新int和new（int）之间c ++的区别是什么？

Question

what's the difference between

int * num = new (int);

and

int * num = new int;

?

Is there a difference at all?

EDIT thx all. ... which one is the most correct answer?

Answer 1

There isn't a difference in the context of your example (using an int type). However, there is a difference if you need to create objects of compound types, where you need to use parenthesized version. ie:

int (**fn_ptr_ok) ()  = new (int (*[10]) ()); // OK
int (**fn_ptr_err) ()  = new int (*[10]) (); // error 

Answer 2

For this particular case, no difference at all. Both are same. Just that first syntax is rarely used, maybe because it looks inconvenient and cryptic, and requires more typing!

Answer 3

There's another difference when you want to create a dynamic array

int n = 2;
int *p = new int[n]; // valid
int *q = new (int[n]); // invalid

The parenthesized version requires a constant size.

Answer 4

In this question's case, the meanings of these two new s are identical. Grammatically, the operand of the former form new () is type-id, and the operand of the latter is new-type-id. As for new-type-id, if (....) appears at the back of the operand, it is interpreted as the argument list of the constructor. That is, If we write new int(1) , the int is initialized to 1. On the other hand, as for type-id, if (....) appears, it is a part of the type. For example, when we new a pointer to a function, we have to use new( type-id ) form. For example, as new( int(*)() ) .

Answer 5

(Not really an answer, but I can't put this into a comment)

Its even more complicated because of "placement new": http://codepad.org/pPKt31HZ

#include <stdio.h>

void* operator new( size_t N, int x ) {
  return new char[N];
}

int main( void ) {
  int x = int();
  int* a = new( int() ) int;
}

Here its interesting that gcc accepts this, but not MS/Intel.