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