一定记得写main（）函数！

记录一下今天遇到的一个问题
eigen.cpp

#include <iostream>
#include <ctime>
#include <Eigen/Core>
#include <Eigen/Dense>

CMakeLists.txt

cmake_minimum_required( VERSION 2.4 )
project( useEigen )

include_directories( "/usr/include/eigen3" )
add_executable( useEigen eigen.cpp )

build的时候报错

/home/safsafsa/projects/useEige/build> make -j2
-- Configuring done
-- Generating done
-- Build files have been written to: /home/safsafsa/projects/useEige/build
Scanning dependencies of target useEigen
[ 50%] Building CXX object CMakeFiles/useEigen.dir/eigen.o
[100%] Linking CXX executable useEigen
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
CMakeFiles/useEigen.dir/build.make:94: recipe for target 'useEigen' failed
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/useEigen.dir/all' failed
Makefile:83: recipe for target 'all' failed
make[2]: *** [useEigen] Error 1
make[1]: *** [CMakeFiles/useEigen.dir/all] Error 2
make: *** [all] Error 2
*** 失败：退出代码 2 ***

报错原因：
因为我指示想看eigen3的包找到没，所以eigen.cpp里面没有写main函数,其实报错信息中提示了的，无奈没看懂

解决办法：
加个main函数即可