Why is this undefined behaviour?

Question

Here\'s the sample code:

X * makeX(int index) { return new X(index); }
struct Tmp {
    mutable int count;
    Tmp() : count(0) {}
    const X ** getX() const {          


        

Answer 1

In C++11, this is fine; each clause of an initialiser list is sequenced before the next one, so the evaluation is well-defined.

Historically, the clauses might have been unsequenced, so the two unsequenced modifications of count would give undefined behaviour.

_{(Although, as noted in the comments, it might have been well-defined even then - you can probably interpret the standard as implying that each clause is a full-expression, and there's a seqeuence point at the end of each full-expression. I'll leave it to historians to debate the finer points of obsolete languages.)}