理解C++中nothrow与Nothrowconstant的使用-优快云博客

constant

nothrow

<new>

extern const nothrow_t nothrow;

Nothrow constant

This constant value is used as an argument for operator new and operator new[] to indicate that these functions shall not throw an exception on failure, but return a null pointer instead.

By default, when the new operator is used to allocate memory and the handling function is unable to do so, abad_alloc exception is thrown. But when nothrow is used as argument for new, it returns a null pointer instead.

This constant (nothrow) is just a value of type nothrow_t, which only purpose is to trigger an overloaded version of function operator new or operator new[] that takes an argument of this type.

In C++, the new operator can be overloaded to take more than one parameter: The first parameter passed to theoperator new function is always the size of the element type to be allocated, but more arguments can be passed to this function by enclosing them in parentheses. For example:

int * p = new (x) int;

is a valid expression that, at some point, calls:

operator new (sizeof(int),x);

By default, one of the versions of operator new is overloaded to accept a parameter of type nothrow_t (likenothrow). The value itself is not used, but that version of operator new shall return zero in case of failure instead of throwing an exception.

The same applies for operator new[] and function operator new[] .

Example

// nothrow example
#include <iostream>
#include <new>
using namespace std;

int main () {
  cout << "Attempting to allocate 1 MiB...";
  char* p = new (nothrow) char [1048576];
  if (p==0) cout << "Failed!\n";
  else {
    cout << "Success!\n";
    delete[] p;
  }
  return 0;
}