C++ 如何在堆栈中存储唯一指针，并允许对象将指针从堆栈中取出（并获得所有权）

Question

I am struggling with a failed unit test and cannot fix the problem.

I have an interface class "ContextObject", and a class "Context" that wraps a stack of unique pointers to "ContextObjects". For the test, I also have an implementation "FooContextObject" of the interface class. Shown Below:

struct ContextObject {
    virtual ~ContextObject() = 0;
};
ContextObject::~ContextObject() {}

struct FooContextObject : ContextObject {
    std::string name;
    FooContextObject(const std::string& name) : name(name) {}
    virtual ~FooContextObject() {}
};

The Context class is defined as follows:

struct Context {
    std::stack<std::unique_ptr<ContextObject>> stack;
    void push(std::unique_ptr<ContextObject>&& t) { stack.push(std::move(t)); }
    std::unique_ptr<ContextObject> pop() {
        if(stack.empty()) {
            std::cerr << "Empty stack" << std::endl;
            exit(-4);
        }
        std::unique_ptr<ContextObject> ptr = std::move(stack.top());
        stack.pop();
        return std::move(ptr);
    }
    std::unique_ptr<ContextObject>& peak() { return stack.top(); }
};

And my test is:

void addToContext(Context& ctx) {
    auto obj = std::make_unique<FooContextObject>("foo");
    ctx.push(std::move(obj));
}

void useContextObject(Context& ctx) {
    ContextObject* a = ctx.pop().get();
    auto b = dynamic_cast<FooContextObject*>(a);
    std::cout << b->name << std::endl;
}

int main() {
    Context ctx;
    addToContext(ctx);
    useContextObject(ctx);
}

Can anybody let me know where I am going wrong, and help me get the code to pass the test? Thanks.

Answer 1

ContextObject* a = ctx.pop().get()

Here ctx.pop() returns a unique_ptr which gets destroyed at the end of the expression because it has no owner. You should assign jt to a unique_ptr to keep ownership. Otherwise, you get a dangling pointer and b->name dereferences It, causing undefined behavior

Off topic note: consider changing the signature of push to

void push(std::unique_ptr<ContextObject> t)

This way it is clear to the user of the API that the object passed to push is moved-from