undefined reference to `std::ios_base::Init::Init()’

You can resolve this in several ways:

• Use g++ in stead of gcc: g++ -g -o MatSim MatSim.cpp
• Add -lstdc++: gcc -g -o MatSim MatSim.cpp -lstdc++
• Replace <string.h> by <string>

This is a linker problem, not a compiler issue. The same problem is covered in the question iostream linker error – it explains what is going on.