c++11 stl 学习之 shared_ptr

shared_ptr
智能指针 shared_ptr 的声明初始化方式
由于指针指针使用explicit参数 必须显示声明初始化
shared_ptr<string> pNico = new string("nico"); // ERROR
shared_ptr<string> pNico{new string("nico")}; // OK

也可以使用make_shared()
shared_ptr<string> pNico = make_shared<string>("nico");
shared_ptr<string> pJutta = make_shared<string>("jutta");

智能指针一旦声明
就不能再次分配 除非使用reset()
shared_ptr<string> pNico4;
pNico4 = new string("nico");
//ERROR: no assignment for ordinary pointers
pNico4.reset(new string("nico")); // OK

shared_ptr的使用方式与实际指针使用类似 基本没什么区别

示例 sharedPtrTest1()

//=======================================
对于shared_ptr中的参数 可以指定 删除器 Deleter
示例 sharedPtrTest2()

#include <iostream>
#include <string>
#include <vector>
#include <memory>
#include <fstream> //for ofstream
#include <cstdio> //for remove()

using namespace std;

void sharedPtrTest1()
{
	shared_ptr<string> pNico(new string("nico"));
	shared_ptr<string> pJutta(new string("jutta"));

	(*pNico)[0] = 'N';
	pJutta->replace(0, 1, "J");

	vector<shared_ptr<string>> whoMadeCoffee;
	whoMadeCoffee.push_back(pJutta);
	whoMadeCoffee.push_back(pJutta);
	whoMadeCoffee.push_back(pNico);
	whoMadeCoffee.push_back(pJutta);
	whoMadeCoffee.push_back(pNico);

	for (auto ptr : whoMadeCoffee) {
		cout << *ptr << " ";
	}
	cout << endl;
	// overwrite a name again
	*pNico = "Nicolai";
	// print all elements again
	for (auto ptr : whoMadeCoffee) {
		cout << *ptr << " ";
	}
	cout << endl;
	// print some internal data
	cout << "use_count: " << whoMadeCoffee[0].use_count() << endl;
}

class FileDeleter
{
private:
	std::string filename;
public:
	FileDeleter(const std::string& fn)
		: filename(fn) {
	}
	void operator () (std::ofstream* fp) {
		fp->close(); //close.file
		std::remove(filename.c_str()); //delete file
		cout << "delete file finish" << endl;
	}
};

void sharedPtrTest2()
{
	shared_ptr<std::ofstream> fp(new std::ofstream("tmpfile.txt"),
		FileDeleter("tmpfile.txt"));
}

int _tmain(int argc, _TCHAR* argv[])
{
	sharedPtrTest1();
	sharedPtrTest2();

	return 0;
}

　　

shared_ptr的误用

下面是错误示例

int* p = new int;

shared_ptr<int> sp1(p);

shared_ptr<int> sp2(p);

这将产生错误 对于int指针有两个智能指针的内存管理器对其进行管理

sp1 sp2 都对其关联的资源进行释放

正确的代码如下:

shared_ptr<int> sp1(new int);

shared_ptr<int> sp2(sp1);

还有一种隐蔽的情况 也会发生这种错误：

比如在Person 中增加一个函数setParentsAndTheirKids()

void setParentsAndTheirKids(shared_ptr<Person> m = nullptr,
shared_ptr<Person> f = nullptr)
{
mother = m;
father = f;
if (m != nullptr){
m->kids.push_back(shared_ptr<Person>(this));
}
if (f != nullptr){
f->kids.push_back(shared_ptr<Person>(thid));
}
}

int _tmain(int argc, _TCHAR* argv[])
{
string name("nico");
shared_ptr<Person> mom(new Person(name + "'s mom"));
shared_ptr<Person> dad(new Person(name + "'s dad"));
shared_ptr<Person> kid(new Person(name + "name"));

kid->setParentsAndTheirKids(mom,dad);

return 0;
}

但是如果在自己的类中使用包含this的shared_ptr指针 会增加引用计数

导致无法释放

所以引进enable_shared_from_this类

#include <iostream>
#include <string>
#include <vector>
#include <memory>
using namespace std;

class Person : public std::enable_shared_from_this<Person> {
public:
	string name;
	shared_ptr<Person> mother;
	shared_ptr<Person> father;
	//vector<shared_ptr<Person>> kids;
	vector<weak_ptr<Person>> kids;

	Person(const string& n,
		shared_ptr<Person> m = nullptr,
		shared_ptr<Person> f = nullptr)
		:name(n), mother(m), father(f){
	};

	~Person(){
		cout << "delete " << name << endl;
	}

	//====================================
	void setParentsAndTheirKids(shared_ptr<Person> m = nullptr,
		shared_ptr<Person> f = nullptr)
	{
		mother = m;
		father = f;
		if (m != nullptr){
			m->kids.push_back(shared_ptr<Person>(shared_from_this()));
		}
		if (f != nullptr){
			f->kids.push_back(shared_ptr<Person>(shared_from_this()));
		}
	}
};

int _tmain(int argc, _TCHAR* argv[])
{
	string name("nico");
	shared_ptr<Person> mom(new Person(name + "'s mom"));
	shared_ptr<Person> dad(new Person(name + "'s dad"));
	shared_ptr<Person> kid(new Person(name + "name"));

	kid->setParentsAndTheirKids(mom,dad);

	return 0;
}