自定义 operator new， placement new，以及释放内存

1. new 操作符(new operator)

new 操作符做两件事：分配内存 + 调用构造函数初始化。

2. operator new

通常声明如下：

operator new 操作符的职责仅仅是分配内存，操作符返回一个未经处理（raw）的指针，未初始化的内存。

void* operator new(size_t size);

调用方式如下：

void *rawMemory = operator new(sizeof(string));

注意，显式调用 operator new 时，几乎没有意义，因为不能显式调用构造函数。

2.1 对指定类型的 operator new, operator delete 进行重载

#include <cstddef>
#include <iostream>
 
// class-specific allocation functions
struct X {
  static void* operator new(std::size_t count) {
    std::cout << "custom new for size " << count << '\n';
    return ::operator new(count);
  }
 
  static void* operator new[](std::size_t count) {
    std::cout << "custom new[] for size " << count << '\n';
    return ::operator new[](count);
  }

  static void operator delete(void* ptr, bool b){
    std::cout << "custom placement delete called, b = " << b << '\n';
    ::operator delete(ptr);
  }
};

int main() {
  X* p1 = new X;
  delete p1;
  X* p2 = new X[10];
  delete[] p2;
}

3. placement new

placement new 是一种特殊的 new 操作符，它允许在已分配的内存上调用构造函数。

alignas(T) unsigned char buf[sizeof(T)];

// Construct a “T” object, placing it directly into your pre-allocated storage at memory address “buf”.
T* tptr = new(buf) T;

tptr->~T(); // 显式调用析构函数

参考

• operator new, operator new[]
• operator delete, operator delete[]
• new expression – Placement new