std::nothrow(3)

NAME
std::nothrow - std::nothrow

Synopsis
   Defined in header <new>
   struct nothrow_t {}; (until C++11)
   struct nothrow_t { explicit nothrow_t() = default; }; (since C++11)
   extern const std::nothrow_t nothrow;
   std::nothrow_t is an empty class type used to disambiguate the overloads of throwing
   and non-throwing allocation functions. std::nothrow is a constant of it.

Example
// Run this code
 #include <iostream>
 #include <new>
 int main()
 {
     try {
         while (true) {
             new int[100000000ul]; // throwing overload
         }
     } catch (const std::bad_alloc& e) {
         std::cout << e.what() << '\n';
     }
     while (true) {
         int* p = new(std::nothrow) int[100000000ul]; // non-throwing overload
         if (p == nullptr) {
             std::cout << "Allocation returned nullptr\n";
             break;
         }
     }
 }

Output:
 std::bad_alloc
 Allocation returned nullptr
  Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 2510 C++11 the default constructor was non-explicit, which made explicit
                       could lead to ambiguity

See also
   operator new allocation functions
   operator new[] (function)