[CMake] Suggestion for CMake platform/compiler detection

[Tanguy Krotoff]

Sylvain Benner wrote:
> So can you explain what do you mean by "inheritance system" please ?

Some explanations here:
http://dev.openwengo.org/trac/openwengo/trac.cgi/wiki/OWBuild

you create a library A that depends on cURL and Boost
A uses privately cURL and uses Boost publicly, example:

file A.h:
#include <boost.h>
int toto();

file A.cpp:
#include <curl.h>
int toto() {...}

This means that B uses A but B really don't care A uses curl internaly
on the other hand B cares that A is using boost since #include <boost.h> 
is inside A.h

so in A' compilation script you will write:
ow_create_library(A)
ow_use_public_libraries(boost)
ow_use_private_libraries(curl)

in B's script:
ow_create_executable(B)
ow_use_private_libraries(A)

B will inherit the public interface from A: boost include path, 
definitions, flags, link libraries... but won't inherits all the curl stuff

See the schema attached

In OOP you write something like this:

class A {
public:
	int boost();
private:
	int curl();
}

class B : public class A {
public:
	//int boost() inherited
}


What is nice with this approach is that you can represent your build 
system as a UML diagram! See the UML diagram attached.
You can easily communicate with your team/dev community about the build 
system because you use a standard representation that everybody understands.


This example does not seem straightforward but public/private notion 
permits to optimize compilation and to separate things in a nice and 
clean way. For example, on a huge project you want to shorten the 
include path because the shorter it is the faster the compilation will be.

+ now you add the dll (=> so shared library under UNIX) problematic...
Imagine you want to choose with a simple variable if A is a dll or a 
static lib. if A is a dll, B won't have to link with boost and curl. If 
A is a static lib, B has to link with boost and curl.
With this kind of inheritance no problem, inside 
ow_use_public_libraries() you check whether the child libs are static or 
shared and propagate or not the link libraries.

Imagine that A depends on curl, ffmpeg, boost, tinyxml...
C depends on Qt, directx, ALSA....
and B the executable depends on A and C
starts to be complex and you really would like the compilation system 
take care of it for yourself...
Here is a schema of such a program:
http://www.cynapses.org/tmp/wengophone/wengophone.png

I don't know if you understand my explanation. if you have already 
encounter the problematic you will understand.

-- 
Tanguy Krotoff <tanguy.krotoff at wengo.fr>
http://openwengo.org
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