如何避免“多重定义"?全局常数错误?

本文介绍了如何避免“多重定义"?全局常数错误?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我正在使用Windows API编写C程序.每个主要功能都有其自己的文件，并且原型具有一个标头，并包含诸如此类的内容:

I'm writing a C program using the Windows API. Each major function has its own file, and there is one header for the prototypes and includes and whatnot:

// Headers & global constants #pragma once #define _WIN32_LEAN_AND_MEAN #include <Windows.h> #include <WindowsX.h> #include <Windef.h> #define szClassName TEXT("EthicsPresentationWnd") // Prototypes LRESULT CALLBACK WindowProc(HWND hWnd, UINT Msg, WPARAM wParam, LPARAM lParam); BOOL CALLBACK FontProc1(HWND hWnd, LPARAM lParam); int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd); ATOM RegisterWindow(HINSTANCE hInstance);

让我烦恼的是#define szClassName行.我不喜欢使用宏，而是希望有一个适当的全局变量wchar_t szClassName[]，但是如果这样做，链接器会抱怨每个包含标头的模块中的乘法定义变量.

The thing I'm irritated about is the #define szClassName line. I dislike using macros and would prefer to have a proper global variable, wchar_t szClassName[], but if I do that then the linker complains about multiply defined variables in each of the modules that include the header.

我以为#pragma once指令可以阻止这种情况，但事实并非如此.

I thought the #pragma once directive would prevent this, but it didn't.

这个问题有解决方案吗?

Is there any solution to this problem?

推荐答案

解决方案是使用单独的声明和定义...

The solution to this is to have a separate declaration and definition...

标题(* .h；对不起，我不知道WinAPI类型名称，请根据需要进行调整):

Header (*.h; sorry, I don't know WinAPI type names, adapt as necessary):

extern const char szClassName[];

实施(* .c或* .cpp)

Implementation (*.c or *.cpp)

const char szClassName[] = "hello, world"

您正在看到问题，因为每次您的* .c或* .cpp文件之一包含标头(即使包含include防护！)时，都会声明一个新符号szClassName；这会使链接器感到困惑(请参阅下文).

You're seeing the problem because a new symbol szClassName is being declared each time one of your *.c or *.cpp files includes the header (even with the include guards!); and that makes the linker confused (see below).

请注意，这将使sizeof(szClassName)不再起作用.

Do note that this will make sizeof(szClassName) not work anymore.

预处理后，编译器基本上会看到以下内容:

After preprocessing, the compiler is basically seeing this:

• 文件"a.c":const char someSymbol[] = <some text, don't care what right now>;
• 文件"b.c":const char someSymbol[] = <some text, don't care if it's the same>;
• 文件"c.c":const char someSymbol[] = <some text, ditto>;

• file "a.c": const char someSymbol[] = <some text, don't care what right now>;
• file "b.c": const char someSymbol[] = <some text, don't care if it's the same>;
• file "c.c": const char someSymbol[] = <some text, ditto>;

当链接器链接目标文件(例如，"a.obj"，"b.obj"和"c.obj")时，它会看到使用新值(至少就链接器而言)---并因此失败并显示错误.

When the linker is linking the object files (say, "a.obj", "b.obj" and "c.obj"), it sees the same symbol being defined with a new value (at least as far as the linker is concerned) --- and thus it fails with an error.