C++: overriding new & delete

整理自：Thinking in C++

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1. Intro

When you create a new-expression such as new T, two things occur:

1. storage is allocated using operator new,
2. then the constructor is called.

In a delete-expression such as delete pt;:

1. the destructor is called,
2. then storage is deallocated using operator delete.

The constructor and destructor calls are never under your control, but you can overload the storage allocation parts of new and delete.

When you overload operator new, you also replace the behavior when it runs out of memory, so you must decide what to do in your operator new:

• return zero, or
• write a loop to call the new_handler and retry allocation, or
• (typically) throw a bad_alloc exception

Digress: set_new_handler()

当 new 申请不到内存的时候，其实是会执行一个叫做 new_handler 的 function，这是一个无参的 void 函数，默认的行为是 throw 一个 exception。我们可以用 #include <new> 的 set_new_handler() 函数来设置一个新的 new_handler，只要这个 new_handler 是一个无参的 void 函数就行。看例子：

#include <iostream>
#include <cstdlib>
#include <new>
using namespace std;

int count = 0;

void out_of_memory() {
    cerr << "memory exhausted after " << count
         << " allocations!" << endl;
    exit(1);
}

// DANGEROUS. Better try it on a virtual machine
int main() {
    set_new_handler(out_of_memory);
    while(1) {
        count++;
        new int[1000]; // Exhausts memory
    }
}

The behavior of the new_handler is tied to operator new, so if you overload operator new, the new_handler will not be called by default. If you still want the new_handler to be called you’ll have to write the code to do so inside your overloaded operator new.

2. Overriding global new & delete

• The overloaded new takes an argument of size_t, and return a void*.
• The operator delete takes an argument of void*, and returns a void.

下面给个没有啥实际意义的例子，主要是演示怎么写：

#include <cstdio>
#include <cstdlib>

void* operator new(size_t sz) {
    printf("operator new: %d Bytes\n", sz);
    
	void* m = malloc(sz);
    if(!m) {
    	puts("out of memory");
	}
    return m;
}

void operator delete(void* m) {
    puts("operator delete");
    free(m);
}

Notice that printf() and puts() are used rather than iostream. This is because when an iostream object is created, it calls new to allocate memory–it’a deadlock.

3. Overriding new & delete for a class

Although you don’t have to explicitly say static, when you overload new and delete for a class, you’re creating static member functions.

例子见书上 P620

4. Overriding new & delete for arrays

重载 operator new[] 和 operator delete[]，语法和前面一样，只是 operator new[] 的参数 (size_t size) 是 size of the entire array。

例子见书上 P624

5. Placement new

首先声明一点：placement new 并不是指传 additional parameter 给 new，而是利用 new 可以接受 additional parameter 这个特性来实现 construct an object on memory that’s already allocated，i.e. 用 already allocated memory 来 replace new 出来的 memory。

new 接收 additional parameter 的语法是：

T::operator new(size_t, ...) {
	...
}

T* pt = new(...) X(1024);

有啥新添的参数，在 new object 的时候全部加到 new 的括号里。

关于 placement new，可以参阅：

• 深入探究 C++ 的 new/delete 操作符
• What uses are there for “placement new”?
• FAQ: What is “placement new” and why would I use it?
  • FAQ: Is there a placement delete?

注意销毁 placement new 出来的对象需要显示调用 destructor pt->T::~T();，也只有这里会这么用。详情请看书或者看上述的参考链接。