2.5.7.1 Strings

SWI-Prolog string handling has evolved over time. The functions that create atoms or strings using char* or wchar_t* are "old school"; similarly with functions that get the string as char* or wchar_t*. The PL_get_unify_put_[nw]chars() family is more friendly when it comes to different input, output, encoding and exception handling.

Roughly, the modern API is PL_get_nchars(), PL_unify_chars() and PL_put_chars() on terms. There is only half of the API for atoms as PL_new_atom_mbchars() and PL-atom_mbchars(), which take an encoding, length and char*.

For return values, char* is dangerous because it can point to local or stack memory. For this reason, wherever possible, the C++ API returns a std::string, which contains a copy of the the string. This can be slightly less efficient that returning a char*, but it avoids some subtle and pervasive bugs that even address sanitizers can't detect.^{16If we wish to minimize the overhead of passing strings, this can be done by passing in a pointer to a string rather than returning a string value; but this is more cumbersome and modern compilers can often optimize the code to avoid copying the return value.}