c++ - 错误 LNK2001 和错误 LNK2019 (C++) -- 请求了解这些错误

Question

好吧，我刚刚完成了最后一个编译器错误(我是这么认为的)，然后出现了这些错误:

1>GameEngine.obj : error LNK2001: unresolved external symbol "public: static double WeaponsDB::PI" (?PI@WeaponsDB@@2NA)
1>Component.obj : error LNK2001: unresolved external symbol "public: static double WeaponsDB::PI" (?PI@WeaponsDB@@2NA)
1>Coordinate.obj : error LNK2019: unresolved external symbol "public: static double WeaponsDB::PI" (?PI@WeaponsDB@@2NA) referenced in function "public: double __thiscall Coordinate::distanceFrom(class Coordinate *)" (?distanceFrom@Coordinate@@QAENPAV1@@Z)
1>Driver.obj : error LNK2001: unresolved external symbol "public: static double WeaponsDB::PI" (?PI@WeaponsDB@@2NA)
1>Environment.obj : error LNK2001: unresolved external symbol "public: static double WeaponsDB::PI" (?PI@WeaponsDB@@2NA)
1>Environment.obj : error LNK2001: unresolved external symbol "public: static bool Environment::spyFlag" (?spyFlag@Environment@@2_NA)
1>Environment.obj : error LNK2001: unresolved external symbol "private: static class Environment * Environment::instance_" (?instance_@Environment@@0PAV1@A)
1>Environment.obj : error LNK2019: unresolved external symbol "public: static void __cdecl Environment::spyAlertOver(void)" (?spyAlertOver@Environment@@SAXXZ) referenced in function "public: void __thiscall Environment::notificationOfSpySuccess(void)" (?notificationOfSpySuccess@Environment@@QAEXXZ)
1>GameDriver.obj : error LNK2019: unresolved external symbol "public: static void __cdecl MainMenu::gameOver(int)" (?gameOver@MainMenu@@SAXH@Z) referenced in function "public: static void __cdecl GameDriver::run(void)" (?run@GameDriver@@SAXXZ)
1>GameDriver.obj : error LNK2019: unresolved external symbol "public: static void __cdecl GameDriver::gatherInput(void)" (?gatherInput@GameDriver@@SAXXZ) referenced in function "public: static void __cdecl GameDriver::run(void)" (?run@GameDriver@@SAXXZ)
1>GameDriver.obj : error LNK2019: unresolved external symbol "public: static void __cdecl GameDriver::ticker(void)" (?ticker@GameDriver@@SAXXZ) referenced in function "public: static void __cdecl GameDriver::run(void)" (?run@GameDriver@@SAXXZ)
1>GameDriver.obj : error LNK2001: unresolved external symbol "public: static int GameDriver::ticks" (?ticks@GameDriver@@2HA)
1>GameDriver.obj : error LNK2001: unresolved external symbol "public: static bool GameDriver::evaluatingInputFlag" (?evaluatingInputFlag@GameDriver@@2_NA)
1>GameDriver.obj : error LNK2001: unresolved external symbol "public: static bool GameDriver::keyQuitFlag" (?keyQuitFlag@GameDriver@@2_NA)
1>GameDriver.obj : error LNK2001: unresolved external symbol "public: static bool GameDriver::keyToggleWeaponRightFlag" (?keyToggleWeaponRightFlag@GameDriver@@2_NA)
1>GameDriver.obj : error LNK2001: unresolved external symbol "public: static bool GameDriver::keyToggleWeaponLeftFlag" (?keyToggleWeaponLeftFlag@GameDriver@@2_NA)
1>GameDriver.obj : error LNK2001: unresolved external symbol "public: static bool GameDriver::keyFireFlag" (?keyFireFlag@GameDriver@@2_NA)
1>GameDriver.obj : error LNK2001: unresolved external symbol "public: static bool GameDriver::keyLeftFlag" (?keyLeftFlag@GameDriver@@2_NA)
1>GameDriver.obj : error LNK2001: unresolved external symbol "public: static bool GameDriver::keyRightFlag" (?keyRightFlag@GameDriver@@2_NA)
1>GameDriver.obj : error LNK2001: unresolved external symbol "public: static bool GameDriver::keyUpFlag" (?keyUpFlag@GameDriver@@2_NA)
1>GameDriver.obj : error LNK2001: unresolved external symbol "public: static bool GameDriver::keyDownFlag" (?keyDownFlag@GameDriver@@2_NA)
1>GUI_Env.obj : error LNK2001: unresolved external symbol "private: static struct BITMAP * GUI_Env::buffer" (?buffer@GUI_Env@@0PAUBITMAP@@A)
1>GUI_Info.obj : error LNK2001: unresolved external symbol "private: static struct BITMAP * GUI_Info::buffer" (?buffer@GUI_Info@@0PAUBITMAP@@A)
1>MenuDriver.obj : error LNK2019: unresolved external symbol "public: static void __cdecl MainMenu::displayMenu(void)" (?displayMenu@MainMenu@@SAXXZ) referenced in function "public: static void __cdecl MenuDriver::start(void)" (?start@MenuDriver@@SAXXZ)
1>SpaceObjectFactory.obj : error LNK2001: unresolved external symbol "private: static class SpaceObjectFactory * SpaceObjectFactory::_instance" (?_instance@SpaceObjectFactory@@0PAV1@A)
1>Spy.obj : error LNK2019: unresolved external symbol "public: virtual bool __thiscall UnFormationable::sameTypeOfSpaceObjectAs(class SpaceObject *)" (?sameTypeOfSpaceObjectAs@UnFormationable@@UAE_NPAVSpaceObject@@@Z) referenced in function "public: virtual bool __thiscall Spy::sameTypeOfSpaceObjectAs(class SpaceObject *)" (?sameTypeOfSpaceObjectAs@Spy@@UAE_NPAVSpaceObject@@@Z)
1>WeaponsDB.obj : error LNK2001: unresolved external symbol "private: static class WeaponsDB * WeaponsDB::_instance" (?_instance@WeaponsDB@@0PAV1@A)
1>C:\Users\Owner\Desktop\Bosconian\code\Bosconian\Debug\Bosconian.exe : fatal error LNK1120: 23 unresolved externals

好的，这是一个简短的概述。

PI 是 WeaponsDB 中的静态常量，其他类使用 Wea​​ponsDB::PI 和适当的#include 引用(这有什么问题？)

大多数其他错误源于 allegro 游戏库中定时器的静态变量和静态方法。

是什么导致了这些错误，我该如何消除它们？

提前致谢

----------------编辑--------------------

根据要求，声明和定义了 WeaponsDB::PI。它在 WeaponsDB.h 中声明:

public:
    static double PI;

但它是在另一个类Driver.cpp中定义的:

WeaponsDB::PI = 4*atan(1.0);

如果这是我的代码的问题之一，我很想知道为什么会导致错误。

Answer 1

大多数情况下，当链接器无法检测到静态成员时，这是因为您忘记了在某处真正定义它，正如之前指出的那样:

// header
class X {
   static const int y;
};

// cpp
const int X::y = 1;

但在您的情况下，因为您不仅缺少静态变量而且还缺少所有其他成员，我敢打赌您没有链接项目中的实现文件 (.cpp)。您必须提供一个编译单元来定义 header 中声明的符号，并使环境编译并将其链接在一起。如果符号属于外部库(来自您当前项目)中的编译单元，那么您必须记得链接到该库。