如何解决“error LNK2019：unresolved external symbol”？ [英] How can I resolve "error LNK2019: unresolved external symbol"?

问题描述

我有这个MFC应用程序，我工作，需要有一个嵌入式数据库。所以我去寻找一个漂亮，快速的可嵌入数据库，并且偶然碰到SQLite。

我创建了一个数据库，我创建了一个静态库项目

在库项目中，我创建了一个类 DBClass 使用方法 AddFeedToDB（CFeed f）。库项目使用来自codeproject（ cppsqlite3.lib ）的 .lib 文件。

当编译静态库时，没有检测到错误，但是当我尝试在主项目中使用库项目文件时，我得到这些类型的错误：

 错误LNK2019：未解析的外部符号public：void __thiscall 
 CppSQLite3DB :: close（void）（？close @ CppSQLite3DB @@ QAEXXZ 
引用in functionpublic：int __thiscall 
 CTalkingFeedsDB :: AddFeedToDB（class CFeed，char const *）（？
 AddFeedToDB @ CTalkingFeedsDB @@ QAEHVCFeed @@ PDB @ Z 
 
解决方案
 
发生在我不止一次，我认为符号 XXX （即？close @ CppSQLite3DB @@ QAEXXZ ） 在导入库中，而实际符号是 __ impXXX （即 __ imp？close @ CppSQLite3DB @@ QAEXXZ ）。
 
 
 
然后在编译步骤中找到链接器错误的原因：编译器将生成？close @ CppSQLite3DB @@ QAEXXZ 符号，它应该 生成 __ imp？这通常意味着函数声明本身没有 __ declspec（dllimport）。这可能是由一些预处理器符号未定义引起的。或 __ declspec 根本不存在... 
 
I've got this MFC application I'm working on that needs to have an embedded database. So I went hunting for a slick, fast "embeddable" database for it and stumbled accross SQLite.

I created a DB with it, and I created a static library project with Visual Studio 2008. the library project will be used in another main project.

In the library project, I created a class DBClass with a method AddFeedToDB(CFeed f). The library project uses the .lib file from codeproject (cppsqlite3.lib).

When compiling the static library, no error is detected, but when I try to use the library project file in the main project, I get these type of errors:
error LNK2019: unresolved external symbol "public:void __thiscall
   CppSQLite3DB::close(void)" (?close@CppSQLite3DB@@QAEXXZ 
   referenced in function "public: int __thiscall
   CTalkingFeedsDB::AddFeedToDB(class CFeed,char const*)" (?
   AddFeedToDB@CTalkingFeedsDB@@QAEHVCFeed@@PDB@Z
What am I missing?
解决方案
It happened to me more than once that I thought symbol XXX (i.e. ?close@CppSQLite3DB@@QAEXXZ) was in the import lib, while the actual symbol was __impXXX (i.e. __imp?close@CppSQLite3DB@@QAEXXZ).

The reason for the linker error is then to be found in the compilation step: the compiler will generate the ?close@CppSQLite3DB@@QAEXXZ symbol to be imported, where it should generate __imp?close@CppSQLite3DB@@QAEXXZ. This often means that the function declaration itself didn't have __declspec( dllimport ). Which may be caused by some preprocessor symbol not being defined. Or the __declspec not being there at all...

                        
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