LazyValue

struct Counter {
        static size_t constructed;
        Counter() { ++constructed; }
        Counter(const Counter&) { ++constructed; }
        Counter(Counter&&) = default;
        Counter& operator=(const Counter&) { ++constructed; return *this; }
        Counter& operator=(Counter&&) = default;
        ~Counter() = default;
        size_t value = 0;
    };
    size_t Counter::constructed = 0;

void Test(){
Counter::constructed = 0;
LazyValue<Counter> val([]() { return Counter(); });
assert(Counter::constructed == 0u);
assert(!val.HasValue());
 val.Get();
assert(Counter::constructed == 1u);
assert(val.HasValue());
Counter counter = val.Get();
assert(Counter::constructed == 2u);
essert(counter.value == 0);
}