function pointers

[Jeffrey Baker]

Hi,
I can't get my code to link. I get
Example error LNK2019: unresolved external symbol "void __cdecl f(void *)"
(?f@@YAXPAX@Z) referenced in function _main
and
Example fatal error LNK1120: 1 unresolved externals

Here is the code:
#include <iostream>

using std::cout;

using std::endl;

void f( void(*)); // void(*) is the function pointer argument

void x();

void y();

int main()

{

f(x); // calls x function

f(y); // calls y function

return 0;

}

void f(void *c); // pointer function


void x()

{

cout << "x() is chosen" << endl;

}

void y()

{

cout << "y() is chosen" << endl;

}

Thanks,
jb

 

[BigBrian]

Hi,
I can't get my code to link. I get
Example error LNK2019: unresolved external symbol "void __cdecl f(void *)"
(?f@@YAXPAX@Z) referenced in function _main
and
Example fatal error LNK1120: 1 unresolved externals

Here is the code:
#include <iostream>

using std::cout;

using std::endl;

void f( void(*)); // void(*) is the function pointer argument

void x();

void y();

int main()

{

f(x); // calls x function

f(y); // calls y function

return 0;

}

void f(void *c); // pointer function


void x()

{

cout << "x() is chosen" << endl;

}

void y()

{

cout << "y() is chosen" << endl;

}

Thanks,
jb

I don't see where f() defined? Your linker isn't either.

-Brian

 

[Thomas Tutone]

I can't get my code to link. I get
Example error LNK2019: unresolved external symbol "void __cdecl f(void *)"
(?f@@YAXPAX@Z) referenced in function _main
and
Example fatal error LNK1120: 1 unresolved externals

Here is the code:

#include <iostream>
using std::cout;
using std::endl;

void f( void(*)); // void(*) is the function pointer argument

Change the above line to:

void f(void(*)());

void x();

void y();

int main()
{
f(x); // calls x function
f(y); // calls y function
return 0;
}

void f(void *c); // pointer function

Change the above line to:

void f(void (* c)())
{
c();
}

void x()
{
cout << "x() is chosen" << endl;
}

void y()
{
cout << "y() is chosen" << endl;
}

Best regards,

Tom

 

[Jeffrey Baker]

I don't see where f() defined? Your linker isn't either.

Isn't void f( void(*)); the defined function?
Jeff

 

[Jakob Bieling]

Isn't void f( void(*)); the defined function?

Nope, its declaring it. In other words, saying "there is such a
function". You still need to tell the compiler what this function is
supposed to do, when it is called, by actually defining it:

void f (void (*))
{
// code here
}

Note the missing semicolon and the added curly braces.

hth

 

[Thomas Tutone]

Jeffrey Baker wrote:

[snip]

Isn't void f( void(*)); the defined function?

No, that's a declaration, not a definition.

Best regards,

Tom

 

[Jeffrey Baker]

Nope, its declaring it. In other words, saying "there is such a
function". You still need to tell the compiler what this function is
supposed to do, when it is called, by actually defining it:

void f (void (*))
{
// code here
}

Note the missing semicolon and the added curly braces.

I added the braces and it linked correctly. It didn't run like it should.
f(x) and f(y) in "int main()" did not call the
void x()
{
cout << "x() is chosen" << endl;
} and

void y()
{
cout << "y() is chosen" << endl;
}

Jeff

 

[Kai-Uwe Bux]

This void(*) is not a function pointer. Try:

void f( void (c) ( void ) ) {
c();
}



[snip]

I added the braces and it linked correctly. It didn't run like it should.
f(x) and f(y) in "int main()" did not call the
void x()
{
cout << "x() is chosen" << endl;
} and

void y()
{
cout << "y() is chosen" << endl;
}

Jeff

Best

Kai-Uwe Bux

 

[Andrey Tarasevich]

I can't get my code to link. I get
Example error LNK2019: unresolved external symbol "void __cdecl f(void *)"
(?f@@YAXPAX@Z) referenced in function _main
and
Example fatal error LNK1120: 1 unresolved externals

Here is the code:
#include <iostream>

using std::cout;
using std::endl;

void f( void(*)); // void(*) is the function pointer argument

'void(*)' is not a function pointer argument. 'void(*)' is equivalent to
'void*', which is a regular "data" pointer, not a function pointer.

void x();
void y();

int main()
{
f(x); // calls x function
f(y); // calls y function

Error 1: This will not compile, because 'x' and 'y' in this context are values
of type 'void(*)()', while 'f' expects an argument of type 'void*'. You did not
mention this error in you original message, which most likely means that the
code you are trying to compile is different from the code you posted here.
Please, always post real code.

return 0;
}

void f(void *c); // pointer function

That's another declaration of the same function 'f'.

void x()
{
cout << "x() is chosen" << endl;
}

void y()
{
cout << "y() is chosen" << endl;
}

Error 2: You provided two declarations for 'f' but no definition. That would be
the reason for the linker error message.

Correct the above errors and it would work.

 

[Jeffrey Baker]

This void(*) is not a function pointer. Try:

void f( void (c) ( void ) ) {
c();
}

Thanks - my varation of code that is true too and not different:
#include <iostream>

using std::cout;

using std::endl;

void f( void(*)());

//void f( void (*c)());


void x();

void y();

int main()

{

f(x);

f(y);



return 0;

}

void f( void (*c) ( /*void */) ) {


c(); //----> this is what I didn't see with the argument ()'s as prototype
and declaration.

}


void x()

{

cout << "x() is chosen" << endl;

}

void y()

{

cout << "y() is chosen" << endl;

}

Best Regards,
Jeff

[snip]

I added the braces and it linked correctly. It didn't run like it should.
f(x) and f(y) in "int main()" did not call the
void x()
{
cout << "x() is chosen" << endl;
} and

void y()
{
cout << "y() is chosen" << endl;
}

Jeff

Best

Kai-Uwe Bux