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<new> operators and enums

  • Artikkeli

enum align_val_t

enum class align_val_t : size_t {};

operator delete

The function called by a delete expression to deallocate storage for individual objects.

void operator delete(void* ptr) noexcept;
void operator delete(void *, void*) noexcept;
void operator delete(void* ptr, const std::nothrow_t&) noexcept;

Parameters

ptr
The pointer whose value is to be rendered invalid by the deletion.

Remarks

The first function is called by a delete expression to render the value of ptr invalid. The program can define a function with this function signature that replaces the default version defined by the C++ Standard Library. The required behavior is to accept a value of ptr that's nullptr or that was returned by an earlier call to operator new.

The default behavior for a nullptr value in ptr is to do nothing. Any other value of ptr must be a value returned earlier by a new call as previously described. The default behavior for a non-null value of ptr is to reclaim storage allocated by the earlier call. It's unspecified under what conditions part or all of such reclaimed storage is allocated by a subsequent call to operator new, or to any of the calloc, malloc, or realloc functions.

The second function is called by a placement delete expression corresponding to a new expression of the form new( std::size_t ). It does nothing.

The third function is called by a placement delete expression corresponding to a new expression of the form new( std::size_t, const std::nothrow_t& ). The program can define a function with this function signature that replaces the default version defined by the C++ Standard Library. The required behavior is to accept a value of ptr that is nullptr or that was returned by an earlier call to operator new. The default behavior is to evaluate delete( ptr ).

Example

See operator new for an example that uses operator delete.

operator delete[]

The function called by a delete expression to deallocate storage for an array of objects.

void operator delete[](void* ptr) noexcept;
void operator delete[](void *, void*) noexcept;
void operator delete[](void* ptr, const std::nothrow_t&) noexcept;

Parameters

ptr
The pointer whose value is to be rendered invalid by the deletion.

Remarks

The first function is called by an delete[] expression to render the value of ptr invalid. The function is replaceable because the program can define a function with this function signature that replaces the default version defined by the C++ Standard Library. The required behavior is to accept a value of ptr that is nullptr or that was returned by an earlier call to operator new[]. The default behavior for a null value of ptr is to do nothing. Any other value of ptr must be a value returned earlier by a call as previously described. The default behavior for such a non-null value of ptr is to reclaim storage allocated by the earlier call. It's unspecified under what conditions part or all of such reclaimed storage is allocated by a subsequent call to operator new, or to any of the calloc, malloc, or realloc functions.

The second function is called by a placement delete[] expression corresponding to a new[] expression of the form new[]( std::size_t ). It does nothing.

The third function is called by a placement delete[] expression corresponding to a new[] expression of the form new[]( std::size_t, const std::nothrow_t& ). The program can define a function with this function signature that replaces the default version defined by the C++ Standard Library. The required behavior is to accept a value of ptr that is nullptr or that was returned by an earlier call to operator new[]. The default behavior is to evaluate delete[]( ptr ).

Example

See operator new[] for examples of the use of operator delete[].

operator new

The function called by a new-expression to allocate storage for individual objects.

void* operator new(std::size_t count);
void* operator new(std::size_t count, const std::nothrow_t&) noexcept;
void* operator new(std::size_t count, void* ptr) noexcept;

Parameters

count
The number of bytes of storage to be allocated.

ptr
The pointer to be returned.

Return value

A pointer to the lowest byte address of the newly allocated storage. Or ptr, if using the third form of the function.

Remarks

The first function is called by a new expression to allocate count bytes of storage suitably aligned to represent any object of that size. This function is replaceable. It means the program can define an alternate function with this function signature that replaces the default version defined by the C++ Standard Library.

The required behavior is to return a non-null pointer only if storage can be allocated as requested. Each such allocation yields a pointer to storage disjoint from any other allocated storage. The order and contiguity of storage allocated by successive calls is unspecified. The initial stored value is unspecified. The returned pointer points to the start (the lowest byte address) of the allocated storage. If count is zero, the value returned doesn't compare equal to any other value returned by the function.

The default behavior is to execute a loop. Within the loop, the function first attempts to allocate the requested storage. Whether the attempt involves a call to malloc is unspecified. If the allocation attempt is successful, the function returns a pointer to the allocated storage. Otherwise, the function calls the designated function of type new_handler. If the called function returns, the loop repeats. The loop terminates when an attempt to allocate the requested storage is successful or when a called function doesn't return.

The required behavior of a function of type new_handler is to perform one of the following operations:

  • Make more storage available for allocation and then return.

  • Call either abort or exit.

  • Throw an object of type bad_alloc.

The default behavior of a new_handler function is to throw an object of type bad_alloc. A nullptr value designates the default new_handler function.

The order and contiguity of storage allocated by successive calls to operator new is unspecified, as are the initial values stored there.

To free storage allocated by the first form of operator new, call operator delete.

The second function is called by a placement new expression to allocate count bytes of storage suitably aligned to represent any object of that size. This function is replaceable. It means the program can define an alternate function with this function signature that replaces the default version defined by the C++ Standard Library.

The default behavior is to return operator new( count ) if that function succeeds. Otherwise, it returns nullptr.

To free storage allocated by the second form of operator new (that is, if it didn't return nullptr), call operator delete.

The third function is called by a non-allocating placement new expression, of the form new ( ptr ) T. Here, ptr consists of a single object pointer. It can be useful for constructing an object at a known address. The function returns ptr. You must call the destructor explicitly on this object.

If you call non-allocating placement new, don't call delete. Instead, call the deallocator for the memory you provided, if necessary, after you call the destructor for the object.

For information on throwing or non-throwing behavior of new, see The new and delete operators.

Example

// new_op_new.cpp
// compile with: /EHsc
#include<new>
#include<iostream>

using namespace std;

class MyClass {
    int member{ 0 };
public:
    MyClass() {
        cout << "MyClass at 0x" << this << " constructed.\n";
    };

    ~MyClass() {
        cout << "MyClass at 0x" << this << " destroyed.\n";
    };
};

int main( )
{
    // The first form of new / delete
    MyClass* fPtr1 = new MyClass;
    delete fPtr1;

    // The second form (fail returns nullptr) of new / delete
    MyClass* fPtr2 = new(nothrow) MyClass[2];
    if (fPtr2)
        delete fPtr2;

    // The third form (non-allocating placement) of new / delete
    char x[sizeof(MyClass)]; // x is automatic
    MyClass* fPtr3 = new(&x[0]) MyClass;
    fPtr3->~MyClass(); // Requires explicit destructor call
    // no delete because x is on the stack
}

operator new[]

The allocation function called by a new expression to allocate storage for an array of objects.

void* operator new[](std::size_t count);
void* operator new[](std::size_t count, const std::nothrow_t&) noexcept;
void* operator new[](std::size_t count, void* ptr) noexcept;

Parameters

count
The number of bytes of storage to be allocated for the array object.

ptr
The pointer to be returned.

Return value

A pointer to the lowest byte address of the newly allocated storage. Or ptr, when the third form is used.

Remarks

The first function is called by a new[] expression to allocate count bytes of storage suitably aligned to represent any array object of that size or smaller. The program can define a function with this function signature that replaces the default version defined by the C++ Standard Library. The required behavior is the same as for operator new. The default behavior is to return operator new( count ) if it succeeds. Otherwise, it throws a std::bad_alloc exception (or an exception derived from std::bad_alloc). To free storage allocated by this form of operator new[], call operator delete[].

The second function is called by a placement new[] expression to allocate count bytes of storage suitably aligned to represent any array object of that size. The program can define a function with this function signature that replaces the default version defined by the C++ Standard Library. The default behavior is to return operator new( count ) if that function succeeds. Otherwise, it returns nullptr. To free storage allocated by this form of operator new[], call operator delete[]. For more information on the throwing or non-throwing behavior of new, see The new and delete operators.

The third function is called by a non-allocating placement new[] expression, of the form new( ptr ) T[ N ]. This form doesn't allocate memory. It constructs the objects in the memory passed in through the ptr parameter. The function returns ptr. You must call the destructor explicitly for each object created. You're responsible for providing sufficient memory for ptr. Don't invoke delete[] on the value returned by the new expression. Instead, deallocate ptr, if necessary, after you call the destructors.

Example

// new_op_array.cpp
// compile with: /EHsc
#include <new>
#include <iostream>

using namespace std;

class MyClass {
    int member{ 0 };
public:
    MyClass() {
        cout << "MyClass at 0x" << this << " constructed.\n";
    };

    ~MyClass() {
        cout << "MyClass at 0x" << this << " destroyed.\n";
    };
};

int main() {
    // The first form of array new / delete
    MyClass* fPtr1 = new MyClass[2];
    delete[] fPtr1;

    // The second form (fail returns nullptr) of array new / delete
    MyClass* fPtr2 = new(nothrow) MyClass[2];
    if (fPtr2)
        delete[] fPtr2;

    // The third form (non-allocating placement) of array new / delete
    char x[2 * sizeof(MyClass) + sizeof(int)]; // x is automatic

    MyClass* fPtr3 = new(&x[0]) MyClass[2];
    fPtr3[1].~MyClass(); // Requires explicit destructor calls
    fPtr3[0].~MyClass(); // Recommended in reverse construction order
    // Don't delete[] fPtr3 here.
    // delete[] &x[0] not required because x is on the stack
}