Returning a const reference brokes dependency among header files?

[Kermit Mei]

Hello all, I wrote four simple c++ source files for this test. Look
please:
/*************1* **************/
/// file A.h
#ifndef A_H
#define A_H

class A {
public:
void set(int i);
const int & get();
private:
int a;
};

#endif

/**************2 *************** */
///file A.cpp
#include "A.h"


inline void A::set(int i) {
a = i;
}

inline const int & A::get() {
return a;
}


/*************** **3************ *******/
///fileB.h
#ifndef B_H
#define B_H
#include "A.h"

class A;

class B
{
public:
B();

const int & getB();
private:
A ab;
};

#endif

/*************** ****4********** ******/
/// file B.cpp
#include "B.h"

B::B()
{ ab.set(19); }

const int & B::getB()
{ return ab.get(); }


/*************** ****main.cpp*** ******/
#include "B.h"
#include <iostream>

using namespace std;

int main()
{
B ab;
cout << ab.getB() << endl;
}

/*************** *************** *******/
Now, compile them and the errors occurs:

c++ -Wall -o main main.cpp A.cpp B.cpp

/tmp/ccB4KNX2.o: In function `B::getB()':
B.cpp:(.text+0x d): undefined reference to `A::get()'
/tmp/ccB4KNX2.o: In function `B::B()':
B.cpp:(.text+0x 29): undefined reference to `A::set(int)'
/tmp/ccB4KNX2.o: In function `B::B()':
B.cpp:(.text+0x 45): undefined reference to `A::set(int)'
collect2: ld returned 1 exit status

>

It's funny that if you merge them into one head file, then
you can run it:

/************* ab.h ************/
#ifndef T_H
#define T_H

class A {
public:
void set(int i);
const int & get();
private:
int a;
};

class B
{
public:
B();
const int & getB();
private:
A ab;
};

#endif


/*********** ab.cpp *************** **/
#include "t.h"

inline void A::set(int i) {
a = i;
}

inline const int & A::get() {
return a;
}

B::B()
{ ab.set(19); }

const int & B::getB()
{ return ab.get(); }

/***********main .cpp*********** ****/
#include <iostream>
using namespace std;

#include "ab.h"

int main()
{
B ab;
cout << ab.getB() << endl;;
}

To compile and run them like this:

c++ -Wall -o main main.cpp ab.cpp
ls

ab.cpp ab.h main main.cpp

./main

19

>

/************End *************** ***/

I hope anybody would like to tell me what cause it.
Dose returning a const reference broke the
dependency among header files?

Now, I need your help, if you would like to tell me why,
please cc your e-mail to me.
Thank you,very much!

Kermit

[Sam]

Re: Returning a const reference brokes dependency among headerfiles?

Kermit Mei writes:

inline void A::set(int i) {
a = i;
}
>
inline const int & A::get() {
return a;
}

Now, compile them and the errors occurs:
>

> c++ -Wall -o main main.cpp A.cpp B.cpp

/tmp/ccB4KNX2.o: In function `B::getB()':
B.cpp:(.text+0x d): undefined reference to `A::get()'
/tmp/ccB4KNX2.o: In function `B::B()':
B.cpp:(.text+0x 29): undefined reference to `A::set(int)'
/tmp/ccB4KNX2.o: In function `B::B()':
B.cpp:(.text+0x 45): undefined reference to `A::set(int)'
collect2: ld returned 1 exit status

Remove the inline keyword.


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[Kermit Mei]

Re: Returning a const reference brokes dependency among header files?

On Sep 23, 7:19 am, Sam <s...@email-scan.comwrote:

Kermit Mei writes:

inline void A::set(int i) {
a = i;
}

>

inline const int & A::get() {
return a;
}
Now, compile them and the errors occurs:

>

c++ -Wall -o main main.cpp A.cpp B.cpp

/tmp/ccB4KNX2.o: In function `B::getB()':
B.cpp:(.text+0x d): undefined reference to `A::get()'
/tmp/ccB4KNX2.o: In function `B::B()':
B.cpp:(.text+0x 29): undefined reference to `A::set(int)'
/tmp/ccB4KNX2.o: In function `B::B()':
B.cpp:(.text+0x 45): undefined reference to `A::set(int)'
collect2: ld returned 1 exit status

>
Remove the inline keyword.
>
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< 1KViewDownload

Sam, it works well after remove them.

But, sir, would you like to tell me why it can not be defined as
"inline"?
"inline" is just a suggestion for the compiler, isn't it? Then why
there're
something wrong with its arising? Why the compiler can NOT ignore it
here?

What's more, why it can be compiled with "inline" if A and B are
defined and
declared in a same file together?

Thank you very much!

[Sam]

Re: Returning a const reference brokes dependency among headerfiles?

Kermit Mei writes:

On Sep 23, 7:19 am, Sam <s...@email-scan.comwrote:

>Kermit Mei writes:

inline void A::set(int i) {
a = i;
}

>>

inline const int & A::get() {
return a;
}
Now, compile them and the errors occurs:

>>

> c++ -Wall -o main main.cpp A.cpp B.cpp
/tmp/ccB4KNX2.o: In function `B::getB()':
B.cpp:(.text+0x d): undefined reference to `A::get()'
/tmp/ccB4KNX2.o: In function `B::B()':
B.cpp:(.text+0x 29): undefined reference to `A::set(int)'
/tmp/ccB4KNX2.o: In function `B::B()':
B.cpp:(.text+0x 45): undefined reference to `A::set(int)'
collect2: ld returned 1 exit status

>>
>Remove the inline keyword.
>>
> application_pgp-signature_part
>< 1KViewDownload

>
Sam, it works well after remove them.
>
But, sir, would you like to tell me why it can not be defined as
"inline"?
"inline" is just a suggestion for the compiler, isn't it? Then why
there're
something wrong with its arising? Why the compiler can NOT ignore it
here?

This is a vast oversimplificat ion, but the 'inline' keyword requires that
every module, every "translatio n unit" that references the inlined function,
must have the definition of the function. Here, other modules, other
translation units, do not define the function with the inline keyword.

'inline' is an "all or nothing" proposition. Either you define the inline
function everywhere, and everywhere the function is used, the function's
code must be defined (and it must be the same in all translation units), or
you cannot use the 'inline' keyword, in any definition of the function.
That's why inline functions are typically declared in header files only,
with the function's code written out.

This is a gross oversimplificat ion, but that's the general idea.


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