什么是未定义的引用/未解析的外部符号错误，如何解决它？ [英] What is an undefined reference/unresolved external symbol error and how do I fix it?

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问题描述

什么是未定义的引用/未解析的外部符号错误？什么是常见原因，以及如何修复/防止它们？



随意编辑/添加您自己的 b

解决方案

根据 2.2


语法中的优先级


  1. 映射物理源文件字符，如果
    是必需的，则以实现定义的方式将其转换为基本源字符集
    （引入用于行结束指示符的新行字符）。 [SNIP]

  2. 每个反斜杠字符（\）紧接着一个新行字符的实例都被删除，将实际源代码行拼接到
    形成逻辑源线。 [SNIP]

  3. 源文件被分解为预处理令牌（2.5）和空格字符序列（包括注释）。 [SNIP]

  4. 执行预处理指令，扩展宏调用，并执行_Pragma一元运算符表达式。 [SNIP]

  5. 字符文字或字符串文字中的每个源字符集成员以及每个转义序列和通用字符名
    在字符文字或非原始字符串文字中，转换为
    执行字符集的相应成员;

    [SNIP]

  6. 相邻的字符串字符串字符串。

  7. 分隔符号的白色空格字符不再重要。每个预处理令牌都将转换为令牌。 （2.7）。
    产生的令牌在语法和语义上被分析，
    被翻译为翻译单元。 [SNIP]

  8. 翻译单位和实例化单位合并如下： [SNIP]

  9. 所有外部实体参考都已解决。链接库组件以满足对未在
    当前翻译中定义的实体的外部引用。所有这样的翻译器输出被收集到
    程序映像中，其包含在其
    执行环境中执行所需的信息。    （强调我）

[footnote] 实施必须表现为发生这些单独的阶段，但实际上不同的阶段可能会折叠在一起。


< blockquote>

指定的错误发生在编译的最后阶段，通常称为链接。它基本上意味着你编译一堆实现文件到目标文件或库，现在你想让他们一起工作。



说你定义的符号 a a.cpp 中的 a 。现在， b.cpp 已声明该符号并使用它。在链接之前，它只是假定该符号被定义在某处，但它并不关心在哪里。链接阶段负责找到符号并将其正确链接到 b.cpp （实际上是使用它的对象或库）。



如果你使用MSVS，你会看到项目生成 .lib 文件。它们包含导出符号表和导入符号表。导入的符号将根据链接的库进行解析，并且为使用 .lib （如果有）的库提供导出的符号。



其他编译器/平台也有类似的机制。



常见的错误讯息是错误LNK2001 MSVS 和的，错误LNK1120 ，错误LNK2019



代码：


$ b $ 

  struct X 
 {
 virtual void foo（）; 
}; 
 struct Y：X 
 {
 void foo（）{} 
}; 
 struct A 
 {
 virtual〜A（）= 0; 
}; 
 struct B：A 
 {
 virtual〜B（）{} 
}; 
 extern int x; 
 void foo（）; 
 int main（）
 {
 x = 0; 
 foo（）; 
 Y y; 
 B b; 
}

会使用 gcc 产生以下错误： / p> 

  /home/AbiSfw/ccvvuHoX.o：在main函数中：
 prog.cpp :(。 0x10）：未定义引用`x'
 prog.cpp :(。text + 0x19）：未定义引用`foo（）`
 prog.cpp :(。+ 0x2d）：未定义引用`A ::〜A（）'
 /home/AbiSfw/ccvvuHoX.o：在函数`B ::〜B（）'中：
 prog.cpp :( .text._ZN1BD1Ev [B： ：〜B（）] + 0xb）：未定义引用`A ::〜A（）'
 /home/AbiSfw/ccvvuHoX.o：在函数`B ::〜B b prog.cpp :( .text._ZN1BD0Ev [B ::〜B（）] + 0x12）：未定义引用`A ::〜A（）'
 /home/AbiSfw/ccvvuHoX.o :(。 rodata._ZTI1Y [typeinfo for Y] + 0x8）：未定义引用`typeinfo for X'
 /home/AbiSfw/ccvvuHoX.o:(.rodata._ZTI1B[typeinfo for B] + 0x8）：未定义引用`typeinfo for A'
 collect2：ld返回1退出状态

strong> MSVS ：

  1> test2.obj：error LNK2001：未解析的外部符号void __cdecl foo ）（？foo @@ YAXXZ）
 1> test2.obj：error LNK2001：未解析的外部符号int x（？x @@ 3HA）
 1> test2.obj：error LNK2001：unresolved外部符号public：virtual __thiscall A ::〜A（void）（?? 1A @@ UAE @ XZ）
 1> test2.obj：error LNK2001：unresolved external symbolpublic：virtual void __thiscall X： ：foo（void）（？foo @ X @@ UAEXXZ）
 1> ... \test2.exe：致命错误LNK1120：4未解决的外部
  pre> 
 
