Is it possible to return a pointer to a struct declared in a class?

Question

The following code

class Test{
private:
    struct Node{
        int element;
        Node* next;
    };

    Node* stupidFunction(); // Line 8
};

///////////////////////////////

struct Node;

Node* Test::stupidFunction(){ // Line 15
    Node foo;
    return &foo;
}

///////////////////////////////

int main(){}

will not compile and gives the following error messages:

Line 15: error: prototype for 'Node* Test::studpidFunction()' does not match any in class 'Test'
Line 8: error: candidate is: Test::Node* Test::stupidFunction()

Is it not possible to return a pointer to a struct declared in a class, or am I doing something wrong?

Answer 1

Since it is defined inside Test , Node is a nested type, ie Test::Node . So, the (non-inline) function definition must be written as

Test::Node* Test::stupidFunction() {

However, the implementation, returning the address of a local variable, is seriously incorrect. The variable, here foo , goes out of scope as soon as the function returns and hence the caller is left with a bad pointer.

One option is the following

Test::Node* Test::stupidFunction() {
  Node * pNode = new Node;
  // update pNode
  return pNode;
}

However, this design too has an issue that the caller must ensure the return pointer is delete -ed before it goes out of scope. Otherwise the memory allocated by new Node would be leaked.

A better option is to use a smart pointer

std::shared_ptr<Test::Node> Test::stupidFunction() {
  auto pNode = std::make_shared<Test::Node>();
  // update pNode;
  return pNode;
}

This way, the caller does not need explicit delete . The memory is released as soon as there is no pointer left that points to this resource, ie Node .

Answer 2

the struct node is define in Test Class

class Test{
private:
    struct Node{
        int element;
        Node* next;
    };

    Node* stupidFunction(); // Line 8
};

///////////////////////////////

struct Node;

Test::Node* Test::stupidFunction(){ //use Node which define in class Test
    Node foo;
    return &foo;
}

int main(void)

Answer 3

对于品种，以下作品

auto Test::stupidFunction() -> Node*