49 lines
942 B
C++
49 lines
942 B
C++
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#include <exception>
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#include <functional>
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#include <mutex>
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namespace nix {
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/* A helper class for lazily-initialized variables.
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Lazy<T> var([]() { return value; });
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declares a variable of type T that is initialized to 'value' (in a
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thread-safe way) on first use, that is, when var() is first
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called. If the initialiser code throws an exception, then all
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subsequent calls to var() will rethrow that exception. */
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template<typename T>
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class Lazy
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{
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typedef std::function<T()> Init;
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Init init;
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std::once_flag done;
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T value;
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std::exception_ptr ex;
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public:
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Lazy(Init init) : init(init)
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{ }
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const T & operator () ()
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{
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std::call_once(done, [&]() {
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try {
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value = init();
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} catch (...) {
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ex = std::current_exception();
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}
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});
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if (ex) std::rethrow_exception(ex);
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return value;
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}
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};
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}
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