C++11 不抛异常的new operator

在google cpp style guide里面明确指出：we don't use exceptions

C++11的noexcept关键字为这种选择提供了便利。

C++11以前，提及malloc和new的区别，总是会强调由malloc返回的指针需要检查是不是null，因为空间分配可能

失败，而由new返回的指针不用检查，因为如若分配失败，它会抛出异常，现在又提供了std::nothrow，使得我们

可以人让new不抛异常，而是返回nullptr表示分配失败，这在需要禁用异常的场合显得很实用，具体的例子如下：

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

struct MyClass {
    int data[];
    MyClass() {std::cout << "constructed [" << this << "]\n";}
};

int main () {

    std::cout << "1: ";
    MyClass * p1 = new MyClass;
    // allocates memory by calling: operator new (sizeof(MyClass))
    // and then constructs an object at the newly allocated space

    std::cout << "2: ";
    MyClass * p2 = new (std::nothrow) MyClass;
    // allocates memory by calling: operator new (sizeof(MyClass),std::nothrow)
    // and then constructs an object at the newly allocated space

    std::cout << "3: ";
    new (p2) MyClass;
    // does not allocate memory -- calls: operator new (sizeof(MyClass),p2)
    // but constructs an object at p2

    // Notice though that calling this function directly does not construct an object:
    std::cout << "4: ";
    MyClass * p3 = (MyClass*) ::operator new (sizeof(MyClass));
    // allocates memory by calling: operator new (sizeof(MyClass))
    // but does not call MyClass's constructor

    delete p1;
    delete p2;
    delete p3;

    return ;
}

可见，只要把原来我们习惯的new T，改成 new (std::nothrow) T，就能使得我们的new expression不抛异常