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POIs this a known pitfall of C++11 for loops?
 

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  1. POIs this a known pitfall of C++11 for loops?
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    <p>Let's imagine we have a struct for holding 3 doubles with some member functions:</p> <pre><code>struct Vector { double x, y, z; // ... Vector &amp;negate() { x = -x; y = -y; z = -z; return *this; } Vector &amp;normalize() { double s = 1./sqrt(x*x+y*y+z*z); x *= s; y *= s; z *= s; return *this; } // ... }; </code></pre> <p>This is a little contrived for simplicity, but I'm sure you agree that similar code is out there. The methods allow you to conveniently chain, for example:</p> <pre><code>Vector v = ...; v.normalize().negate(); </code></pre> <p>Or even:</p> <pre><code>Vector v = Vector{1., 2., 3.}.normalize().negate(); </code></pre> <p>Now if we provided begin() and end() functions, we could use our Vector in a new-style for loop, say to loop over the 3 coordinates x, y, and z (you can no doubt construct more "useful" examples by replacing Vector with e.g. String):</p> <pre><code>Vector v = ...; for (double x : v) { ... } </code></pre> <p>We can even do:</p> <pre><code>Vector v = ...; for (double x : v.normalize().negate()) { ... } </code></pre> <p>and also:</p> <pre><code>for (double x : Vector{1., 2., 3.}) { ... } </code></pre> <p>However, the following (it seems to me) is broken:</p> <pre><code>for (double x : Vector{1., 2., 3.}.normalize()) { ... } </code></pre> <p>While it seems like a logical combination of the previous two usages, I think this last usage creates a dangling reference while the previous two are completely fine.</p> <ul> <li>Is this correct and Widely appreciated? </li> <li>Which part of the above is the "bad" part, that should be avoided? </li> <li>Would the language be improved by changing the definition of the range-based for loop such that temporaries constructed in the for-expression exist for the duration of the loop?</li> </ul>
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    1. USndkrempel
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    1. COFor some reason I recall a very similar question being asked before, forgot what it was called though.
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    2. COI consider this a language defect. The life of temporaries is not extended to the entire body of the for-loop, but only for setup of the for loop. It's not just the range syntax which suffers, the classic syntax does as well. In my opinion the life of temporaries in the init statement should extend for the full life of the loop.
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    3. CO@edA-qamort-ora-y: I tend to agree that there is a slight language defect lurking in here, but I think it's specifically the fact that lifetime extension happens implicitly whenever you directly bind a temporary to a reference, but not in any other situation - this seems like a half-baked solution to the underlying problem of temporary lifetimes, although that's not to say it's obvious what a better solution would be. Perhaps an explicit 'lifetime extension' syntax when constructing the temporary, which makes it last until the end of the current block - what do you think?
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