fatal error: Eigen/Dense: No such file or directory

Question

I have installed `libeigen3-dev` in order to compile programs using Eigen 3. when I include a file, such as `Eigen/Dense` I get this error when I try to run `g++`: ``` user@office-debian:~/Documents/prog$ g++ src/main.cpp -MMD -std=c++11 In file included from src/main.cpp:9:0: src/tdefs.h:16:23: fatal error: Eigen/Dense: No such file or directory compilation terminated. ``` Running the following line works fine: g++ -I /usr/include/eigen3/ src/main.cpp -MMD -std=c++11 shouldn't that include directory be automatically found by GCC because I installed the Eigen package through aptitude? Why are boost and OpenGL found automatically when I install the libraries but not Eigen? (Note that eigen is a header-only library, but that shouldn't matter right?) Running `g++ src/main.cpp -MMD -std=c++11 --verbose` produces the following output: ``` Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.7/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/…

Accepted Answer

Run your compiler with the `--verbose` switch: ``` g++ --verbose ... ``` If your includes are relative to one of the paths shown in this output, you don't have to use `-I`. It depends how gcc has been configured, and it depends where that other stuff is installed. Note that `.` is typically not in the -I paths. Later After exchanging a couple of comments it is clear that `/usr/include/eigen3/Eigen/Dense` should be include-able by `#include `, but not by `#include `. Therefore, the addition of the command line option `-I /usr/include/eigen3` is mandatory. Whether some installation selects to install header files into a directory that in one of the ones compiled into gcc, depends on the default, a decision made by the distributor, or a decision made during installation. I'd say that "frequently used" header files (Boost) are well placed into /usr/local/include while some "elitist" stuff would be better off in a directory of its own.

fatal error: Eigen/Dense: No such file or directory

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