uninitialized data and memcpy

Question

Mike Miller says in 240. Uninitialized values and undefined behavior:

The wording in 3.9.1 [basic.fundamental] was carefully crafted to allow use of unsigned char to access uninitialized data so that memcpy and such could be written in C++ without undefined behavior

What is meant by that? Why would one want to access uninitialized data?

Weird, I don’t think this passage would actually be necessary, one could always use placement-new to construct into uninitialised values. — Konrad Rudolph
– Konrad Rudolph, Commented Nov 2, 2012 at 8:04
@chac when you write into, do you use a pointer to do so? This pointer would then be initialized. — Micha Wiedenmann
– Micha Wiedenmann, Commented Nov 2, 2012 at 12:02

MSalters · Accepted Answer · 2012-11-02 10:29:21Z

1

When you're copying a struct which contains padding, memcpy will copy the padding too. Generally that padding is uninitialized.

answered Nov 2, 2012 at 10:29

MSalters

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imreal · Accepted Answer · 2012-11-02 08:05:26Z

0

It means that it is still possible to access memory not initialized by an object constructor (which is the way intended for C++). We can use unsigned char (or byte) to point to raw memory chunks.

Objects are what you should handle, but you can still see memory the way it is.

edited Nov 2, 2012 at 8:05

answered Nov 2, 2012 at 7:59

imreal

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