[llvm-commits] [llvm] r149918 - in /llvm/trunk: include/llvm/Bitcode/ include/llvm/MC/ include/llvm/Support/ lib/Bitcode/Reader/ lib/Bitcode/Writer/ lib/MC/MCDisassembler/ lib/Support/ lib/Target/ARM/Disassembler/ lib/Target/MBlaze/Disassembler/

[Derek Schuff]

The problem is that fetching more data can cause the underlying structure
(e.g. the storage pointer in the example) to be reallocated/resized.
MutableArrayRef type classes are basically just glorified pointers and
designed not to own the data; but the data has to be owned by something
accessible to MemoryObject, and MemoryObject::readBytes would have to call
a non-const method on whatever does own it. The behavior you're describing
is like a const pointer (const char* foo) but the 'const' qualifier on a
method basically means 'const char* const foo'
We could of course keep const on the methods and do our game-playing by
making all the data members of the StreamableMemoryObject mutable, but
that's worse (IMO) than just having another class that's like MemoryObject
but doesn't derive directly from it.

On Tue, Feb 7, 2012 at 12:30 PM, Jim Grosbach <grosbach at apple.com> wrote:

>
> Why can't a const StreamableMemoryObject read in more information to
> satisfy a request? As I understand it, 'const' there is saying that the
> data won't be modified and not necessarily anything about what actions need
> to be taken to read it.
>
> From an implementation detail point of view, consider the following
> contrived example:
>
> $ cat foo.cpp
> #include <iostream>
>
> #include <vector>
>
> class myClass {
>
>   unsigned *storage;
>
> public:
>
>   myClass(unsigned *buffer) { storage = buffer; }
>
>
>
>   void write(unsigned val) const { *storage = val; }
>
>   unsigned read() const { return *storage; }
>
>   void set(unsigned *buffer) { storage = buffer; }
>
> };
>
>
>
> void foo(const myClass *p) {
>
>   p->write(5);
>
>   std::cout << p->read() << std::endl;
>
>   p->write(4);
>
>   std::cout << p->read() << std::endl;
>
>   // Can't do this due to 'const'
>
>   // error: member function 'set' not viable: 'this' argument has type
> 'const myClass', but function is not marked const
>   //unsigned newbuf;
>
>   //p->set(&newbuf);
>
> }
>
>
>
> int main(int argc, char *argv[]) {
>
>   unsigned val;
>
>   myClass X(&val);
>
>   foo(&X);
>
> }
> $ clang++ foo.cpp -o foo
> $ ./foo
> 5
> 4
>
>
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