This is a pointer type that refers to a string’s position within a source file.
The following functions provide an interface to extract and manipulate these references.
Returns the file reference in position index from the message. If
index is out of range, returns
Removes the file reference in position index from the message. It moves all references following index one position backwards.
Adds a reference to the string from file starting at
start_line, if it is not already present for the message. The
(size_t)(-1) for start_line denotes that the line
number is not available.