preprocessor help

brunogon · 06-16-2008, 08:19 AM

Hi,

I´m trying to create a routine to validate my casting. What I want to do is if my structure size is right put some code, if the code should report an error.
Does anyone knows how can I do this at compile time?

Code:

#define COMPILE_TIME_ASSERT(test, a) \
#if (test == 1) \
 *(char*)&(a)\
#else\
"Structure size error!"\
#endif

#define  BYTE(a)    COMPILE_TIME_ASSERT(sizeof(a) == 1, a)

int main
{
    char a,b;
    b = BYTE(a);
    BYTE(a) = a+1;
    return a;
}

Bruno

MacGyver · 06-16-2008, 08:25 AM

Try something like this in your #else construct:

Code:

#error "Structure size error!"\

Edit: Well, that is just for spitting out the error message. I'm not sure about the rest of your stuff.

matsp · 06-16-2008, 08:26 AM

No, you can't. "sizeof()" is done later during the compile than the preprocessor, so you can't do that.

--
Mats

QuantumPete · 06-16-2008, 08:29 AM

The Preprocessor is just a dumb search-and-replace tool. It doesn't evaluate the code in any way. f you need to validate a sizeof() you'll need to have the compiler do it for you.

QuantumPete

brunogon · 06-16-2008, 09:27 AM

So far, I think It is getting the sizeof correctly, the problem is if has any way to define my
#define COMPILE_TIME_ASSERT(test, a) with a preprocessor #if.

As the #if gives me an error, I also tryied use (test) ? *(char*)&(a) : "error"; despite it compiles okay, it add the whole code on the define and not just the *(char*)&(a) or "error"

Elysia · 06-16-2008, 10:08 AM

I don't think this sort of thing can be done in C, or at least not easily.
However, it can be done easily in C++.

C_ntua · 06-16-2008, 10:13 AM

Quote:

Originally Posted by Elysia

I don't think this sort of thing can be done in C, or at least not easily.
However, it can be done easily in C++.

How would it be done in C++ (curious, curious, don't really use C++)

Elysia · 06-16-2008, 10:20 AM

Psuedo code, but...
Something along the lines of:

Code:

int main()
{
    char mychar;
    int myint;
    char* c = safe_cast<char>(mychar); // Compiles
    c = safe_cast<char>(myint); // Won't compile
}

template<typename NewType, typename OriginalType>
NewType& safe_cast(Original& Data)
{
    STATIC_ASSERT(sizeof(NewType) == sizeof(OriginalType));
    return (NewType)Data;
}

This is dependant on boost's STATIC_ASSERT (or what it's called; can't remember 100%) macro.
There are more ways to do this, of course, but this is the easy way.

I haven't checked if this will compile, so it may contain a few errors, but the theory is sound.

And in you don't want to use boost, you can just the compiler to complain.
Something like:

Code:

int main()
{
    char mychar;
    int myint;
    char* c = safe_cast<char>(mychar); // Compiles
    c = safe_cast<char>(myint); // Won't compile
}

template<bool bFail>
struct dummy { static const bool bFail; };

template<>
struct dummy<true> { static const bool bFail = 'a'; }; // Causes a compile error about conversion from char to bool on purpose

template<>
struct dummy<false> { static const bool bFail = false; };

template<typename NewType, typename OriginalType>
NewType& safe_cast(Original& Data)
{
    dummy< sizeof(NewType) != sizeof(OriginalType) >::bFail;
    return (NewType)Data;
}

Balbanes · 06-16-2008, 10:21 AM

A workaround I've used before is something like:

Code:

#define COMPILE_ASSERT(_cond)   typedef char _Dummy[(_cond)]

Then you'd use it like:

Code:

int main()
{
     COMPILE_ASSERT(sizeof(char) == 1);
}

You'd have to check if your compiler supports the definition of zero-element arrays (I remember some Linux compilers did). In which case, you'd redefine the macro to:

Code:

#define COMPILE_ASSERT(_cond)   typedef char _Dummy[(_cond) - 1]

Of course, you won't get a proper error message when the assertion fails, but eh.

matsp · 06-16-2008, 10:24 AM

Quote:

Originally Posted by brunogon

So far, I think It is getting the sizeof correctly, the problem is if has any way to define my
#define COMPILE_TIME_ASSERT(test, a) with a preprocessor #if.

As the #if gives me an error, I also tryied use (test) ? *(char*)&(a) : "error"; despite it compiles okay, it add the whole code on the define and not just the *(char*)&(a) or "error"

You also can't put preprocessor commands in a macro (well, you can, but it doesn't do what you want). The preprocessor FIRST examines for #if, #include, #define. It then replaces the COMPILE_TIME_ASSERT() with whatever you define it as - so it will replace your macro with #if .... Then the compiler proper comes to work, and it says "Huh? What's this #if - I've never heard of that... "

So, you have to use regular if-statements to make the compiler see what you want. If it's even a little bit clever, it will then optimize away your test all together unless it's failing, and if it fails, then it will produce ONLY the code to perform your error message.

