[C++] 重载new和delete——控制内存分配

string *sp=new string("aaaa");
string *arr=new string[10];//string采用默认初始化

delete sp;
delete []arr;

#ifndef _NEWANDDELETE_H_
#define _NEWANDDELETE_H_
#include<new>
#include<cstdlib>
using namespace std;

class NewAndDelete {
public:
    /**
    * 以下是标准库函数中的operate new和operator delete
    * 前面4个会抛出异常，后面4个不会抛出异常
    * 这几个运算符函数是隐式静态的，因为operator new在构造对象之前，用来分配空间；operator delete在对象销毁之后，用来释放空间
    * operator new 或者operator new[]的返回类型必须是void*，第一个形参类型必须是size_t并且没有默认实参，size_t代表要分配的空间的大小
    * operator delete或者operator delete[]返回类型必须是void，第一个形参类型必须是void*，用指向待释放内存的指针来初始化
    * 这两个函数的行为与allocator类中allocate()和deallocate()很类似
    */
    static void *operator new(size_t);
    static void *operator new[](size_t);
    static void operator delete(void*)noexcept;
    static void operator delete[](void*)noexcept;

    static void *operator new(size_t, nothrow_t&)noexcept;
    static void *operator new(size_t, nothrow_t&)noexcept;
    static void operator delete(void*, nothrow_t&)noexcept;
    static void operator delete[](void*, nothrow_t&)noexcept;
};

//实现示例——使用malloc和free
void* NewAndDelete::operator new(size_t size) {
    if (void *mem = malloc(size))
        return mem;
    else
        throw bad_alloc();//抛出异常
}
void NewAndDelete::operator delete(void *mem)noexcept {
    free(mem);
}
void* NewAndDelete::operator new(size_t size, nothrow_t& no)noexcept {//不抛出异常，返回空指针
    void *p;
    if (void *mem = malloc(size))
        return mem;
    else return p;
}
void NewAndDelete::operator delete(void *mem, nothrow_t& no)noexcept {
    free(mem);
}
#endif