Re: c++ code compiling

[Frank Schafer <frank dot schafer at setuza dot cz>]

Rupert Wood wrote:

> Hua Qiaowen wrote:
>
> > In fact, i don't wanna give out a clear declaration of the function
> > be called.
>
> But that's wrong - for at least two reasons:
>
> 1. One of the philosophies of C++ (IIRC) is that as much compile-time
>    checking is performed as possible. If you don't give the compiler
>    complete type information, it can't perform full compile time
>    type checking. Even if the compiler would accept it, it would be
>    considered bad style.
>
> 2. C++ generates 'mangled' or 'decorated' function names. In the
>    object file,
>
>        int f(void) { return 1; }
>
>    will be symbol _Z1fv not _f (on my system at least).
>
>    This is because C++ supports type overloading; it's valid to have
>    two functions:
>
>        int   f(int);
>        char* f(char*);
>
>    in the same program, and the compiler will sort out which one to
>    call. These overloaded functions need to have different symbol
>    names. The _Z and the v in the above example represent (I think)
>    return type and argument type respectively, and extra decorators
>    can be added for class names, namespaces and other modifiers
>    (const, etc.) If you don't declare a function, there's no way the
>    compiler can know what mangled name to generate a call to.
>
> > I know if i code it with "C", this error is just a warning and i can
> > successfully link the programm as long as i provide the obj file
> > with this function's implementation when linking. So, do you know if
> > g++ compiler has some compilation options can be used to bypass this
> > "error"?
>
> There is no flag to turn this off. If you'd like to see for yourself (or
> change this) then look in gcc/cp/lex.c: searching for 'first use this
> function' will take you to the right place.
>
> If you want to link against an C-style undecorated symbol then you have
> to declare a function:
>
>     extern "C" int f(void);
>
> or similar. You can declare a block of these:
>
>     extern "C" {
>         int f(void);
>         char* g(int);
>     };
>
> etc. but you need the "C" to specify non-mangled names.
>
> I have a hunch that you don't want to provide a delcaration because you
> don't know what it is - that you have someone else's object or library
> that you'd like to link to and play with but you don't know the exact
> function arguments and return types. You may be better off reverse
> engineering the function you'd like to call rather than repeatedly
> trying and guessing, although the above 'extern "C"' form will link
> against regular C functions if you really want.
>
> If you just want to omit a declaration to be lazy then tough luck: C++
> won't allow this. (Or at least g++ won't - I'm not going to check the
> standard.)
>
> Rup.

Hi Rup,

thanks for this excellent explanation of C++. I was too lazy, to write this
all down in MY reply.

You're right, it's WRONG what Qiao wants to do, but it's not only wrong, it
even doesn't make sense. ( I hope Qiao reads this reply too. I've already
deleted his mail. )
Why the hick should I link a function if I don't know its interface?
The interface declaration is done by including the header files for the
library in question. If the headers aren't available, You won't be able to
write a progam based on the lib.

Frank
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