I presume Elysia is referring to the template meta-programming stuff you can do in C++, where templates ARE expanded at compile time when sizeof() and such is known to the compiler.

--
Mats

--
Mats

Elysia · 06-16-2008, 10:28 AM

Quote:

Originally Posted by matsp

I presume Elysia is referring to the template meta-programming stuff you can do in C++, where templates ARE expanded at compile time when sizeof() and such is known to the compiler.

Indeed. It's potent stuff.

brunogon · 06-16-2008, 10:45 AM

Thanks everybody,

Unfortunately I can´t use C++ code, I was trying to do some decent cast validation, but I think it is not possible.
Tks again for your help

brunogon · 06-16-2008, 11:11 AM

Anyway, If I try implement this during execution, and not at the preprocessor, would it be possible? Any ideas?

I would need something that makes posbile to use it exactly as *(char*)&(a).
I mean a = safe_cast(b), safe_cast(b) = a

Cheers

Elysia · 06-16-2008, 11:14 AM

It would be possible at runtime, yes. Perhaps you can use...
if (sizeof(a) != sizeof(b)) abort();
...type of macro.
It's runtime, but it's better than nothing.
(C++ can do it compile time.)

06-16-2008, 08:19 AM	#1
brunogon Registered User Join Date: Jun 2008 Posts: 4	preprocessor help Hi, I´m trying to create a routine to validate my casting. What I want to do is if my structure size is right put some code, if the code should report an error. Does anyone knows how can I do this at compile time? Code: #define COMPILE_TIME_ASSERT(test, a) \ #if (test == 1) \ (char)&(a)\ #else\ "Structure size error!"\ #endif #define BYTE(a) COMPILE_TIME_ASSERT(sizeof(a) == 1, a) int main { char a,b; b = BYTE(a); BYTE(a) = a+1; return a; } Bruno
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06-16-2008, 08:25 AM	#2
MacGyver Deathray Engineer Join Date: Mar 2007 Posts: 3,211	Try something like this in your #else construct: Code: #error "Structure size error!"\ Edit: Well, that is just for spitting out the error message. I'm not sure about the rest of your stuff. __________________ Last edited by MacGyver; 06-16-2008 at 08:29 AM.
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06-16-2008, 08:26 AM	#3
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	No, you can't. "sizeof()" is done later during the compile than the preprocessor, so you can't do that. -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
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06-16-2008, 08:29 AM	#4
QuantumPete Technical Lead Join Date: Aug 2007 Location: London, UK Posts: 723	The Preprocessor is just a dumb search-and-replace tool. It doesn't evaluate the code in any way. f you need to validate a sizeof() you'll need to have the compiler do it for you. QuantumPete __________________ "No-one else has reported this problem, you're either crazy or a liar" - Dogbert Technical Support "Have you tried turning it off and on again?" - The IT Crowd
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06-16-2008, 09:27 AM	#5
brunogon Registered User Join Date: Jun 2008 Posts: 4	So far, I think It is getting the sizeof correctly, the problem is if has any way to define my #define COMPILE_TIME_ASSERT(test, a) with a preprocessor #if. As the #if gives me an error, I also tryied use (test) ? (char)&(a) : "error"; despite it compiles okay, it add the whole code on the define and not just the (char)&(a) or "error"
brunogon is offline

06-16-2008, 10:21 AM	#9
Balbanes Registered User Join Date: Jun 2008 Posts: 1	A workaround I've used before is something like: Code: #define COMPILE_ASSERT(_cond) typedef char _Dummy[(_cond)] Then you'd use it like: Code: int main() { COMPILE_ASSERT(sizeof(char) == 1); } You'd have to check if your compiler supports the definition of zero-element arrays (I remember some Linux compilers did). In which case, you'd redefine the macro to: Code: #define COMPILE_ASSERT(_cond) typedef char _Dummy[(_cond) - 1] Of course, you won't get a proper error message when the assertion fails, but eh.
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06-16-2008, 10:45 AM	#12
brunogon Registered User Join Date: Jun 2008 Posts: 4	Thanks everybody, Unfortunately I can´t use C++ code, I was trying to do some decent cast validation, but I think it is not possible. Tks again for your help
brunogon is offline

06-16-2008, 11:11 AM	#13
brunogon Registered User Join Date: Jun 2008 Posts: 4	Anyway, If I try implement this during execution, and not at the preprocessor, would it be possible? Any ideas? I would need something that makes posbile to use it exactly as (char)&(a). I mean a = safe_cast(b), safe_cast(b) = a Cheers Last edited by brunogon; 06-16-2008 at 11:13 AM.
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