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;
}