C2039：类不是命名空间的成员

Question

Mage/Interface/Context.h

#pragma once

#include <Mage/Interface/Element.h>
#include <Mage/Renderer/RenderingContext.h>
#include <Mage/Renderer/VertexBuffer.h>

#include <glm/glm.hpp>

namespace Mage {
    namespace Interface {
        class Context {

        protected:
            RenderingContext* ctx;
            VertexBuffer* vbo;
            glm::mat4 projection;
            Mage::Interface::Frame* uiParent;

        public:
            Context(RenderingContext* ctx);
            ~Context();

            void render();
            Mage::Interface::Frame* createFrame();
        };
    }
}

Mage/Interface/Element.h

#pragma once

#include <vector>

#include <Mage/Interface/Context.h>

#include <glm/glm.hpp>

namespace Mage {
    namespace Interface {
        class Element {

        protected:
            Mage::Interface::Context* ctx;
            std::vector<Element*> children;
            glm::vec3 position;
            float scale;

        public:
            virtual void draw();

            void attach(Element* child) {
                this->children.push_back(child);
            }

            inline glm::vec3 getPosition() {
                return this->position;
            }

            float getScale() {
                return this->scale;
            }
        };

        // Frame is an untextured, single colour quad. Frame may contain other
        // Elements.
        class Frame : public Element {

        public:
            Frame();
            Frame(glm::vec3 pos);
            Frame(float width, float height);
            Frame(glm::vec3 pos, float width, float height);
        };
    }
}

这给了我以下错误：

Error   C2039   'Context': is not a member of 'Mage::Interface' Mage2D  c:\users\jesse\documents\visual studio 2015\projects\mage2d\include\mage\interface\element.h    14

Error   C2238   unexpected token(s) preceding ';'   Mage2D  c:\users\jesse\documents\visual studio 2015\projects\mage2d\include\mage\interface\element.h    14

Error   C2143   syntax error: missing ';' before '*'    Mage2D  c:\users\jesse\documents\visual studio 2015\projects\mage2d\include\mage\interface\element.h    14

Error   C4430   missing type specifier - int assumed. Note: C++ does not support default-int    Mage2D  c:\users\jesse\documents\visual studio 2015\projects\mage2d\include\mage\interface\element.h    14

当我取出Mage::Interface::Context* ctx 时，代码编译得很好。我想我一定漏掉了一个分号，但我看不到它——我觉得这一切都很好。

Answer 1

你有一个循环依赖。 Element.h 包含 Context.h，而 Context.h 包含 Element.h，那是行不通的。

解决这个问题的方法是前向声明类型，而不是尽可能包含它们的标头，这也将减少编译时间。