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Comment by tspiteri | Hacker News Reader

That says used, not read. And indeterminate, not unspecified. My point was on reading a value, not on acting on the value. I know it looks like nit-picking, and you should just not read uninitialized memory, but I don't think it's consistent with the standard to say that reading uninitialized memory is always UB.

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What kind of read would not constitute "using" a value? And per your previous post an object that was not initialised is indeterminate, not just unspecified. So yes, reading uninitialized memory is always UB.

A read into a character type.

From 6.2.6.1¶5:

>Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. ... Such a representation is called a trap representation.

A read from uninitialized memory is not always UB.

What you quoted doesn't actually say anything about what happens when a trap representation is read into a character type. Such a read is still "using" the value at least in the everyday sense of the word, so in the absence of something explicitly to the contrary, as far as I can see the part of the standard that states that using an uninitialised value is UB stil applies.

Per http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_451.htm , the current standard is unclear in some respects, but the latest committee view is that that under the current standard any library function (including memcpy) may exhibit undefined behaviour when called with uninitialized memory, even when the uninitialized memory is of character type or is propagated through values of character type.

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