c++,c++11,shared-ptr,c++-standard-library,make-shared , std::shared_ptr<type>(new DerivedType(…)) != std::make_shared<type>(DerivedType(…))?
std::shared_ptr(new DerivedType(…)) != std::make_shared(DerivedType(…))?
Question:
Tag: c++,c++11,shared-ptr,c++-standard-library,make-shared
I haven't found any issues quite like this yet: but if someone finds one then sorry. I've been trying to use std::shared_ptr to greatly simplify memory management, however I've come across what must be some sort of bug.
When I create a DerivedType pointer with std::make_shared<type>(DerivedType(...)) It can only be addressed as a Type rather than a DerivedType.
Yet when I use the syntax std::shared_ptr<Type>(new DerivedType) the vfptr table lists the correct entries and it can be cast to a DerivedType without a problem.
I believe there should be no difference. Is this a bug in my understanding? or an actual bug?
Thanks for your help. Luke
Answer:
You must pass to std::make_shared the parameters that you'd pass to your type's constructor, and they are forwarded.
You can then convert the resulting pointer to a base pointer implicitly.
std::shared_ptr<Type> p = std::make_shared<DerivedType>(...);
Let's dig a bit in the "why" in your question.
std::shared_ptr<Type>(new DerivedType);
This does work, and does nothing noteworthy. However, std::make_shared is usually preferred, because the latter can allocate std::shared_ptr's bookkeeping data along with your object, and thus is faster and cheaper, as well as exception-safe*.
std::make_shared<Type>(DerivedType(...))
You saw it : it doesn't work. What happens here is that you create a DerivedType instance. std::make_shared happily forwards it to the constructor of Type.
Overload resolution happens, and Type's copy constructor (or its move constructor, if available) is called, with a reference to your DerivedType's base part. And a plain Type object gets instantiated and pointed to. The original DerivedType vanishes. This issue as a whole is called slicing.
* Note on exception safety : Shoud you use a function of the form :
void func(std::shared_ptr<A> a, std::shared_ptr<B> b);
... constructing the std::shared_ptrs like so :
func(std::shared_ptr<A>(new A), std::shared_ptr<B>(new B);
... can leak if, for example, new A is called, then new B, and the latter throws an exception. The A has not yet been wrapped into a smart pointer, and is unrecoverable.
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