C++

Reference

<new>

constant
<new>

std::nothrow

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 attempt to allocate memory and the handling function is unable to do so, a bad_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, with the only purpose of triggering an overloaded version of the function operator new (or operator new[]) that takes an argument of this type.

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

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int * p = new (x) int;

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

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operator new (sizeof(int),x);

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

The same applies to the new[] operator and function operator new[].

Example

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// nothrow example
#include <iostream>     // std::cout
#include <new>          // std::nothrow

int main () {
  std::cout << "Attempting to allocate 1 MiB... ";
  char* p = new (std::nothrow) char [1048576];

  if (!p) {             // null pointers are implicitly converted to false
    std::cout << "Failed!\n";
  }
  else {
    std::cout << "Succeeded!\n";
    delete[] p;
  }

  return 0;
}

Possible output:

Attempting to allocate 1 MiB... Succeeded!


See also

operator new
Allocate storage space (function)