#include "test_runner.h"

#include <functional>
#include <memory>
#include <string>
using namespace std;

template <typename T>
class LazyValue {
public:
    explicit LazyValue(std::function<T()> init)
        : init_ { init }
    {
    }

    bool HasValue() const { return bool(Value); }
    const T& Get() const
    {
        if (Value)
            return *Value.get();
        Value = make_unique<T>(init_());
        return *Value.get();
    }

private:
    mutable unique_ptr<T> Value;
    std::function<T()>& init_;
};

void UseExample()
{
    const string big_string = "Giant amounts of memory";

    LazyValue<string> lazy_string([&big_string] { return big_string; });

    ASSERT(!lazy_string.HasValue());
    ASSERT_EQUAL(lazy_string.Get(), big_string);
    ASSERT_EQUAL(lazy_string.Get(), big_string);
}

void TestInitializerIsntCalled()
{
    bool called = false;

    {
        LazyValue<int> lazy_int([&called] {
            called = true;
            return 0;
        });
    }
    ASSERT(!called);
}

int main()
{
    TestRunner tr;
    RUN_TEST(tr, UseExample);
    RUN_TEST(tr, TestInitializerIsntCalled);
    return 0;
}