There are 2 main obstacles for any program to be portable: hardware and language variants
Hardware: If program is dependent on a hardware, it cannot run on different hardware.
Language variants: If computer program is written in a language variant not recognized by other platform, then such program is not portable. To solve the problem related to different language variants a standard should be created and this happened for C++ in 1998.
Summary: Computer program (code) is portable, if it can be compiled for different platform without any changes. This can happen for C++ programs.