为什么::operator 关键字添加在new 之前用于内存分配？

Question

在理解事物的过程中，我遇到了以下代码行。

int *p2 = (int *) ::operator new (sizeof(int)); 
*p2 = 100;
delete p2;

我理解逻辑和任务，但是为什么在new之前添加这个"::operator"关键字？

关于这个我应该如何进一步阅读？

我知道以下几行：

p2 = new int;
*p2 = 100;
delete p2;

Answer 1

operator new is a basic memory allocation function. new 为一个对象分配内存并初始化它.

对于像 int 这样的内置类型没有太大区别，但是对于像 std:string.[=17= 这样的非 POD(*) 类型来说区别是至关重要的]

int *p2 = (int*) :: operator new(sizeof(int));
int i = *p2;  // ERROR.  *p2 is not (yet) an int - just raw, uninitialized memory.
*p2 = 100;   // Now the memory has been set.

std::string *s2 = (std::string*) :: operator new(sizeof(std::string));
std::string str = *s2;  // ERROR: *s2 is just  raw uninitialized memory.
*s2 = "Bad boy";        // ERROR: You can only assign to a properly
                        //        constructed string object.
new(s2) std::string("Constructed");  // Construct an object in the memory
                                     // (using placement new).

str = *s2;  // Fine.
*s2 = "Hello world";  // Also fine.

*: POD代表"plain old data".

为什么::operator 关键字添加在new 之前用于内存分配？

why ::operator keyword added before new for memory allocation?

c++

memory

dynamic-memory-allocation