Creates and initializes objects with dynamicstorage duration, that is, objects whose lifetime is not necessarily limited by the scope in which they were created.

[edit]Syntax

[edit]Explanation

Thenew expression attempts to allocate storage and then attempts to construct and initialize either a single unnamed object, or an unnamed array of objects in the allocated storage. Thenew expression returns a prvalue pointer to the constructed object or, if an array of objects was constructed, a pointer to the initial element of the array.

Syntax(1) or(3) is required iftype includes parentheses:

newint(*[10])();// error: parsed as (new int) (*[10]) ()new(int(*[10])());// okay: allocates an array of 10 pointers to functions

In addition,type is parsed greedily: it will be taken include every token that can be a part of a declarator:

newint+1;// okay: parsed as (new int) + 1, increments a pointer returned by new intnewint*1;// error: parsed as (new int*) (1)

Thenew-initializer is not optional if

• type is anarray of unknown bound,

double* p= newdouble[]{1,2,3};// creates an array of type double[3]auto p= newauto('c');// creates a single object of type char. p is a char* auto q= newstd::integralauto(1);// OK: q is an int*auto q= newstd::floating_pointauto(true)// ERROR: type constraint not satisfied auto r= newstd::pair(1,true);// OK: r is a std::pair<int, bool>*auto r= newstd::vector;// ERROR: element type can't be deduced

[edit]Dynamic arrays

Iftype is an array type, all dimensions other than the first must be specified as positiveintegral constant expression(until C++14)converted constant expression of typestd::size_t(since C++14), but (only when using un-parenthesized syntaxes(2) and(4)) the first dimension may bean expression of integral type, enumeration type, or class type with a single non-explicit conversion function to integral or enumeration type(until C++14)any expression convertible tostd::size_t(since C++14). This is the only way to directly create an array with size defined at runtime, such arrays are often referred to asdynamic arrays:

int n=42;double a[n][5];// errorauto p1= newdouble[n][5];// OKauto p2= newdouble[5][n];// error: only the first dimension may be non-constantauto p3= new(double[n][5]);// error: syntax (1) cannot be used for dynamic arrays

The first dimension of zero is acceptable, and the allocation function is called.

[edit]Allocation

Thenew expression allocates storage by calling the appropriateallocation function. Iftype is a non-array type, the name of the function isoperator new. Iftype is an array type, the name of the function isoperator new[].

As described inallocation function, the C++ program may provide global and class-specific replacements for these functions. If thenew expression begins with the optional:: operator, as in::new T or::new T[n], class-specific replacements will be ignored (the function islooked up in globalscope). Otherwise, ifT is a class type, lookup begins in the class scope ofT.

When calling the allocation function, thenew expression passes the number of bytes requested as the first argument, of typestd::size_t, which is exactlysizeof(T) for non-arrayT.

Array allocation may supply unspecified overhead, which may vary from one call tonew to the next, unless the allocation function selected is the standard non-allocating form. The pointer returned by thenew expression will be offset by that value from the pointer returned by the allocation function. Many implementations use the array overhead to store the number of objects in the array which is used by thedelete[] expression to call the correct number of destructors. In addition, if thenew expression is used to allocate an array ofchar,unsignedchar, orstd::byte(since C++17), it may request additional memory from the allocation function if necessary to guarantee correct alignment of objects of all types no larger than the requested array size, if one is later placed into the allocated array.

[edit]Placementnew

Ifplacement-args are provided, they are passed to the allocation function as additional arguments. Such allocation functions are known as "placementnew", after the standard allocation functionvoid*operator new(std::size_t,void*), which simply returns its second argument unchanged. This is used to construct objects in allocated storage:

// within any block scope...{// Statically allocate the storage with automatic storage duration// which is large enough for any object of type “T”.    alignas(T)unsignedchar buf[sizeof(T)];     T* tptr= new(buf) T;// Construct a “T” object, placing it directly into your// pre-allocated storage at memory address “buf”.     tptr->~T();// You must **manually** call the object's destructor// if its side effects is depended by the program.}// Leaving this block scope automatically deallocates “buf”.

Note: this functionality is encapsulated by the member functions of theAllocator classes.

new T;// calls operator new(sizeof(T))// (C++17) or operator new(sizeof(T), std::align_val_t(alignof(T))))new T[5];// calls operator new[](sizeof(T)*5 + overhead)// (C++17) or operator new(sizeof(T)*5+overhead, std::align_val_t(alignof(T))))new(2,f) T;// calls operator new(sizeof(T), 2, f)// (C++17) or operator new(sizeof(T), std::align_val_t(alignof(T)), 2, f)

If a non-throwing allocation function (e.g. the one selected bynew(std::nothrow) T) returns a null pointer because of an allocation failure, then thenew expression returns immediately, it does not attempt to initialize an object or to call a deallocation function. If a null pointer is passed as the argument to a non-allocating placementnew expression, which makes the selected standard non-allocating placement allocation function return a null pointer, the behavior is undefined.

