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[flang] Update FortranForCProgrammers.md 

Refine the text describing differences between C and Fortran pointers to correct an erroneous statement (that pointers cannot point to allocatables).  Fixes bug flang-compiler/f18#461.

Original-commit: flang-compiler/f18@eaa3c10aa8
Reviewed-on: https://github.com/flang-compiler/f18/pull/464
This commit is contained in:
Peter Klausler 2019-05-15 13:37:10 -07:00 committed by GitHub
parent 74f5d6e308
commit 849597f4f9
1 changed files with 11 additions and 2 deletions
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flang/documentation/FortranForCProgrammers.md
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@ -260,14 +260,23 @@ little like C++17's `std::visit()` on a discriminated union.
Pointers
--------
Pointers are objects in Fortran, not data types.
Pointers can point to data, arrays, and subprograms, but not to other pointers
or to an allocatable.
Pointers can point to data, arrays, and subprograms.
A pointer can only point to data that has the `TARGET` attribute.
Outside of the pointer assignment statement (`P=>X`) and some intrinsic
functions and cases with pointer dummy arguments, pointers are implicitly
dereferenced, and the use of their name is a reference to the data to which
they point instead.
Unlike C, a pointer cannot point to a pointer *per se*, nor can they be
used to implement a level of indirection to the management structure of
an allocatable.
If you assign to a Fortran pointer to make it point at another pointer,
you are making the pointer point to the data (if any) to which the other
pointer points.
Similarly, if you assign to a Fortran pointer to make it point to an allocatable,
you are making the pointer point to the current content of the allocatable,
not to the metadata that manages the allocatable.
Unlike allocatables, pointers do not deallocate their data when they go
out of scope.