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copied!<p>I'm assuming that you will want to initialize a <em>global</em> variable, since for a local variable it just seems like a very awkward choice to forgo three lines of plain and understandable statements in order to have a constant of questionable value.</p> <p>At the global scope, we can't have errors in the initialization, so we'll have to handle them somehow. Here are some ideas.</p> <p>First, a templated little construction helper:</p> <pre><code>template <typename T> T cinitialize(std::istream & is) noexcept { T x; return (is && is >> x) ? x : T(); } int const X = cinitialize<int>(std::cin); </code></pre> <p>Note that global initializers must not throw exceptions (under pain of <code>std::terminate</code>), and that the input operation may fail. All told, it's probably pretty bad design to initialize global variables from user input in such a fashion. Perhaps a fatal error would be indicated:</p> <pre><code>template <typename T> T cinitialize(std::istream & is) noexcept { T x; if (!(is && is >> x)) { std::cerr << "Fatal error while initializing constants from user input.\n"; std::exit(1); } return x; } </code></pre> <p>Just to clarify my position after some discussion in the comments: In a local scope I would <em>never</em> resort to such an awkward crutch. Since we're processing <em>external, user-supplied</em> data, we basically have to live with failure as part of the normal control flow:</p> <pre><code>void foo() { int x; if (!(std::cin >> x)) { /* deal with it */ } } </code></pre> <p>I leave it up to you to decide whether that's too much to write or too hard too read.</p>

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