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While I have std::tr1::shared_ptr<T> available in my compiler, I don't
have make_shared.
Can someone point me to a proper implementation of make_shared? I see
that I need to use varargs to provide arguments to constructor of T.
But I don't have variadic templates available in my compiler as well.
316
Description. It constructs an object of type T passing args to its constructor, and returns an object of type shared_ptr that owns and stores a pointer to it.
The difference is that std::make_shared performs one heap-allocation, whereas calling the std::shared_ptr constructor performs two.
Whenever possible, use the make_shared function to create a shared_ptr when the memory resource is created for the first time. make_shared is exception-safe. It uses the same call to allocate the memory for the control block and the resource, which reduces the construction overhead.
One reason is because make_shared allocates the reference count together with the object to be managed in the same block of memory.
If your compiler don't give an implementation of make_shared and you can't use boost, and you don't mind the lack of single-allocation optimization both for the object and the reference counter then make_shared is something like this:
Without variadic template support:
// zero arguments version
template <typename T>
inline shared_ptr<T> make_shared()
{
return shared_ptr<T>(new T());
}
// one argument version
template <typename T, typename Arg1>
inline shared_ptr<T> make_shared(Arg1&& arg1)
{
return shared_ptr<T>(new T(std::forward<Arg1>(arg1)));
}
// two arguments version
template <typename T, typename Arg1, typename Arg2>
inline shared_ptr<T> make_shared(Arg1&& arg1, Arg2&& arg2)
{
return shared_ptr<T>(new T(std::forward<Arg1>(arg1),
std::forward<Arg2>(arg2)));
}
// ...
If your compiler don't support r-value references, then make 2 versions for each arguments count: one const Arg& and one Arg&
With variadic template support:
template <typename T, typename... Args>
inline shared_ptr<T> make_shared(Args&&... args)
{
return shared_ptr<T>(new T( std::forward<Args>(args)... ));
}
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