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I did lots of things to fix these errors so I'm not sure which of them did, but I think it was one of these:

1. (I think it was what solved the problem) Uninstalling and reinstalling gcc and g++ to a specific version (in my case 5.4.0 for both).
2. Doing this: export CC=/usr/local/bin/gcc export CXX=/usr/local/bin/g++ or run catkin_make like: CXX=g++5 CC=gcc5 catkin_make.

It was important to delete the build and devel folders, and then the catkin_make worked.

I did lots of things to fix these errors so I'm not sure which of them did, but I think it was one of these:

1. (I think it was what solved the problem) Uninstalling and reinstalling gcc and g++ to a specific version (in my case 5.4.0 for both).
2. Doing this: export CC=/usr/local/bin/gcc export CXX=/usr/local/bin/g++ or run catkin_make like: CXX=g++5 CC=gcc5 catkin_make.. Like @gvdhoorn said, this should not be necessary, so I think that 1. solved my problem

It was important to delete the build and devel folders, and then the catkin_make worked.

I did lots of things to fix these errors so I'm not sure which of them did, but I think it was one of these:

1. (I think it was what solved the problem) Uninstalling and reinstalling gcc and g++ to a specific version (in my case 5.4.0 for both).
2. Doing this: export CC=/usr/local/bin/gcc export CXX=/usr/local/bin/g++ or run catkin_make like: CXX=g++5 CC=gcc5 catkin_make. Like @gvdhoorn said, this should not be necessary, so I think that 1. solved my problem

It was important to delete the build and devel folders, and then the catkin_make worked.

EDIT For more information, in this answer https://answers.ros.org/question/291910/linking-problem-with-catkin_libraries/ someone gave me the clue: "it looks like all of the unresolved symbols involve std::string, which changed between gcc 4.9 and gcc 5". I think that I had a problem because I had g++ and gcc of some versions at the same time and I wasn't using the right version, like I said below. I uninstalled the "bad" version but probably there is another way to fix it.

I did lots of things to fix these errors so I'm not sure which of them did, but I think it was one of these:

1. (I think it was what solved the problem) Uninstalling and reinstalling gcc and g++ to a specific version (in my case I had 4.8, 4.9 and 5.4 installed, and I uninstalled the two 4.*, now I have only 5.4.0 for both).
2. Doing this: export CC=/usr/local/bin/gcc export CXX=/usr/local/bin/g++ or run catkin_make like: CXX=g++5 CC=gcc5 catkin_make. Like @gvdhoorn said, this should not be necessary, so 1. solved my problem

It was important to delete the build and devel folders, and then the catkin_make worked.

EDIT For more information, in this answer https://answers.ros.org/question/291910/linking-problem-with-catkin_libraries/ someone gave me the clue: "it looks like all of the unresolved symbols involve std::string, which changed between gcc 4.9 and gcc 5". I think that I had a problem because I had g++ and gcc of some versions at the same time and I wasn't using the right version, like I said below. I uninstalled the "bad" version but probably there is another way to fix it.

I did lots of things to fix these errors so I'm not sure which of them did, but I think it was one of these:

1. (I think it was what solved the problem) Uninstalling and reinstalling gcc and g++ to a specific version (in my case I had 4.8, 4.9 and 5.4 installed, and so I uninstalled the two 4.*, now I have only all and reinstalled 5.4.0 for both).
2. Doing this: export CC=/usr/local/bin/gcc export CXX=/usr/local/bin/g++ or run catkin_make like: CXX=g++5 CC=gcc5 catkin_make. Like @gvdhoorn said, this should not be necessary, so 1. solved my problem

It was important to delete the build and devel folders, and then the catkin_make worked.

EDIT For more information, in this answer https://answers.ros.org/question/291910/linking-problem-with-catkin_libraries/ someone gave me the clue: "it looks like all of the unresolved symbols involve std::string, which changed between gcc 4.9 and gcc 5". I think that I had a problem because I had g++ and gcc of some versions at the same time and I wasn't using the right version, like I said below. I uninstalled the "bad" version but probably there is another way to fix it.