当 operator new 没有能力为调配出咱们所申请的 memory，会抛出一个 std::bad_alloc exception。某些老编译器则是返回 0 ，古代编译器依然能够这样做：new (nothrow) Foo;

抛出 exception 之前会先（不止一次）调用一个可有 client 指定的 handler, 以下是 new handler 的模式和设定办法：

typedef void (*new_handler)();new_handler set_new_handler(new_handler p) throw();

• 设计良好的 new_handler 只有两个抉择：

  • 让更多的 memory 可用
  • 调用 abort() 或 exit()

void *operator_new(size_t size, const std::nothrow_t &_THROW0()){    // try to allocate size bytes    void *p;    while ((p = malloc(size)) == 0)    {        // buy more memory or return null pointer        _TRY_BEGIN            if (_callnew_h(size) == 0) break;        _CATCH(std::bad_alloc) return 0;        _CATCH_END    }    return (p);}

#include <new>#include <iostream>#include <cassert>using namespace std;void onMoreMemory(){    cerr << "out of memory" << endl;    abort();}int main(){    set_new_handler(onMoreMemory);    int *p = new int[10000];    assert(p);    cout << p << endl;    p = new int[100000000000000];    assert(p);    cout << p << endl;    return 0;}

输入[gcc version 7.5.0]

0x55ad89dd7e70out of memoryAborted (core dumped)

本例中 new handler 中如果没有调用 abort(), 执行后 cerr 会一直呈现 "out of memory", 需强制中断。这样的体现是正确的，示意当 operator new 无奈满足申请量时，会一直调用 new handler 直到失去足够 memory.

default、delete

default、delete 不仅实用于构造函数（拷贝结构、挪动结构）和赋值函数（拷贝赋值、挪动赋值）

class Foo {public:    Foo() = default;    Foo(const Foo&) = delete;    Foo& operator=(const Foo&) = delete;    ~Foo() = default;};

理论测试：

#include <iostream>using namespace std;class Foo {public:    long _x;public:    Foo(long x = 0) : _x(x)    { }    // static void *operator new(size_t size) = default;                   // error: only special member functions may be defaulted    // static void operator delete(void *pdead, size_t size) = default;    // error: only special member functions may be defaulted    static void *operator new[](size_t size) = delete;    static void operator delete[](void *pdead, size_t size) = delete;};int main(){    // Foo *p1 = new Foo[10];  // error: call to deleted function 'operator new[]'    // delete []  p1;          // error: attempt to use a deleted function    return 0;}