我可以通过 C++ 中的（无符号）char* 读取任何可读的有效 memory 位置吗？

Question

My search foo seems lacking today.

I would like to know if it is legal according to std C++ to inspect "any" memory location via an (unsigned(?)) char*. By any location I mean any valid address of an object or array (or inside an array) inside the program.

By way of example:

void passAnyObjectOrArrayOrSomethingElseValid(void* pObj) {
   unsigned char* pMemory = static_cast<unsigned char*>(pObj)
   MyTypeIdentifyier x = tryToFigureOutWhatThisIs(pMemory);
}

Disclaimer : This question is purely academical. I do not intend to put this into production code! By legal I mean if it's really legal according to the standard, that is if it would work on 100% of all implementations. (Not just on x86 or some common hardware.)

Sub-question: Is static_cast the right tool to get from the void* address to the char* pointer?

Answer 1

C++ assumes strict aliasing, which means that two pointers of fundamentally different type do not alias the same value.

However, as correctly pointed out by bdonlan, the standard makes an exception for char and unsigned char pointers.

Thus, in general this is undefined behaviour for any pointer type to read any deliberate address (which might be any type), but for the particular case of unsigned char as in the question it is allowed (ISO 14882:2003 3.10(15)).

static_cast does compile-time type checking, so it is unlikely to always work. In such a case, you will want reinterpret_cast .

Answer 2

Per ISO/IEC 9899:1999 (E) §6.5/7:

7. An object shall have its stored value accessed only by an lvalue expression that has one of the following types:

• a type compatible with the effective type of the object,
• [...]
• a character type

So it is legal (in C) to dereference and examine a (valid) pointer via unsigned char . However, the contents you'll find there are unspecified; tryToFigureOutWhatThisIs has no well-defined way of actually figuring out what it's looking at. I don't have a copy of the C++ spec here, but I suspect it uses the same definition, in order to maintain compatibility.

Answer 3

You can only use a char* , not an unsigned char* . Using an unsigned char* will break strict aliasing rules and invoke undefined behaviour, but there is an exception for char* . However, trying to actually do anything with the memory you read is very highly dubious and very likely to do something undefined. That's why it's rarely done in idiomatic C++ code.