Allow constructing RMatrix from a non-const buffer

[galexv]

No description provided.

[kevinushey]

Thanks!

[jwood000]

I had the same issue, however my fix was to implement a similar pattern as the more common constructor:

RcppParallel/inst/include/RcppParallel/RMatrix.h

Lines 199 to 205 in cb514e8

	template <typename Source>
	inline explicit RMatrix(const Source& source)
	: data_(const_cast<Source&>(source).begin()),
	nrow_(source.nrow()),
	ncol_(source.ncol())
	{
	}

Thus, I implemented the following:

   inline RMatrix(const T* data, std::size_t nrow, std::size_t ncol) 
      : data_(const_cast<T*>(data)), nrow_(nrow), ncol_(ncol) 
   {
   }

I had experimented with the current fix (removing the const altogether), however I ended up using the fix above.

I'm not sure which is better, but my gut feeling is that keeping const T* with const_cast is better.

[galexv]

This is a possibility, but my gut feeling is just the opposite: i do not like const_casts 🙂, and for a reason. IMHO, basically, the constructor above lies to me promising to create an object that cannot change my data (via the provided pointer). For example, i could have passed a pointer to an array in some kind of read-only memory, and the compiler would not say a word, resulting in undefined behavior (UB) (see, e.g., https://stackoverflow.com/q/4042626).

Case in point: a simplified code at https://godbolt.org/z/WP5qes uses const_cast<int*> from a const-data pointer at line 30, then tries to assign to the pointed-to location (line 31), which results in a crash when compiled without optimization ("-O0"). Without the forced const_cast the corresponding piece of code (lines 17-18) produces a compilation error (as it should happen when a user tries to do something grossly incorrect).

A side note: When the program is compiled with "-O1", the compiler deduces the result without actually running any memory access code, ignoring the incorrect assignment, so we get different results at different optimization levels, as is common when a UB is invoked.

[Sorry for double posting, accidentally posted it from a temporary account 😔 ]