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    <p>This is basically a question of how to handle <a href="http://en.wikipedia.org/wiki/Partial_function" rel="nofollow">non-total functions</a>—functions that are not necessarily defined for all possible inputs. There are many ways to go about it, though none of them is type-safe:</p> <ol> <li><p>Have two separate functions for retrieving and erasing the top element of the stack, much like <a href="http://en.cppreference.com/w/cpp/container/stack/top" rel="nofollow"><code>std::stack::top()</code></a> and <a href="http://en.cppreference.com/w/cpp/container/stack/pop" rel="nofollow"><code>std::stack::pop()</code></a>.</p></li> <li><p>Use a reference parameter and return a <code>bool</code> indicating whether an element was returned:</p> <pre><code>bool pop(T&amp; value) { if (stack.empty()) return false; value = stack.back(); stack.pop_back(); return true; } </code></pre> <p>The caller can write:</p> <pre><code>T value; if (stack.pop(value)) { use(value); } </code></pre></li> <li><p>Return a <a href="http://www.boost.org/doc/libs/1_53_0/libs/optional/doc/html/boost_optional/examples.html#boost_optional.examples.optional_return_values" rel="nofollow"><code>boost::optional&lt;T&gt;</code></a>.</p></li> <li><p>Return a default-constructed <code>T</code> instance. This requires that <code>T</code> be default-constructible, which is already imposed by some <a href="http://en.cppreference.com/w/cpp/container/vector" rel="nofollow"><code>std::vector</code></a> member functions.</p></li> <li><p>Throw an exception, documenting that a precondition was violated.</p></li> <li><p>Leave the behaviour of popping an empty stack undefined.</p></li> </ol> <p>I recommend #1, #2, or #3, depending on preference and what fits best with the surrounding code.</p>
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