另一个'x未在此范围内声明'

Question

this is my first question here.

Writing some code, i receive this error from g++: "Entity was not declared in this scope", in this context:

#ifndef Psyco2D_GameManager_
#define Psyco2D_GameManager_

#include <vector>
#include "Entity.h"

namespace Psyco2D{
    class GameManager{J
    private:
        std::vector<Entity> entities;
    };
}

#endif

This is the content of Entity.h:

#ifndef Psyco2D_Entity_
#define Psyco2D_Entity_

#include <string>
#include "GameManager.h"
#include "EntityComponent.h"


namespace Psyco2D{

    class Entity{
        friend class GameManager;

    private:
        /* Identificatore */
        std::string _name;

        /* Components list */
        std::map<const std::string, EntityComponent*> components;

    protected:
        Entity(const std::string name);

    public:
        inline const std::string getName() const{
            return this->_name;
        }

        void addComponent(EntityComponent* component, const std::string name);

        EntityComponent* lookupComponent(const std::string name) const;

        void deleteComponent(const std::string name);

    };

}

#endif

If i use std::vector<class Entity> instead of std::vector<Entity> it works.

Why?

Thanks to all =)

Answer 1

The problem is you have a cyclic dependency. Take out #include "GameManager.h" in Entity.h , since you don't actually need it in that header. (Up-vote this answer , which first pointed it out.)

Note the guards are actually the problem; but don't take them out! You just need to minimize the includes you have, and declare (and not define) types when possible. Consider what happens when you include Entity.h : As some point it includes GameManager.h , which in turn includes Entity.h . At this point, Entity.h already has its header guard defined, so it skips the contents. Then the parsing of GameManager.h continues, where upon it runs into Entity , and rightfully complains it is not defined. (Indeed, this is still the process of including GameManager.h in the first inclusion of Entity.h , far before Entity is defined!)

Note your numerous edits demonstrate why it's important to post real code, not re-synthesized code. You need real details to get real answers.

---

_Old:

Entity is in the Psyco2D namespace. You need to specify that:

class GameManager{
private:
    std::vector<Psyco2D::Entity> entities;
};

Answer 2

Assuming the first snippet is part of GameManager.h you have a circular header dependency. I believe you can fix this by changing the GameManager.h include in Entity.h to class GameManager; instead.

Additionally as GMan noted, Entity is in a namespace and you need to qualify Entity with the namespace name.

Answer 3

删除Psyco2D命名空间，它将工作而不声明“类实体”。