1.控制内存分配的new和delete等

目录

• C++内存管理的架构
• C++ memory primitives
  • memory primitives 测试
    • new在编译器中的执行过程
    • delete在编译器中的执行过程
    • array new在编译器中的执行过程
    • placement new在编译器中的执行过程
• C++分配内存途径
  • C++ 应用程序分配内存的途径
  • C++ 容器分配内存的途径
• 重载new/class::operator new/::operator new
  • 重载全局::operator new /::operator delete
  • 重载全局/::operator new[] /::operator delete[]
  • 重载类内的operator new /operator delete
  • 重载类内的operator new[] /operator delete[]
    • 定义类Foo
    • operator new等重载实现
    • 主函数测试
    • 测试结果
    • 测试全局的operator new和operator delete
    • 全局的operator new和operator delete测试结果
  • 重载new()和delete()
    • 重载new()和delete()实现

C++内存管理的架构

C++ memory primitives

memory primitives 测试

#include <cstdlib>
#include <iostream>
#include "memory_primitives_test.h"
#include <vector>
#include <memory>
namespace memory_primitives_test {
	void memory_primitives_test_function() {

		std::cout << "malloc test start" << std::endl;
		void *p1 = malloc(512);  //512bytes
		free(p1);
		std::cout << "malloc test is over" << std::endl;
		//2.new的测试
		std::cout << "new test start" << std::endl;
		std::vector<int>* p2 = new std::vector<int>;
		delete p2;
		std::cout << "new test is over" << std::endl;

		std::cout << "operator new test start" << std::endl;
		void *p3 = ::operator new (512); //直接执行全局作用域中的operator new
		::operator delete(p3);
		std::cout << "operator new test is over" << std::endl;

		//测试allocators,其接口虽然有标准规格，但是不同的厂商实现了不同的接口	
		//一般不会通过分配器直接拿内存
#ifdef _MSC_VER
		std::cout << "allocator test start" << std::endl;
		int *p4 = std::allocator<int>().allocate(3, (int*)0); //分配3个ints
		std::allocator<int>().deallocate(p4, 3);
		std::cout << "allocator test is over" << std::endl;
#endif

#ifdef __BORLANDC__
		int *p4 = allocator<int>().allocate(5);
		allocator<int>().deallocate(p4, 5);
#endif

#ifdef __GNUC__
		void *p4 = alloc::allocate(512);
		alloc::deallocate(p4, 512);
#endif

	}

}

new在编译器中的执行过程

Complex *pc = new Complex(1,2);

被编译器解释成以下代码：并且只有编译器才能做到如下所示的执行过程：

Complex *pc;
try {
    void *mem = operator new(sizeof(Complex));
    pc = static_cast<Complex*>(mem);
    pc->Complex::Complex(1,2);
}
catch (std::bad_alloc) {
    //若失败，就不执行构造函数了
    
}

delete在编译器中的执行过程

delete pc;

被编译器解释成以下代码：并且只有编译器才能做到如下所示的执行过程：

pc->~Complex();  //先析构
operator delete(pc);  //再释放内存

其中operator delete又会调用free

void__cdecl opeartor delete(void *p) _THROW0() {
    //free an allocated object
    free(p);
}

array new在编译器中的执行过程

string *psa = new string[3];
delete []psa;

array new得到的是绿色的部分加上一个小的cookie(记录一些琐碎的东西，比如数组的长度等)

如上图所示，在 $ delete $时如果没有加上 $ [ ] $,回收还是正常回收的，但是析构函数只会被调用一次，即析构的可能是数组的第一个，也可能是数组的最后一个，这会造成内存泄露。

而如果是下面这个例子：

Complex* pca = new Complex[3]; //调用3次构造函数

delete [] pca; //调用三次析构函数

其中并没有指针指向析构函数，所以它的析构函数根本没有用，在析构时是可以不用加 $ [] $ 的，是不会造成内存泄露的，但是为了格式的统一，还是要加上[]

placement new在编译器中的执行过程

   placement new允许我们将object建立于allocated memory中。
没有所谓的placement delete，因为placement new根本没有分配memory，

		char *buf = new char[sizeof(Complex) * 3];
		Complex *pc = new(buf)Complex(1,2);

		delete [] buf;

	//被编译器转为
		Complex *pc;
		try {
			void *mem = operator new(sizeof(Complex), buf);
			pc = static_cast<Complex*>(mem);
			pc->Complex::Complex(1, 2);
		}
		catch (std::bad_alloc)
		{
			//若分配失败，则不执行constructor
		}

   所以 placemenet new或指new(p)或指::operator new(size,void*)

C++分配内存途径

C++ 应用程序分配内存的途径

C++ 容器分配内存的途径

重载new/class::operator new/::operator new

重载全局::operator new /::operator delete

//重载::operator new /::operator delete /::operator new[] /::operator delete[]
void* myAlloc(size_t size) {
	return malloc(size); //返回一个指针指向malloc的内存
}

void myFree(void *ptr) {
	return free(ptr);
}

inline void * operator new(size_t size) {
	std::cout << "global new() 
 ";
	return myAlloc(size);
}

inline void operator delete(void* ptr) {
	std::cout << "global delete() 
";
	return myFree(ptr);
}

