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John Dumais · Guest

Hello,

I have been trying to develop a realtively simple policy template class
and ran into a situation where I think I don't understand template
syntax when the class function definitions are in a .cpp file. As an
example, this compiles, links and runs...

---------------------------------------------------------

Policy.h:

#if !defined(POLICY_INCLUDED)
#define POLICY_INCLUDED

#include <string>

struct LoggingBase
{
virtual ~LoggingBase() {}

virtual void SetLogName(const std::string &logName) = 0;
virtual void Append(const std::string &msg) = 0;
};

struct NoLogging : public LoggingBase
{
virtual void SetLogName(const std::string &logName) {}
virtual void Append(const std::string &msg) {}
};

class FileLogging : public LoggingBase
{
std::string m_logName;

public:
virtual ~FileLogging() {}

virtual void SetLogName(const std::string &logName);
virtual void Append(const std::string &msg);
};

template <typename LogPolicy>
class HasPolicy
{
LogPolicy l;

public:
HasPolicy() { l.SetLogName("logName"); } // <--- implementation

void DoSomething() { l.Append("msg"); } // <--- implementation
};

#endif

---------------------------------------------------------

PolicyImpl.cpp:

#include "Policy.h"

void FileLogging::SetLogName(const std::string &logName)
{
m_logName = logName;
}

void FileLogging::Append(const std::string & msg)
{
// do something here
}

---------------------------------------------------------

TestProgram.cpp:

#include "Policy.h"

int main(int argc, char *argv[])
{
HasPolicy p.DoSomething();

HasPolicy<NoLogging> n;
n.DoSomething();

return 0;
}

The problem comes in when I try to move the implementation of class
functions in the HasPolicy class to a .cpp file. If I put the following
in the PolicyImpl.cpp file, the application compiles but produces
unresolved externals of the form...

HasPolicy<class FileLogging>::HasPolicy<class FileLogging>(void)
HasPolicy<struct NoLogging>::HasPolicy<struct NoLogging>(void)
void HasPolicy<class FileLogging>::DoSomething(void)
void HasPolicy<struct NoLogging>::DoSomething(void)

What I put in PolicyImpl.cpp that causes the problem is this...

template <typename LogPolicy>
HasPolicy<LogPolicy>::HasPolicy()
{
l.SetLogName("foo");
}

template <typename LogPolicy>
void HasPolicy<LogPolicy>::DoSomething()
{
l.Append("foo");
}

What I'm doing appears to be wrong, but I can't hit upon the correct syntax.

Any suggestions are most appreciated.

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lampard · Guest

For templates the implementation should be in the header file rather
than cpp file that is what I do regularly.
Why should it be in header file I do not have any good answer but wish
some one will provide.
I think the compiler need to know this information to expand the
templates for the class usage in your case LogPolicy.

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Bo Persson · Guest

"lampard" <devendermarri (AT) hotmail (DOT) com> skrev i meddelandet
news:1122694395.113117.239120 (AT) g47g2000cwa (DOT) googlegroups.com...

Ulrich Eckhardt · Guest

John Dumais wrote: