Using a basic function of CGAL, e.g. `square` - c++

Question

I am starting to work my way through CGAL and have a question about the Algebraic foundations package.
How do you use the square function defined here?
I tried the following
// tmp.cpp
#include <CGAL/number_utils.h>
int main() {
double dist = 0.002;
auto sq_dist = CGAL::square(dist);
return 0;
}
The compiler throws the following error message
$ make tmp
Consolidate compiler generated dependencies of target tmp
[ 14%] Building CXX object CMakeFiles/tmp.dir/tmp.cpp.o
In file included from /home/USER/info/cgal/tmp/tmp.cpp:1:
/home/USER/info/cgal/Algebraic_foundations/include/CGAL/number_utils.h: In instantiation of ‘typename CGAL::Algebraic_structure_traits<Type_>::Square::result_type CGAL::square(const AS&) [with AS = double; typename CGAL::Algebraic_structure_traits<Type_>::Square::result_type = CGAL::Null_tag; typename CGAL::Algebraic_structure_traits<Type_>::Square = CGAL::Null_functor]’:
/home/USER/info/cgal/tmp/tmp.cpp:5:30: required from here
/home/USER/info/cgal/Algebraic_foundations/include/CGAL/number_utils.h:71:18: error: no match for call to ‘(CGAL::Algebraic_structure_traits<double>::Square {aka CGAL::Null_functor}) (const double&)’
71 | return square( x );
| ~~~~~~^~~~~
make[3]: *** [CMakeFiles/tmp.dir/build.make:146: CMakeFiles/tmp.dir/tmp.cpp.o] Error 1
make[2]: *** [CMakeFiles/Makefile2:83: CMakeFiles/tmp.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:90: CMakeFiles/tmp.dir/rule] Error 2
make: *** [Makefile:124: tmp] Error 2
It is mentioned in this github issue.
Update: Following the comment by #MarcGlisse I changed the header include to
// tmp.cpp
#include <CGAL/number_utils.h>
#include <CGAL/basic.h>
int main() {
double dist = 0.002;
auto sq_dist = CGAL::square(dist);
return 0;
}
Compilation runs through smoothly but linking throws new errors
$ make tmp
[ 14%] Building CXX object CMakeFiles/tmp.dir/tmp.cpp.o
[ 28%] Linking CXX executable tmp
/usr/bin/ld: CMakeFiles/tmp.dir/tmp.cpp.o: in function `main':
/home/USER/info/cgal/STL_Extension/include/CGAL/exceptions.h:88: multiple definition of `main'; /usr/bin/ld: DWARF error: section .debug_str is larger than its filesize! (0x32f0bc2 vs 0x2447b28)
/usr/bin/ld: DWARF error: section .debug_str is larger than its filesize! (0x32f0bc2 vs 0x2447b28)
/usr/bin/ld: DWARF error: section .debug_str is larger than its filesize! (0x32f0bc2 vs 0x2447b28)
...
The referenced line STL_Extension/include/CGAL/exceptions.h:88 can be found here.
Update: I found the issue (see my answer) but it doesnt make sense to me. Why am I not able to have multiple main functions in different files in the same folder?
I built the following way
cd ${CGAL_DIR}/tmp
cgal_create_CMakeLists -s tmp
cmake -DCMAKE_BUILD_TYPE=Debug .
make tmp

Answer 1

The following example (cgal_ex.cpp) works for me:
#include <iostream>
#include <CGAL/Simple_cartesian.h>
typedef CGAL::Simple_cartesian<double> Kernel;
typedef Kernel::Point_2 Point_2;
int main()
{
Point_2 p(0,0), q(3,4);
std::cout << "p = " << p << std::endl;
std::cout << "q = " << q.x() << " " << q.y() << std::endl;
std::cout << "sqdist(p,q) = "
<< CGAL::squared_distance(p,q) << std::endl;
return 0;
}
g++ cgal_ex.cpp; a.out
p = 0 0
q = 3 4
sqdist(p,q) = 25
Reference: [https://doc.cgal.org/latest/Manual/tutorial_hello_world.html][1]
Adding example of a Makefile for two files having main in the same directory:
MODULES=cgal_ex cgal_ex1
%: %.cpp
g++ -O2 -o $# $<
strip $#
%: %.c
gcc -o $# $<
strip $#
clean:
-rm -rf $(MODULES)
all: $(MODULES)
Usage example:
make all -f Makefile.cgal
g++ -O2 -o cgal_ex cgal_ex.cpp
strip cgal_ex
g++ -O2 -o cgal_ex1 cgal_ex1.cpp
strip cgal_ex1
Usage example with cgal_create_CMakeLists.
Source: https://github.com/CGAL/releases/blob/main/scripts/cgal_create_CMakeLists
Usage example:
Create new directory, which has 3 files only:
cgal_create_CMakeLists cgal_ex.cpp cgal_ex1.cpp
Run the following commands:
cmake $PWD
make
Results:
-- The C compiler identification is GNU 9.3.0
-- The CXX compiler identification is GNU 9.3.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Using header-only CGAL
-- Targetting Unix Makefiles
-- Using /usr/bin/c++ compiler.
-- Found GMP: /usr/lib/x86_64-linux-gnu/libgmp.so
-- Found MPFR: /usr/lib/x86_64-linux-gnu/libmpfr.so
-- Found Boost: /usr/lib/x86_64-linux-gnu/cmake/Boost-1.71.0/BoostConfig.cmake (found suitable version "1.71.0", minimum required is "1.48")
-- Boost include dirs: /usr/include
-- Boost libraries:
-- Using gcc version 4 or later. Adding -frounding-math
-- Found Boost: /usr/lib/x86_64-linux-gnu/cmake/Boost-1.71.0/BoostConfig.cmake (found version "1.71.0")
CMake Warning at /usr/lib/x86_64-linux-gnu/cmake/CGAL/CGAL_enable_end_of_configuration_hook.cmake:99 (message):
=======================================================================
Call Stack (most recent call first):
CMakeLists.txt:9999 (CGAL_run_at_the_end_of_configuration)
-- Configuring done
-- Generating done
-- Build files have been written to: /home/<username>/cgal
Scanning dependencies of target cgal_ex1
[ 25%] Building CXX object CMakeFiles/cgal_ex1.dir/cgal_ex1.cpp.o
[ 50%] Linking CXX executable cgal_ex1
[ 50%] Built target cgal_ex1
Scanning dependencies of target cgal_ex
[ 75%] Building CXX object CMakeFiles/cgal_ex.dir/cgal_ex.cpp.o
[100%] Linking CXX executable cgal_ex
[100%] Built target cgal_ex
[1]: https://doc.cgal.org/latest/Manual/tutorial_hello_world.html
[2]: https://github.com/CGAL/releases/blob/main/scripts/cgal_create_CMakeLists

Answer 2

The issue arises because of the -s tmp argument.
The solution is to call it in either of the following formats
cgal_create_CMakeLists tmp
cgal_create_CMakeLists
The documentation gives an explanation
$ cgal_create_CMakeLists -h
Usage: cgal_create_CMakeLists [-s source] ...
...
-s source If this parameter is given the script will create one single executable for 'source' with all source files; otherwise it creates one executable for each main'ed source.
...
There is no reasonable way to create one executable for files containing multiple main functions.