重载全局/::operator new[] /::operator delete[]

inline void* operator new[](size_t size) {
	std::cout << "global new() 
";
	return myAlloc(size);
}
inline void operator delete[](void *ptr) {
	std::cout << "global delete [] 
";
	myFree(ptr);
}

重载类内的operator new /operator delete

必须是static的

重载类内的operator new[] /operator delete[]

必须是static

定义类Foo

重载类的operator new和operator delete 以及operator new []和operator delete[]，注意重载的operator new等为static的

	//重载载类内的operator new和operator delete
	class Foo {
	public:
		int _id;
		long _data;
		std::string _st;
	public:
		Foo() : _id(0) { std::cout << "default cotr. this = " << this << "   id=" << _id << std::endl; }
		Foo(int i):_id(i) { std::cout << "default cotr. this = " << this << "   id=" << _id << std::endl; }
		~Foo() { std::cout << "dtor. this= " << this << "  id= " << _id << std::endl; }

		static void *operator new(size_t size);
		static void operator delete(void* pdead, size_t size);
		static void *operator new[](size_t size);
		static void operator delete[](void*pdead, size_t size);
	};

operator new等重载实现

	//Foo内的operator new
	void *Foo::operator new(size_t size) {
		Foo* p = (Foo*)malloc(size);
		std::cout << "Foo operator new" << std::endl;;
		return p;
	}
	//Foo内的operator delete
	void Foo::operator delete(void* pdead, size_t size) {
		std::cout << "Foo opeartor delete" << std::endl;
		free(pdead);
	}
	//Foo内的operator new[]
	void *Foo::operator new[](size_t size) {
		Foo* p = (Foo*)malloc(size);
		std::cout << "Foo operator new[]" << std::endl;;
		return p;
	}
	//Foo内的operator delete[]
	void Foo::operator delete[](void* pdead, size_t size) {
		std::cout << "Foo opeartor delete[]" << std::endl;
		free(pdead);
	}

主函数测试

	std::cout << "sizeof(Foo)" << sizeof(Foo) << std::endl;
	//测试operator new和operator delete
	Foo *p = new Foo(7);
	delete p;
	//测试operator new[] 和operator delete[]
	Foo* pArray = new Foo[5];
	delete[] pArray;

测试结果

sizeof(Foo)36
Foo operator new
default cotr. this = 0063EE50   id=7
dtor. this= 0063EE50  id= 7
Foo opeartor delete
Foo operator new[]
default cotr. this = 006412CC   id=0
default cotr. this = 006412F0   id=0
default cotr. this = 00641314   id=0
default cotr. this = 00641338   id=0
default cotr. this = 0064135C   id=0
dtor. this= 0064135C  id= 0
dtor. this= 00641338  id= 0
dtor. this= 00641314  id= 0
dtor. this= 006412F0  id= 0
dtor. this= 006412CC  id= 0
Foo opeartor delete[]
请按任意键继续. . .

测试全局的operator new和operator delete

	std::cout << "sizeof(Foo)" << sizeof(Foo) << std::endl;
	//测试operator new和operator delete
	Foo *p = ::new Foo(7);
	::delete p;
	//测试operator new[] 和operator delete[]
	Foo* pArray = ::new Foo[5];
	::delete[] pArray;

全局的operator new和operator delete测试结果

sizeof(Foo)36
default cotr. this = 003EEE50   id=7
dtor. this= 003EEE50  id= 7
default cotr. this = 003F12CC   id=0
default cotr. this = 003F12F0   id=0
default cotr. this = 003F1314   id=0
default cotr. this = 003F1338   id=0
default cotr. this = 003F135C   id=0
dtor. this= 003F135C  id= 0
dtor. this= 003F1338  id= 0
dtor. this= 003F1314  id= 0
dtor. this= 003F12F0  id= 0
dtor. this= 003F12CC  id= 0
请按任意键继续. . .

发现并没有进入Foo类内的operator new和operator delete。

重载new()和delete()

我们可以重载class member operator new()，写出多个版本，前提是每个版本的声明都具有不同的参数列，且其中第一个参数必须是size_t,其余参数是以new所指定的placement arguments为初值，出现new (...)小括号内的便是所谓的placement arguments。

Foo *pf = new(300,'c')Foo;

我们也可以重载class member operator delete()，写出多个版本，但它们绝对不会被delete调用，只有当new所调用的ctor抛出exception时，才会调用这些重载的operator delete().它只可能这样被调用，主要用于未能完全创建成功的object所占用的memory.

重载new()和delete()实现

	//重载new()和delete()
	void* Foo::operator new(size_t size, void *start) {
		return start;
	}
	void* Foo::operator new(size_t size, long extra) {
		return malloc(size + extra);
	}
	void* Foo::operator new(size_t size, long extra, char init) {
		return malloc(size + extra);
	}

	//如果调用构造函数失败，才会调用如下的构造函数
	void Foo::operator delete (void*, size_t) {
		std::cout << "operator delete(void*,size_t)" << std::endl;
	}
	void Foo::operator delete(void*, void*) {
		std::cout << "operator delete(void*,void*)" << std::endl;
	}
	void Foo::operator delete(void*, long) {
		std::cout << "operator delete(void*,long)" << std::endl;
	}
	void Foo::operator delete(void*, long, char) {
		std::cout << "operator delete(void*,long,char)" << std::endl;
	}