141 lines
3.4 KiB
Markdown
141 lines
3.4 KiB
Markdown
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## 使用c++11解决内存泄露的问题
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### 4.1 shared_ptr共享的智能指针
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1. 初始化<br>
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优先通过make_shared来构造智能指针
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```
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std::shared_ptr<int> p(new int(1));
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std::shared_ptr<int> p2=p;
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std::shared_ptr<int> ptr;
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ptr.reset(new int(1));
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```
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2. 获取原始指针<br>
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```
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std::shared_ptr<int> ptr(new int(1));
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int *p=ptr.get();
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```
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3. 指定删除器
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```
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void DeleteIntPtr(int *p){
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delete p;
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}
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std::shared_ptr<int> p(new int,DeleteIntPtr);
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//当然也可以用lambda
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std::shared_ptr<int> p(new int,[](int* p){delete p;});
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```
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#### 4.2.1 使用shared_ptr需要注意的问题
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1. 不要用一个原始指针初始化多个shared_ptr
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```
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int* ptr=new int;
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std::shared_ptr<int> p1(ptr);
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std::shared_ptr<int> p2(ptr);
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```
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2. 不要在函数实参中创建shared_ptr
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```
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function(shared_ptr<int>(new int),g()) //有缺陷，不同编译器的执行顺序不一样
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shared_ptr<int> p(new int());
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f(p,g());
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```
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3. 通过shared_from_this()返回的this指针，不要将指针作为shared_ptr返回出来，因为this本子上是个裸指针。
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4. 避免循环引用
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```
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struct A;
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struct B;
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struct A{
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std::shared_ptr<B> bptr;
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};
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struct B{
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std::shared_ptr<A> bptr;
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};
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void TestPtr(){
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{
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std::shared_ptr<A> ap(new A);
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std::shared_ptr<B> bp(new B);
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ap->bptr=bp;
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bp->aptr=ap;
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//计数不会归零，无法正确销毁
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}
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}
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```
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### 4.2 unique_ptr独占的智能指针
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它不允许其他的智能指针共享内部的指针，不允许通过复制将unique_ptr赋值给另一个unique_ptr<br>
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但是可以通过std::move将拥有权转移
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```
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std::shared_ptr<T> myPtr(new T);
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std::shared_ptr<T> myOtherPtr=std::move(myPtr);
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```
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### 4.3 weak_ptr弱引用的智能指针
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弱引用指针weak_ptr是用来监视shared_ptr的生命周期的，不会使引用计数加1.同时他可以解决返回this指针和解决循环引用的问题。
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#### 4.3.1 weak_ptr基本用法
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1. 通过use_count()方法来获得当前观测资源的引用计数
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```
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std::shared_ptr<int> sp(new int(10));
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std::weak_ptr<int> wp(sp);
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std::cout<<wp.use_count()<<std::endl;
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```
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2. 通过expired()来判断资源是否释放
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```
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std::shared_ptr<int> sp(new int(10));
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std::weak_ptr<int> wp(sp);
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if(wp.expired())
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{
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std::cout<<"weak_ptr无效,资源释放”<<std::endl;
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}else{
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std::cout<<"weak_ptr有效"<<std::endl;
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}
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```
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3. 通过lock方法获取所监视的shared_ptr
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#### 4.3.2 weak_ptr返回this指针
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通过派生std::enable_shared_from_this类，并通过shader_from_this来返回智能指针
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#### 4.3.3 weak_ptr解决循环引用问题
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```
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struct A;
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struct B;
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struct A{
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std::shared_ptr<B> bptr;
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};
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struct B{
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std::weak_ptr<A> bptr;
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//改成weak_ptr就能正常销毁了
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};
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void TestPtr(){
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{
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std::shared_ptr<A> ap(new A);
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std::shared_ptr<B> bp(new B);
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ap->bptr=bp;
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bp->aptr=ap;
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}
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}
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```
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### 4.4通过智能指针管理第三方库分配的内存
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```
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void* p=GetHandle()->Create();
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std::shared_ptr<void> sp(p,[this](void*p){GetHandle()->Release(p);});
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```
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包装公共函数
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```
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std::shared_ptr<void> Guard(void* p)
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{
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return std::shared_ptr<void> sp(p,[this](void *p){GetHandle()->Release(p);});
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}
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```
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但是不够安全
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```
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void *p=GetHandle()->Create();
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Guard(p);
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//Guard(p);是个右值，如果没有被赋予给auto指针，这句结束就会释放从而导致p提前释放
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```
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可以使用宏来解决
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```
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#define GUARD(p) std::shared_ptr<void> p##p(p,
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void *p=GetHandle()->Create();
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Guard(p);
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```
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