using templates as substitutions for #ifdef

[Chris Goller]

Imagine a cross platform library. A particular object wraps up
operating system specific functionality and presents it in a uniform
manner.

Generally, people use ifdefs to change the functionality of a
particular piece of code to match that of the native operating system
(#ifdef _WINDOWS, etc)

I'm wondering if it is possible to use templates to accomplish this
same functionality.

Basically, there might be something like:

enum os_type
{
LINUX,
SOLARIS,
WINDOWS
};

template <os_type os>
class os_wrapper
{
void os_specific_function();
};

template<>
class os_wrapper<WINDOWS>
{
void os_specific_function()
{
windows_specific_function();
}
};

template<>
class os_wrapper<SOLARIS>
{
void os_specific_function()
{
solaris_specific_function();
}

};

This way the compiler wouldn't need to compile the class unless it was
instantiated with an operating system as a template argument.

However, the code above doesn't seem to work at least in g++.

So, my question is it possible with some template kung fu to wrap OS
specific functions so that the compiler will compile them ONLY if they
have been instantiated and ignore the code otherwise?

Chris

[Thomas Matthews]

Chris Goller wrote:

Imagine a cross platform library. A particular object wraps up
operating system specific functionality and presents it in a uniform
manner.

Generally, people use ifdefs to change the functionality of a
particular piece of code to match that of the native operating system
(#ifdef _WINDOWS, etc)

I'm wondering if it is possible to use templates to accomplish this
same functionality.

Basically, there might be something like:

enum os_type
{
LINUX,
SOLARIS,
WINDOWS
};

template <os_type os>
class os_wrapper
{
void os_specific_function();
};

template<>
class os_wrapper<WINDOWS>
{
void os_specific_function()
{
windows_specific_function();
}
};

template<>
class os_wrapper<SOLARIS>
{
void os_specific_function()
{
solaris_specific_function();
}

};

This way the compiler wouldn't need to compile the class unless it was
instantiated with an operating system as a template argument.

However, the code above doesn't seem to work at least in g++.

So, my question is it possible with some template kung fu to wrap OS
specific functions so that the compiler will compile them ONLY if they
have been instantiated and ignore the code otherwise?

Chris

That is one method. I prefer having files containing OS specific
implementations of functions, and letting the linker handle it all.
The build script would choose which modules to link together.

This helps push the OS specifics to their lowest level.

--
Thomas Matthews

[Chris Goller]

> That is one method. I prefer having files containing OS specific

implementations of functions, and letting the linker handle it all.
The build script would choose which modules to link together.
This is of course the traditional way of handling things. However,
there are numerous benefits to letting the compiler rather than the
linker know what is going on. Take the following example.

Many libraries do mostly the same thing for all platforms. In fact,
this is good design when developing on multiple platforms. They try to
limit the OS-specific code to a few basic functions. Now, let's say
I've developed a library that works on all most major operating
systems. However, a user decides that they want to use this library on
a 37-bit machine that isn't supported. Lets say that modifying the
library itself would require only a few changes in one or two functions
to make it work. The rest of the library would work just fine
unmodified.

The problem is that the user would have to make their own local copy of
the library with just their changes, and change the build the process
to accomodate their new OS. This of course now means that all updates
to the library will require the user to reinsert his changes and build
process changes.

However, if this library uses templates to specify the OS then the user
can use *template specialization* to implement changes to just the
functions he needs to modify. No changes to the build process are
needed.
This helps push the OS specifics to their lowest level.

No, it actually pushes them to the highest level. The build process
affects all portions of the program. Why even bother specifying the OS
when the code itself is completely OS independent?

The point of course is not just all this. The point is that I can't
seem to get the method of using templates to work. That is the
question.

Chris

[Jordan Stewart]

The following works as expected (it displays "solaris specific
function") on gcc 3.4.2 (MinGW, on Win XP):

#include <iostream>

void windows_specific_function() {
std::cout << "windows specific function" << std::endl;
};

void solaris_specific_function() {
std::cout << "solaris specific function" << std::endl;
};
enum os_type
{
LINUX,
SOLARIS,
WINDOWS
};

template <os_type os>
class os_wrapper
{
public:
void os_specific_function();

};

template<>
class os_wrapper<WINDOWS>
{
public:
void os_specific_function()
{
windows_specific_function();
}

};

template<>
class os_wrapper<SOLARIS>
{
public:
void os_specific_function()
{
solaris_specific_function();
}

};

const os_type operating_system = SOLARIS;

int main () {

os_wrapper<operating_system> os;

os.os_specific_function();

return 0;
}
- Jordan

[Jordan Stewart]

oops, I've just realised that my example doesn't really mean much:
- You're still going to need conditional compilation for #includes (I
don't think you'll be getting around that with template voodoo though
:)
- You'll still get compilation errors because of missing function
declarations for os specific functions (which you could get around by
providing declarations, but that creates far more work that this is all
worth)...

- Jordan