Re: [Widelands-public] Problems with union Owner_Info

[Erik <fr...@ho...>]

Christoph Mallon wrote:
> Erik wrote:
>   
>> Christoph Mallon wrote:
>>     
>>> Erik wrote:
>>>   
>>>       
>>>> Christoph Mallon wrote:
>>>>     
>>>>         
>>>>> add a compile time assert to verify that the size of the struct is 1.
>>>>>   
>>>>>       
>>>>>           
>>>> What do you expect a compile time assert in C++ to look like? Would 
>>>> something like this hack please you?:
>>>> #define UMP_META_ASSERT(const_expr) typedef bool 
>>>> Meta_Assertion[(const_expr) ? 1 : -1]
>>>>     
>>>>         
>>> This looks ok to me.
>>> Though i would use extern int instead of a typedef, because in C a 
>>> typedef must be declared at most once. (Yes, i know, there is no C in 
>>> Widelands, but this change costs nothing)
>>>       
>> I tried do "extern int COMPILE_ASSERT" and it fails as it should when 
>> the argument is false. But it also fails when the argument is true:
>> error: storage class specified for 'COMPILE_ASSERT'
>> So I have to use typedef bool COMPILE_ASSERT. It works as intended.
>>     
> Please show exactly what you're doing and don't try to describe it.
>   
#define compile_assert(x) extern int COMPILE_ASSERT[(x) ? 1 : -1]
struct F {
    compile_assert(true);
};


g++ says:
 fel: storage class specified for "COMPILE_ASSERT"

#define compile_assert(x) typedef bool COMPILE_ASSERT[(x) ? 1 : -1]
struct F {
    compile_assert(true);
};

g++ says nothing

Your suggestion does not allow compile_assert inside struct 
declarations, which is what I want.