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GRalphE
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 Posted: Fri Jun 27, 2003 2:03 pm Post subject: How to fix Undefined Reference compile error? |
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The code can be found at:
http://www.anora.org/grellsworth/muvie/index.html
Just click on the "Download MUVIE Development Snapshot" link to get
the Gzipped Tarball.
The problem is:
In Muvie.c, in main() I have:
#include "Data.h"
#include "Muvie.h"
int main(int argc, char **argv)
{
stringstream Stream;
Logger = new Log();
SetValue(32, Stream);
delete Logger;
pthread_exit(NULL);
return 0;
}
In Data.c, I have a function:
template <class T>
void SetValue(const T &value, stringstream &ss)
{
ss.clear();
ss.str("");
ss << value;
}
I have the function prototype (declaration) in Data.h which is
#included in Muvie.c
I get the following compile error:
../objs/Muvie.o: In function `main':
/home/muvie/muvie/src/Muvie.c:30: undefined reference to `void
SetValue
std::char_traits&)'
collect2: ld returned 1 exit status
make: *** [Muvie] Error 1
I don't see what the problem is, since Data.h is #included in Muvie.c
where main() is.
How can I fix this?
Sincerely,
GRalphE
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Thomas Mang
Guest
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| Posted: Fri Jun 27, 2003 8:16 pm Post subject: Re: How to fix Undefined Reference compile error? |
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GRalphE schrieb:
| Quote: | #include "Data.h"
#include "Muvie.h"
int main(int argc, char **argv)
{
stringstream Stream;
Logger = new Log();
SetValue(32, Stream);
delete Logger;
pthread_exit(NULL);
return 0;
}
In Data.c, I have a function:
template
void SetValue(const T &value, stringstream &ss)
{
ss.clear();
ss.str("");
ss << value;
}
|
Move this function definition out of the *.c - file into the header
file.
Remember, templates are special - the compiler has to see their
definitions from outside the current file, in order to instantiate it.
At least under the inclusion-model - which I strongly guess you are
using.
regards,
Thomas
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Thomas Mang
Guest
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| Posted: Sun Jun 29, 2003 11:37 am Post subject: Re: How to fix Undefined Reference compile error? |
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GRalphE schrieb:
| Quote: | OK, I should have known that...
But what if the template was inside a class (like I intended it to be
in the beginning)?
=========================
In MyClass.h :
class MyClass {
public:
template
void SetValue(const T &value);
private:
stringstream Value;
};
========================
In My Class.c :
template
void MyClass::SetValue(const T &value)
{
Value.clear();
Value.str("");
Value << value;
return;
}
=======================
Would I still have to put the template definition within the class in
this
case, also?
|
No, not necessarily inside the class definition (inline function - you
can do that, but probably only if it's a very small function), but you
have to place it into the header file.
Imagine some file, say "main.c", calls your template function:
MyClass myClass;
myClass.SetValue(23);
Now suppose the definition of your template function is in a "MyClass.c".
How should the compiler be able to instantiate the function with an int
as parameter?
Short answer: It can't.
However, if you place the definition of your template function into the
header file, there is no problem. Why? Because in order to use class
MyClass, "main.c" MUST include the header-file "MyClass.h". Otherwise
simple statements such as
MyClass myClass;
won't compile either. And as a result of #including the header file, the
compiler is able to see the definition for your template functions and is
able to instantiate them.
Conclusion:
When using the "inclusion model" of templates, place both the declaration
AND the definition of template functions into the header file. Other
files can then #include this file and instantiate the templates.
The only exception is, of course, if a template function / class is NOT
INTENDED to be used by other files - then you can place it into the *.c -
file.
regards,
Thomas
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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