How to compile a c++ program in Linux?

Use g++

g++ -o hi hi.cpp

g++ is for C++, gcc is for C although with the -libstdc++ you can compile c++ most people don't do this.

You have to use g++ (as mentioned in other answers). On top of that you can think of providing some good options available at command line (which helps you avoid making ill formed code):

g++   -O4    -Wall hi.cpp -o hi.out
     ^^^^^   ^^^^^^
  optimize  related to coding mistakes

For more detail you can refer to man g++ | less.

As the other answers say, use g++ instead of gcc.

Or use make: make hi