Редактиране

Споделяне чрез

<new> functions

  • Статия

get_new_handler

new_handler get_new_handler() noexcept;

Remarks

Returns the current new_handler.

launder

template <class T>
    constexpr T* launder(T* ptr) noexcept;

Parameters

ptr
The address of a byte in memory that holds an object whose type is similar to T.

Return Value

A value of type T* that points to X.

Remarks

Also referred to as a pointer optimization barrier.

Used as a constant expression when the value of its argument may be used in a constant expression. A byte of storage is reachable through a pointer value that points to an object if within the storage occupied by another object, an object with a similar pointer.

Example

struct X { const int n; };

X *p = new X{3};
const int a = p->n;
new (p) X{5}; // p does not point to new object because X::n is const
const int b = p->n; // undefined behavior
const int c = std::launder(p)->n; // OK

nothrow

Provides an object to be used as an argument for the nothrow versions of new and delete.

extern const std::nothrow_t nothrow;

Remarks

The object is used as a function argument to match the parameter type std::nothrow_t.

Example

See operator new and operator new[] for examples of how std::nothrow_t is used as a function parameter.

set_new_handler

Installs a user function that is to be called when operator new fails in its attempt to allocate memory.

new_handler set_new_handler(new_handler Pnew) throw();

Parameters

Pnew
The new_handler to be installed.

Return Value

0 on the first call and the previous new_handler on subsequent calls.

Remarks

The function stores Pnew in a static new handler pointer that it maintains, then returns the value previously stored in the pointer. The new handler is used by operator new.

Example

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

using namespace std;
void __cdecl newhandler( )
{
   cout << "The new_handler is called:" << endl;
   throw bad_alloc( );
   return;
}

int main( )
{
   set_new_handler (newhandler);
   try
   {
      while ( 1 )
      {
         new int[5000000];
         cout << "Allocating 5000000 ints." << endl;
      }
   }
   catch ( exception e )
   {
      cout << e.what( ) << endl;
   }
}

Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
Allocating 5000000 ints.
The new_handler is called:
bad allocation