new expression
Creates and initializes objects with dynamic storage duration, that is, objects whose lifetime is not necessarily limited by the scope in which they were created.
Syntax
::(optional) new ( type ) initializer(optional)
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(1) | ||||||||
::(optional) new new-type initializer(optional)
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(2) | ||||||||
::(optional) new (placement-params) ( type ) initializer(optional)
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(3) | ||||||||
::(optional) new (placement-params) new-type initializer(optional)
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(4) | ||||||||
new int(*[10])(); // error: parsed as (new int) (*[10]) () new (int (*[10])()); // okay: allocates an array of 10 pointers to functions
- In addition, new-type is greedy: it will include every token that can be a part of a declarator:
new int + 1; // okay: parsed as (new int) + 1, increments a pointer returned by new int new int * 1; // error: parsed as (new int*) (1)
The initializer is not optional if
- type or new-type is an array of unknown bound
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(since C++11) |
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(since C++17) |
double* p = new double[]{1,2,3}; // creates an array of type double[3] auto p = new auto('c'); // creates a single object of type char. p is a char* auto q = new std::integral auto(1); // OK: q is a int* auto q = new std::floating_point auto(true) // ERROR: type constraint not satisfied auto r = new std::pair(1, true); // OK: r is a std::pair<int, bool>* auto r = new std::vector; // ERROR: element type can't be deduced
Explanation
The new 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. The new-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.
If type is an array type, all dimensions other than the first must be specified as positive integral constant expression (until C++14)converted constant expression of type std::size_t (since C++14), but (only when using un-parenthesized syntaxes (2) and (4)) the first dimension may be an 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 to std::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 as dynamic arrays:
int n = 42; double a[n][5]; // error auto p1 = new double[n][5]; // OK auto p2 = new double[5][n]; // error: only the first dimension may be non-constant auto p3 = new (double[n][5]); // error: syntax (1) cannot be used for dynamic arrays
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The behavior is undefined if the value in the first dimension (converted to integral or enumeration type if needed) is negative. |
(until C++11) |
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In the following cases the expression specifying the first dimension is erroneous:
If the value in the first dimension is erroneous for any of these reasons,
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(since C++11) |
The first dimension of zero is acceptable, and the allocation function is called.
Note: std::vector offers similar functionality for one-dimensional dynamic arrays.
Allocation
The new-expression allocates storage by calling the appropriate allocation function. If type is a non-array type, the name of the function is operator new. If type is an array type, the name of the function is operator new[].
As described in allocation function, the C++ program may provide global and class-specific replacements for these functions. If the new-expression begins with the optional :: operator, as in ::new T or ::new T[n], class-specific replacements will be ignored (the function is looked up in global scope). Otherwise, if T is a class type, lookup begins in the class scope of T.
When calling the allocation function, the new-expression passes the number of bytes requested as the first argument, of type std::size_t, which is exactly sizeof(T) for non-array T.
Array allocation may supply unspecified overhead, which may vary from one call to new to the next, unless the allocation function selected is the standard non-allocating form. The pointer returned by the new-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 the delete[] expression to call the correct number of destructors. In addition, if the new-expression is used to allocate an array of char, unsigned char, or std::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.
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New-expressions are allowed to elide or combine allocations made through replaceable allocation functions. In case of elision, the storage may be provided by the compiler without making the call to an allocation function (this also permits optimizing out unused new-expression). In case of combining, the allocation made by a new-expression E1 may be extended to provide additional storage for another new-expression E2 if all of the following is true: 1) The lifetime of the object allocated by E1 strictly contains the lifetime of the object allocated by E2,
2) E1 and E2 would invoke the same replaceable global allocation function
3) For a throwing allocation function, exceptions in E1 and E2 would be first caught in the same handler.
Note that this optimization is only permitted when new-expressions are used, not any other methods to call a replaceable allocation function: delete[] new int[10]; can be optimized out, but operator delete(operator new(10)); cannot. |
(since C++14) |
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During an evaluation of a constant expression, a call to an allocation function is always omitted. Only new-expressions that would otherwise result in a call to a replaceable global allocation function can be evaluated in constant expressions. |
(since C++20) |
Placement new
If placement-params are provided, they are passed to the allocation function as additional arguments. Such allocation functions are known as "placement new", after the standard allocation function void* 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) unsigned char 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 the Allocator classes.
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When allocating an object whose alignment requirement exceeds |
(since C++17) |
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 by new(std::nothrow) T) returns a null pointer because of an allocation failure, then the new-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 placement new-expression, which makes the selected standard non-allocating placement allocation function return a null pointer, the behavior is undefined.
Construction
The object created by a new-expression is initialized according to the following rules:
- For non-array
type, the single object is constructed in the acquired memory area.
- If initializer is absent, the object is default-initialized.
- If initializer is a parenthesized list of arguments, the object is direct-initialized.
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(since C++11) |
- If type or new-type is an array type, an array of objects is initialized.
- If initializer is absent, each element is default-initialized
- If initializer is an empty pair of parentheses, each element is value-initialized.
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(since C++11) |
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(since C++20) |
If initialization terminates by throwing an exception (e.g. from the constructor), if new-expression allocated any storage, it calls the appropriate deallocation function: operator delete for non-array type, operator delete[] for array type. The deallocation function is looked up in global scope if the new-expression used the ::new syntax, otherwise it is looked up in the scope of T, if T is a class type. If the failed allocation function was usual (non-placement), lookup for the deallocation function follows the rules described in delete-expression. For a failed placement new, all parameter types, except the first, of the matching deallocation function must be identical to the parameters of the placement new. The call to the deallocation function is made the value obtained earlier from the allocation function passed as the first argument, alignment passed as the optional alignment argument (since C++17), and placement-params, if any, passed as the additional placement arguments. If no deallocation function is found, memory is not deallocated.
Memory leaks
The objects created by new-expressions (objects with dynamic storage duration) persist until the pointer returned by the new-expression is used in a matching delete-expression. If the original value of pointer is lost, the object becomes unreachable and cannot be deallocated: a memory leak occurs.
This may happen if the pointer is assigned to:
int* p = new int(7); // dynamically allocated int with value 7 p = nullptr; // memory leak
or if the pointer goes out of scope:
void f() { int* p = new int(7); } // memory leak
or due to exception:
void f() { int* p = new int(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 a new-expression is often stored in a smart pointer: std::auto_ptr (until C++17)std::unique_ptr, or std::shared_ptr (since C++11). These pointers guarantee that the delete expression is executed in the situations shown above.
Keywords
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 array new if the element type is not trivially destructible, which is no longer conforming since CWG 2382.
A non-allocating placement array new-expression that creates an array of char, unsigned char, or std::byte (since C++17) can be used to implicitly 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.
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| CWG 74 | C++98 | value in the first dimension must have integral type | enumeration types permitted |
| CWG 299 | C++98 | value in the first dimension must have integral or enumeration type |
class types with a single conversion function to integral or enumeration type permitted |
| CWG 624 | C++98 | the behavior was unspecified when the size of the allocated object would exceed the implementation-defined limit |
no storage is obtained and an exception is thrown in this case |
| CWG 1748 | C++98 | non-allocating placement new need to check if the argument is null |
undefined behavior for null argument |
| CWG 1992 | C++11 | new (std::nothrow) int[N] could throw bad_array_new_length
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changed to return a null pointer |
| P1009R2 | C++11 | the array bound cannot be deduced in a new expression |
deduction permitted |
| CWG 2382 | C++98 | non-allocating placement array new could require allocation overhead |
such allocation overhead disallowed |