Lapack undefined reference - g++

Question

I am new to g++ and lapack, and attempting to use them. I encountered a problem when I tried to compile the following naive code
#include <lapackpp.h>
int main()
{
LaGenMatDouble A;
return 0;
}
If I run the command
$g++ -L/usr/local/lib -llapackpp test2.cpp
where test2.cpp is the name of the cpp file, the terminal would give an error:
test2.cpp:1:22: fatal error: lapackpp.h: No such file or directory
But if I run the command:
$g++ -I/usr/local/include/lapackpp -L/usr/local/lib -llapackpp test2.cpp
the terminal would give an error:
/tmp/ccUi11DG.o: In function `main':
test2.cpp:(.text+0x12): undefined reference to `LaGenMatDouble::LaGenMatDouble()'
test2.cpp:(.text+0x23): undefined reference to `LaGenMatDouble::~LaGenMatDouble()'
collect2: ld returned 1 exit status
BTW, if I run the command
$pkg-config lapackpp --libs
the result is
-L/usr/local/lib -llapackpp
Could you please help me solve this? Thanks in advance!

Answer 1

Lapack requires fortran libraries, so that's where the -lgfortran comes from. Moreover, it appears the exact way to provide that library for the compiler depends on the Linux distriburion. From the documentation:
Requirements
This package requires the packages "blas", "lapack" (without the "++"), and a Fortran compiler. On most Linuxes these are available as pre-compiled binaries under the name "blas" and "lapack". For SuSE 10.x, the Fortran compiler is available as package "gfortran". For SuSE 9.x, the Fortran compiler is available as package "gcc-g77".
Not sure why pkg-config lapackpp --libs does not list -lgfortran
The -I/usr/local/include/lapackpp specifes the lapackpp-related header files. Without it the compiler cannot find lapackpp.h when you try to include it (#include <lapackpp.h>) -- see the compiler error in your question

Answer 2

I finally solved the problem but would still wonder why it has to be so.
The only command that can link cpp file to lapackpp library is:
g++ foo.cpp -o foo -lgfortran -llapackpp -I/usr/local/include/lapackpp
It would not work without -lgfortran, or with -I/usr/local/include/lapackpp replaced by -L/usr/local/lib.
Does anyone have an answer?