std::make_shared, std::make_shared_default_init
Defined in header <memory>
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template< class T, class... Args > shared_ptr<T> make_shared( Args&&... args ); |
(1) | (since C++11) (T is not array) |
template<class T> shared_ptr<T> make_shared( std::size_t N ); |
(2) | (since C++20) (T is U[]) |
template<class T> shared_ptr<T> make_shared(); |
(3) | (since C++20) (T is U[N]) |
template<class T> shared_ptr<T> make_shared( std::size_t N, const std::remove_extent_t<T>& u ); |
(4) | (since C++20) (T is U[]) |
template<class T> shared_ptr<T> make_shared( const std::remove_extent_t<T>& u ); |
(5) | (since C++20) (T is U[N]) |
template<class T> shared_ptr<T> make_shared_default_init(); |
(6) | (since C++20) (T is not U[]) |
template<class T> shared_ptr<T> make_shared_default_init( std::size_t N ); |
(7) | (since C++20) (T is U[]) |
T
and wraps it in a std::shared_ptr using args
as the parameter list for the constructor of T
. The object is constructed as if by the expression ::new (pv) T(std::forward<Args>(args)...), where pv
is an internal void*
pointer to storage suitable to hold an object of type T
. The storage is typically larger than sizeof(T)
in order to use one allocation for both the control block of the shared pointer and the T
object. The std::shared_ptr
constructor called by this function enables shared_from_this
with a pointer to the newly constructed object of type T
. This overload only participates in overload resolution if T is not an array typestd::remove_all_extents_t<T>
are value-initialized as if by placement-new expression ::new(pv) std::remove_all_extents_t<T>(). The overload (2) creates an array of size N
along the first dimension. The array elements are initialized in ascending order of their addresses, and when their lifetime ends are destroyed in the reverse order of their original construction.u
. If U
is not an array type, then this is performed as if by the same placement-new expression as in (1); otherwise, this is performed as if by initializing every non-array element of the (possibly multidimensional) array with the corresponding element from u
with the same placement-new expression as in (1). The overload (4) creates an array of size N
along the first dimension. The array elements are initialized in ascending order of their addresses, and when their lifetime ends are destroyed in the reverse order of their original construction.T
is not an array type and (3) if T
is U[N]
, except that the created object is default-initialized.In each case, the object (or individual elements if T
is an array type) (since C++20) will be destroyed by p->~X(), where p
is a pointer to the object and X
is its type.
Parameters
args | - | list of arguments with which an instance of T will be constructed.
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N | - | array size to use |
u | - | the initial value to initialize every element of the array |
Return value
std::shared_ptr of an instance of type T
.
Exceptions
May throw std::bad_alloc or any exception thrown by the constructor of T
. If an exception is thrown, the functions have no effect. If an exception is thrown during the construction of the array, already-initialized elements are destroyed in reverse order. (since C++20)
Notes
This function may be used as an alternative to std::shared_ptr<T>(new T(args...)). The trade-offs are:
- std::shared_ptr<T>(new T(args...)) performs at least two allocations (one for the object
T
and one for the control block of the shared pointer), while std::make_shared<T> typically performs only one allocation (the standard recommends, but does not require this, all known implementations do this) - If any std::weak_ptr references the control block created by
std::make_shared
after the lifetime of all shared owners ended, the memory occupied byT
persists until all weak owners get destroyed as well, which may be undesirable ifsizeof(T)
is large. - std::shared_ptr<T>(new T(args...)) may call a non-public constructor of
T
if executed in context where it is accessible, whilestd::make_shared
requires public access to the selected constructor. - Unlike the std::shared_ptr constructors,
std::make_shared
does not allow a custom deleter. -
std::make_shared
uses ::new, so if any special behavior has been set up using a class-specific operator new, it will differ from std::shared_ptr<T>(new T(args...)).
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(until C++20) |
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(until C++17) |
A constructor enables shared_from_this
with a pointer ptr
of type U*
means that it determines if U
has an unambiguous and accessible (since C++17) base class that is a specialization of std::enable_shared_from_this, and if so, the constructor evaluates the statement:
if (ptr != nullptr && ptr->weak_this.expired()) ptr->weak_this = std::shared_ptr<std::remove_cv_t<U>>(*this, const_cast<std::remove_cv_t<U>*>(ptr));
Where weak_this
is the hidden mutable std::weak_ptr
member of std::shared_from_this. The assignment to the weak_this
member is not atomic and conflicts with any potentially concurrent access to the same object. This ensures that future calls to shared_from_this() would share ownership with the shared_ptr
created by this raw pointer constructor.
The test ptr->weak_this.expired()
in the exposition code above makes sure that weak_this
is not reassigned if it already indicates an owner. This test is required as of C++17.
Example
#include <iostream> #include <memory> #include <type_traits> struct C { C(int i) : i(i) {} //< constructor needed (until C++20) int i; }; int main() { auto sp = std::make_shared<C>(12); static_assert(std::is_same_v<decltype(sp), std::shared_ptr<C>>); std::cout << sp->i << '\n'; }
Output:
12
See also
constructs new shared_ptr (public member function) | |
creates a shared pointer that manages a new object allocated using an allocator (function template) | |
(C++14)(C++20) |
creates a unique pointer that manages a new object (function template) |
allocation functions (function) |