#include <new> using namespace std; class X { public: void* operator new(size_t,int, int){ /* ... */ } }; // ... int main () { X* ptr = new(1,2) X; }
The placement syntax is commonly used to invoke the global placement new function. The global placement new function initializes an object or objects at the location specified by the placement argument in the placement new expression. This location must address storage that has previously been allocated by some other means, because the global placement new function does not itself allocate memory. In the following example, no new memory is allocated by the calls new(whole) X(8);, new(seg2) X(9);, or new(seg3) X(10); Instead, the constructors X(8), X(9), and X(10) are called to reinitialize the memory allocated to the buffer whole.
#include <new> class X { public: X(int n): id(n){ } ~X(){ } private: int id; // ... }; int main() { char* whole = new char[ 3 * sizeof(X) ]; // a 3-part buffer X * p1 = new(whole) X(8); // fill the front char* seg2 = &whole[ sizeof(X) ]; // mark second segment X * p2 = new(seg2) X(9); // fill second segment char* seg3 = &whole[ 2 * sizeof(X) ]; // mark third segment X * p3 = new(seg3) X(10); // fill third segment p2->~X(); // clear only middle segment, but keep the buffer // ... return 0; }The placement new syntax can also be used for passing parameters to an allocation routine rather than to a constructor.