std::make_shared

Defined in header `<memory>`
template< class T, class... Args > shared_ptr<T> make_shared( Args&&... args );		(since C++11)

Constructs an object of type 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.

Parameters

args - list of arguments with which an instance of T will be constructed.

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, this function has no effect.

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 by T persists until all weak owners get destroyed as well, which may be undesirable if sizeof(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, while std::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::shared_ptr supports array types (as of C++17), but std::make_shared does not. This functionality is supported by boost::make_shared

code such as f(std::shared_ptr<int>(new int(42)), g()) can cause a memory leak if g gets called after new int(42) and throws an exception, while f(std::make_shared<int>(42), g()) is safe, since two function calls are never interleaved.

(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 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

Run this code

#include <iostream>
#include <memory>
 
void foo(const std::shared_ptr<int>& i)
{
    (*i)++;
}
 
int main()
{
    auto sp = std::make_shared<int>(12);
    foo(sp);
    std::cout << *sp << std::endl;
}

Output:

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Date and time
Function objects
initializer_list (C++11)
bitset
hash (C++11)
Relational operators
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
`optional`, `any` and `variant` (C++17)
optional
any
variant
in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t
Pairs and tuples
pair
tuple (C++11)
apply (C++17)
make_from_tuple (C++17)
piecewise_construct_t (C++11)
piecewise_construct (C++11)
integer_sequence (C++14)
Swap, forward and move
swap
exchange (C++14)
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
Elementary string conversions
to_chars (C++17)
from_chars (C++17)
chars_format (C++17)
Type operations
declval (C++11)
as_const (C++17)

Member functions
shared_ptr::shared_ptr
shared_ptr::~shared_ptr
shared_ptr::operator=
Modifiers
shared_ptr::reset
shared_ptr::swap
Observers
shared_ptr::get
shared_ptr::operator*shared_ptr::operator->
shared_ptr::operator[] (C++17)
shared_ptr::use_count
shared_ptr::unique (deprecated)
shared_ptr::operator bool
shared_ptr::owner_before
Non-member functions
std::swap
make_shared
allocate_shared
static_pointer_castdynamic_pointer_castconst_pointer_castreinterpret_pointer_cast (C++17)
get_deleter
operator==operator\|=operator<operator<=operator>operator>=
operator<<
atomic_is_lock_freeatomic_loadatomic_load_explicitatomic_storeatomic_store_explicitatomic_exchangeatomic_exchange_explicitatomic_compare_exchange_weakatomic_compare_exchange_strongatomic_compare_exchange_weak_explicitatomic_compare_exchange_strong_explicit
std::hash