英文:
Memory leak using unique pointer and polymorphism
问题
我知道std::unique_ptr不保证内存安全，尤其在循环依赖的情况下更是如此。然而，在我的情况下，我看不到这个问题。Child 包含 Parent（或者在多态的情况下是 SecondChild），但它们都不包含 Child。
Valgrind 报告了丢失了 4 个字节，所以我认为 SecondChild 没有被销毁。我的代码依赖于多态性，所以我希望能够得到关于重构的建议，而不会改变 Child 中的 Parent*。
#include <iostream>#include <memory>using namespace std;struct Parent{Parent() {cout << "Expr created" << endl;}~Parent() {cout << "Expr destroyed" << endl;}};struct Child : public Parent{std::unique_ptr<Parent> content;};struct SecondChild : public Parent{int val;};std::unique_ptr<Parent> foo(){auto test = make_unique<Child>();auto content_in_child = make_unique<SecondChild>();content_in_child->val = 4;test->content = std::move(content_in_child);return test;}int main(){std::unique_ptr<Parent> high = foo();return 0;}
英文:
I know that std::unique_ptr doesn't guarantee memory safety and especially it is true in the case of circular dependency. However, in my case, I can't see it. Child contain Parent(or SecondChild in case of polymorphism), but neither of them contains Child.
Valgrind reports 4 bytes lost so I suppose that SecondChild was not destroyed. My code really relies on polymorphism so I hope to get advice on refactoring that won't change Parent* in Child.
#include <iostream>#include <memory>using namespace std;struct Parent{Parent() {cout << "Expr created" << endl;}~Parent() {cout << "Expr destroyed" << endl;}};struct Child : public Parent{std::unique_ptr<Parent> content;};struct SecondChild : public Parent{int val;};std::unique_ptr<Parent> foo(){auto test = make_unique<Child>();auto content_in_child = make_unique<SecondChild>();content_in_child->val = 4;test->content = std::move(content_in_child);return test;}int main(){std::unique_ptr<Parent> high = foo();return 0;}
答案1
得分: 7
你的代码存在未定义行为，这是因为父类缺少虚析构函数，而你通过基类指针（即std::unique_ptr<Parent>）删除了Child对象（即foo()中的test）。
未定义行为意味着任何事情都可能发生。在你的情况下，这会导致内存泄漏。
struct Parent {// ....virtual ~Parent() {//^^^^^^ -----> 必须添加 !!// ...}};
阅读更多：
- https://stackoverflow.com/questions/461203/when-to-use-virtual-destructors
- https://stackoverflow.com/questions/353817/should-every-class-have-a-virtual-destructor
英文:
You have undefined behavior, due to the fact that the parent class is missing a virtual destructor, and you are deleting Child objects (i.e. test from foo()) through base class pointers (i.e. std::unique_ptr<Parent>).
Undefined behavior means that anything can happen. In your case, it is a memory leak.
Fix it by adding a virtual destructor.
struct Parent {// ....virtual ~Parent() {//^^^^^^ -----> required !!// ...}};
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