将新对象作为默认参数传递给构造函数

Question

I have a constructor which looks like:

RandomClass(const int16_t var1, const SecondClass& var2);

I need to pass a default argument to the second parameter, so currently I do something like this:

RandomClass(const int16_t var1, const SecondClass& var2 = *(new SecondClass(*(new std::unordered_map<int16_t, double>())));

which is incredibly awkward. Note that I do not want to use an overloaded constructor, or change the second parameter from a reference to a pointer.

What would an elegant way of passing the default parameter be?

Answer 1

You are allowed to bind a const reference to a temporary:

RandomClass(const int16_t var1, const SecondClass& var2 = SecondClass(std::unordered_map<int16_t, double>()));

Of course this assumes that you either copy var2 into a member or do not use it after the constructor exits. If this is not the case, then something is wrong in your design. Perhaps using a raw pointer or a shared/weak_ptr is more appropriate.

Answer 2

The way to do this nicely is to provide two overloads for your constructor:

RandomClass(const int16_t var1, const SecondClass& var2)
{
  // ...
}

RandomClass(const int16_t var1)
{
  const SecondClass var2 = *(new SecondClass(*(new std::unordered_map<int16_t, double>()));
}

However, you have a much worse problem than how it looks. What you have in that second overload is incredibly horrendous because it results in memory leaks - you need to delete everything you new but now you've lost any trace of what you new ed. You would simply do this:

RandomClass(const int16_t var1)
{
  const SecondClass var2{std::unordered_map<int16_t, double>()};
}

Don't use new unless you have to - prefer automatic storage duration.