 0>(ERROR_##msg()));\
> }

As far as I remember the reason for the sizeof is to just evaluate the
type of the expression - which is why this line exists. Not using it
would create a temporary object. This is less efficient. I think the
term "type deduction engine" or something similar has been used in the
chapter regarding sizeof - but I am not sure if I read it somewhere
else.

regards

Lars


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Lars wrote:
> Hi,
>
> > why there use sizeof(...)
> > not just
> >
> > #define STATIC_CHECK(expr,msg) \
> > {\
> > class ERROR_##msg {} ; \
> > CompileTimeChecker<(expr) != 0>(ERROR_##msg()));\
> > }
>
> As far as I remember the reason for the sizeof is to just evaluate the
> type of the expression - which is why this line exists. Not using it
> would create a temporary object. This is less efficient. I think the
> term "type deduction engine" or something similar has been used in the
> chapter regarding sizeof - but I am not sure if I read it somewhere
> else.
>

Hello,

>From C98 standard clause 5.3.3:
The operand is either an expression, which is not evaluated, or a
parenthesized typeid.

All expressions will not be evaluated, sizeof operator concerns type
instead of value.


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