source: doc/proposals/void_ptr.md @ fcf3493

## Options for safer void* handling #
C allows implicit conversions between `void*` and other pointer types, as per 
section 6.3.2.3.1 of the standard. 
Making these implicit conversions explicit in Cforall would provide 
significant type-safety benefits, and is precedented in C++. 
A weaker version of this proposal would be to allow implicit conversions to 
`void*` (as a sort of "top type" for all pointer types), but to make the 
unsafe conversion from `void*` back to a concrete pointer type an explicit 
conversion. 
However, `int *p = malloc( sizeof(int) );` and friends are hugely common 
in C code, and rely on the unsafe implicit conversion from the `void*` return 
type of `malloc` to the `int*` type of the variable - obviously it would be 
too much of a source-compatibility break to disallow this for C code. 
We do already need to wrap C code in an `extern "C"` block, though, so it is 
technically feasible to make the `void*` conversions implicit in C but 
explicit in Cforall. 

As a possible mitigation for calling C code with `void*`-based APIs, pointers-to-dtype are 
calling-convention compatible with `void*`; we could read `void*` in function 
signatures as essentially a fresh dtype type variable, e.g:

        void* malloc( size_t )
                => forall(dtype T0) T0* malloc( size_t )
        void qsort( void*, size_t, size_t, int (*)( const void*, const void* ) )
                => forall(dtype T0, dtype T1, dtype T2)
                   void qsort( T0*, size_t, size_t, int (*)( const T1*, const T2* ) )

In this case, there would be no conversion needed to call `malloc`, just the 
polymorphic type binding.
This should handle many of the uses of `void*` in C.

This feature would even allow us to leverage some of Cforall's type safety to write 
better declarations for legacy C API functions, like the following wrapper for 
`qsort`:

        extern "C" { // turns off name-mangling so that this calls the C library
                // call-compatible type-safe qsort signature
                forall(dtype T)
                void qsort( T*, size_t, size_t, int (*)( const T*, const T* ) );
                
                // forbid type-unsafe C signature from resolving
                void qsort( void*, size_t, size_t, int (*)( const void*, const void* ) )
                        = delete;
        }