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