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POHow to check if a range specified by iterators is valid?
 

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  1. POHow to check if a range specified by iterators is valid?
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    <p>Many of the standard library containers have operations that accept a range specified by iterators. For example, <code>std::list::assign(InputIterator first, InputIterator last)</code>.</p> <p>But because many of these operations do not throw exceptions, if the range specified by <code>[first,last)</code> is not valid, it will cause undefined behaviour.</p> <p>So what <em>is</em> a valid range? I assume it means that <code>first</code> comes before or is equal to <code>last</code> in the container, because when I tried otherwise (i.e. <code>first</code> comes after <code>last</code>), the program's execution would hang.</p> <p>For example:</p> <pre><code>std::list.assign(container.begin(), container.end()); // valid range std::list.assign(container.end(), container.begin()); // invalid range </code></pre> <p>Now with an assumption of what a valid range is, how can we check if a range is valid?</p> <p>I came up with a helper function that checks if <code>first</code> is ever equal to <code>last</code> before it reaches the end of the container. If true, it's a strong guarantee that <code>first</code> does not come after <code>last</code> and therefore the range is valid.</p> <pre><code>template &lt;typename Iterator&gt; bool isValidRange(Iterator first, Iterator last, Iterator end) { for (; first != end; ++first) if (first == last) return true; return false; } </code></pre> <p>It's not very convenient because you have to pass it an iterator to the end of the container in addition to the iterators that specify the range.</p> <p>Is this correct? Is there a better way to do this?</p>
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    1. USDennis
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    1. COTo check whether a range is valid, increment the first iterator until it is equal to the second; then you know it's valid. If it never equals the second, then the range is not valid.
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      1. USPete Becker
    2. COOn a more serious note: don't equate ranges and containers. Containers are one way of creating ranges, but they are not the only way. For example, an iterator into an input stream does not come from a container.
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      1. USPete Becker
    3. CO@PeteBecker In that case, why does something like `list_a.assign(list_b.end(), list_b.begin());` hang the program? `std::list::end` and `std::list::begin` return a bidirectional iterator, so the first iterator will eventually reach the second, meaning it should be valid according to your description.
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      1. POHow to check if a range specified by iterators is valid?
      1. USDennis
 

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