[edit]Initialization

The object created by anew expression is initialized according to the following rules.

Iftype is not an array type, the single object is constructed in the acquired memory area:

• Ifnew-initializer is absent, the object isdefault-initialized.
• Ifnew-initializer is a parentheses-enclosed expression list, the object isdirect-initialized.

Iftype is an array type, an array of objects is initialized:

• Ifnew-initializer is absent, each element isdefault-initialized.

• Even if the first dimension is zero, the semantic constraints of default-initializing a hypothetical element still need to be met.

• Ifnew-initializer is a pair of parentheses, each element isvalue-initialized.

• Even if the first dimension is zero, the semantic constraints of value-initializing a hypothetical element still need to be met.

[edit]Initialization failure

If initialization terminates by throwing an exception (e.g. from the constructor), the program looks up a matching deallocation function, then:

• If a suitable deallocation function can be found, the deallocation function is called to free the memory in which the object was being constructed. After that, the exception continues to propagate in the context of thenew expression.
• If no unambiguous matching deallocation function can be found, propagating the exception does not cause the object’s memory to be freed. It is only appropriate if the called allocation function does not allocate memory, otherwise it is likely to result in a memory leak.

The scope of thelookup of the matching deallocation function is determined as follows:

• If thenew expression does not begin with::, and the allocated type is either a class typeT or an array of class typeT, a search is performed for the deallocation function’s name in the class scope ofT.
• Otherwise, or if nothing is found in theT's class scope, the deallocation function’s name is looked up by searching for it in theglobal scope.

For a non-placement allocation function, the normal deallocation function lookup is used to find the matching deallocation function (seedelete-expression).

For a placement allocation function, the matching deallocation function must have the same number of parameter, and each parameter type except the first is identical to the corresponding parameter type of the allocation function (afterparameter transformations).

• If the lookup finds a single matching deallocation function, that function will be called; otherwise, no deallocation function will be called.
• If the lookup finds a non-placement deallocation function and that function, considered as a placement deallocation function, would have been selected as a match for the allocation function, the program is ill-formed.

In any case, the matching deallocation function (if any) must be non-deleted and(since C++11) accessible from the point where thenew expression appears.

struct S{// Placement allocation function:staticvoid*operator new(std::size_t,std::size_t); // Non-placement deallocation function:staticvoidoperator delete(void*,std::size_t);}; S* p= new(0) S;// error: non-placement deallocation function matches//        placement allocation function

If a deallocation function is called in anew expression (due to initialization failure), the arguments passed to that function are determined as follows:

• The first argument is the value (of typevoid*) returned from the allocation function call.
• Other arguments (only for placement deallocation functions) are theplacement-args passed to the placement allocation function.

If the implementation is allowed to introduce a temporary object or make a copy of any argument as part of the call to the allocation function, it is unspecified whether the same object is used in the call to both the allocation and deallocation functions.

[edit]Memory leaks

The objects created bynew expressions (objects with dynamic storage duration) persist until the pointer returned by thenew expression is used in a matchingdelete-expression. If the original value of pointer is lost, the object becomes unreachable and cannot be deallocated: amemory leak occurs.

This may happen if the pointer is assigned to:

int* p= newint(7);// dynamically allocated int with value 7p= nullptr;// memory leak

or if the pointer goes out of scope:

void f(){int* p= newint(7);}// memory leak

or due to exception:

void f(){int* p= newint(7);    g();// may throw    delete p;// okay if no exception}// memory leak if g() throws

To simplify management of dynamically-allocated objects, the result of anew expression is often stored in asmart pointer:std::auto_ptr(until C++17)std::unique_ptr, orstd::shared_ptr(since C++11). These pointers guarantee that the delete expression is executed in the situations shown above.

[edit]Notes

Itanium C++ ABI requires that the array allocation overhead is zero if the element type of the created array is trivially destructible. So does MSVC.

Some implementations (e.g. MSVC before VS 2019 v16.7) require non-zero array allocation overhead on non-allocating placement arraynew if the element type is not trivially destructible, which is no longer conforming sinceCWG issue 2382.

A non-allocating placement arraynew expression that creates an array ofunsignedchar, orstd::byte(since C++17) can be used toimplicitly create objects on given region of storage: it ends lifetime of objects overlapping with the array, and then implicitly creates objects of implicit-lifetime types in the array.

std::vector offers similar functionality for one-dimensional dynamic arrays.

[edit]Keywords

new

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

• constructor
• copy elision
• default constructor
• delete
• destructor
• initialization
  • aggregate initialization
  • default initialization
  • direct initialization
  • list initialization
  • value initialization

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