c++ : error: member access into incomplete type , 未使用的参数 [-Werror,-Wunused-parameter]

Question

我有以下代码:

class MyNode;

class MyCompare {
  public:
    bool operator()(MyNode *a, MyNode *b) {
        return a->q <= b->q ? true : false;
    }
};

class MyNode {

  public:

    double sum;    
    double q;

    StateNode *parent;
    std::priority_queue<MyNode, std::vector<MyNode>, MyCompare> children;
};

但是出现如下编译错误:

error: member access into incomplete type 'MyNode'
        return a->q <= b->q ? true : false;
                ^
..MyNode.h:35:7: note: forward declaration of 'MyNode'
class MyNode;
      ^
..MyNode.h:39:46: error: unused parameter 'b' [-Werror,-Wunused-parameter]
    bool operator()(MyNode *a, MyNode *b) {

知道我在这里做错了什么吗？谢谢!

Answer 1

如错误信息所述，a->q 和b->q，即class member access operator 的用法要求类型 MyNode 为 complete type .仅前向声明是不够的。

The following types are incomplete types:

• class type that has been declared (e.g. by forward declaration) but not defined;

和

Any of the following contexts requires class T to be complete:

• class member access operator applied to an expression of type T;

您可以将operator()的定义移到MyNode的定义之后，此时MyNode就完成了。例如

class MyNode;

class MyCompare {
  public:
    bool operator()(const MyNode *a, const MyNode *b) const;
};

class MyNode {

  public:

    double sum;    
    double q;

    StateNode *parent;
    std::priority_queue<MyNode, std::vector<MyNode>, MyCompare> children;
};

bool MyCompare::operator()(const MyNode *a, const MyNode *b) const {
    return a->q < b->q;
}