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From: Xiutao Yang on 15 Apr 2008 04:05
hi,all
I have a program about ## ,which is the following.
=========================================================
#include <iostream>
using namespace std;

class CMyClass {
public:
static int m_i;
public:
CMyClass(){
m_i++;
}
};

#define NEWW(i) CMyClass *a_##i=new CMyClass();
#define PPR(i) cout<<a_##i->m_i<<endl
#define F(i) F_##i()
int CMyClass::m_i = 0;


int F_0()
{
cout<<"F0"<<endl;
return 1;
}

int F_1()
{
cout<<"F1"<<endl;
return 1;
}

int F_2()
{
cout<<"F2"<<endl;
return 1;
}

int F_3()
{
cout<<"F3"<<endl;
return 1;
}

void FF(int i){
int a;
a=i;

switch(a){
case 0:{
F(0);
break;
}
case 1:{
F(1);
break;
}
case 2:{
F(2);
break;
}
case 3:{
F(3);
break;
}

}

}

void NEW(int i){
int a;
a=i;

NEWW(i);
PPR(i);

}

int main()
{
for(int i=0;i<10;i++){
NEW(i);
}
for(int i=0;i<4;i++){
FF(i);
}

return 0;
}

===========================
The question is the I can call NEWW(i) in function void NEW(int i)
But failed in processing function void FF(int i)?
If we write void FF(int i) as:
void FF(int i){
F(i);
}
It will fail.

Compiler : VS2005


From: Ian Collins on 15 Apr 2008 04:09
Xiutao Yang wrote:
> hi,all
> I have a program about ## ,which is the following.
> =========================================================
> #include <iostream>
> using namespace std;
>
> class CMyClass {
> public:
> static int m_i;
> public:
> CMyClass(){
> m_i++;
> }
> };
>
> #define NEWW(i) CMyClass *a_##i=new CMyClass();
> #define PPR(i) cout<<a_##i->m_i<<endl
> #define F(i) F_##i()

Why are you doing something so diabolical?

--
Ian Collins.
From: Ben Bacarisse on 15 Apr 2008 07:40
"Xiutao Yang" <xiutao.yang(a)ia.ac.cn> writes:

> I have a program about ## ,which is the following.
<snip>

> #define F(i) F_##i()

<snip more pre-processor mangling>

> The question is the I can call NEWW(i) in function void NEW(int i)
> But failed in processing function void FF(int i)?
> If we write void FF(int i) as:
> void FF(int i){
> F(i);
> }
> It will fail.

That is because FF(i) expands to F_i() and there is no F_i defined.
Presumably in NEWW(i) the value of i does not matter. You seem to
want the pre-processor (which runs once when you program is built) to
know about the value of variables at run-time. It just can't.

Back up a bit and explain what you are doing and why you think this is
the right solution.

--
Ben.
From: Xiutao Yang on 15 Apr 2008 10:55
thx,all
Now, I found the reason.
void NEW(int i){
NEWW(i);
PPR(i);
}
wil be compiled as
void NEW(int i){
CMyClass *a_##i=new CMyClass();
cout<<a_##i->m_i<<endl

}
Of course, It works well :P

thx again.

"Ben Bacarisse" <ben.usenet(a)bsb.me.uk> ??????:874pa3jqhd.fsf(a)bsb.me.uk...
> "Xiutao Yang" <xiutao.yang(a)ia.ac.cn> writes:
>
>> I have a program about ## ,which is the following.
> <snip>
>
>> #define F(i) F_##i()
>
> <snip more pre-processor mangling>
>
>> The question is the I can call NEWW(i) in function void NEW(int i)
>> But failed in processing function void FF(int i)?
>> If we write void FF(int i) as:
>> void FF(int i){
>> F(i);
>> }
>> It will fail.
>
> That is because FF(i) expands to F_i() and there is no F_i defined.
> Presumably in NEWW(i) the value of i does not matter. You seem to
> want the pre-processor (which runs once when you program is built) to
> know about the value of variables at run-time. It just can't.
>
> Back up a bit and explain what you are doing and why you think this is
> the right solution.
>
> --
> Ben.


From: Xiutao Yang on 15 Apr 2008 10:57
sorry, it should be

void NEW(int i){
CMyClass *a_i=new CMyClass();
cout<<a_i->m_i<<endl

}


"Xiutao Yang" <xiutao.yang(a)ia.ac.cn> д����Ϣ����:fu2fli$39m$1(a)news.cn99.com...
> thx,all
> Now, I found the reason.
> void NEW(int i){
> NEWW(i);
> PPR(i);
> }
> wil be compiled as
> void NEW(int i){
> CMyClass *a_##i=new CMyClass();
> cout<<a_##i->m_i<<endl
>
> }
> Of course, It works well :P
>
> thx again.
>
> "Ben Bacarisse" <ben.usenet(a)bsb.me.uk> ??????:874pa3jqhd.fsf(a)bsb.me.uk...
>> "Xiutao Yang" <xiutao.yang(a)ia.ac.cn> writes:
>>
>>> I have a program about ## ,which is the following.
>> <snip>
>>
>>> #define F(i) F_##i()
>>
>> <snip more pre-processor mangling>
>>
>>> The question is the I can call NEWW(i) in function void NEW(int i)
>>> But failed in processing function void FF(int i)?
>>> If we write void FF(int i) as:
>>> void FF(int i){
>>> F(i);
>>> }
>>> It will fail.
>>
>> That is because FF(i) expands to F_i() and there is no F_i defined.
>> Presumably in NEWW(i) the value of i does not matter. You seem to
>> want the pre-processor (which runs once when you program is built) to
>> know about the value of variables at run-time. It just can't.
>>
>> Back up a bit and explain what you are doing and why you think this is
>> the right solution.
>>
>> --
>> Ben.
>
>


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