 
常见原因包括：
 
 
  
What are undefined reference/unresolved external symbol errors? What are common causes and how to fix/prevent them?



Feel free to edit/add your own.
 解决方案 
Compiling a C++ program takes place in several steps, as specified by 2.2 (credits to Keith Thompson for the reference):

  
The precedence among the syntax rules of translation is specified by the following phases [see footnote].
    
  
  1. Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set
  (introducing new-line characters for end-of-line indicators) if
  necessary. [SNIP]
    2. 
  
  2. Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines to
  form logical source lines. [SNIP]
    3. 
  
  3. The source file is decomposed into preprocessing tokens (2.5) and sequences of white-space characters (including comments). [SNIP]
    4. 
  
  4. Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed. [SNIP]
    5. 
  
  5. Each source character set member in a character literal or a string literal, as well as each escape sequence and universal-character-name
  in a character literal or a non-raw string literal, is converted to
  the corresponding member of the execution character set; [SNIP]
    6. 
  
  6. Adjacent string literal tokens are concatenated.
    7. 
  
  7. White-space characters separating tokens are no longer significant. Each preprocessing token is converted into a token. (2.7). The
  resulting tokens are syntactically and semantically analyzed and
  translated as a translation unit. [SNIP]
    8. 
  
  8. Translated translation units and instantiation units are combined as follows: [SNIP]
    9. 
  
  9. All external entity references are resolved. Library components are linked to satisfy external references to entities not defined in the
  current translation. All such translator output is collected into a
  program image which contains information needed for execution in its
  execution environment. (emphasis mine)
    10. 
  
[footnote] Implementations must behave as if these separate phases occur, although in practice different phases might be folded together.


The specified errors occur during this last stage of compilation, most commonly referred to as linking. It basically means that you compiled a bunch of implementation files into object files or libraries and now you want to get them to work together.



Say you defined symbol a in a.cpp. Now, b.cpp declared that symbol and used it. Before linking, it simply assumes that that symbol was defined somewhere, but it doesn't yet care where. The linking phase is responsible for finding the symbol and correctly linking it to b.cpp (well, actually to the object or library that uses it).



If you're using MSVS, you'll see that projects generate .lib files. These contain a table of exported symbols, and a table of imported symbols. The imported symbols are resolved against the libraries you link against, and the exported symbols are provided for the libraries that use that .lib (if any).



Similar mechanism exist for other compilers/platforms.



Common error messages are error LNK2001, error LNK1120, error LNK2019 for MSVS and undefined reference to symbolName for gcc.



The code:
struct X
{
   virtual void foo();
};
struct Y : X
{
   void foo() {}
};
struct A
{
   virtual ~A() = 0;
};
struct B: A
{
   virtual ~B(){}
};
extern int x;
void foo();
int main()
{
   x = 0;
   foo();
   Y y;
   B b;
}

would generate the following errors with gcc:
/home/AbiSfw/ccvvuHoX.o: In function `main':
prog.cpp:(.text+0x10): undefined reference to `x'
prog.cpp:(.text+0x19): undefined reference to `foo()'
prog.cpp:(.text+0x2d): undefined reference to `A::~A()'
/home/AbiSfw/ccvvuHoX.o: In function `B::~B()':
prog.cpp:(.text._ZN1BD1Ev[B::~B()]+0xb): undefined reference to `A::~A()'
/home/AbiSfw/ccvvuHoX.o: In function `B::~B()':
prog.cpp:(.text._ZN1BD0Ev[B::~B()]+0x12): undefined reference to `A::~A()'
/home/AbiSfw/ccvvuHoX.o:(.rodata._ZTI1Y[typeinfo for Y]+0x8): undefined reference to `typeinfo for X'
/home/AbiSfw/ccvvuHoX.o:(.rodata._ZTI1B[typeinfo for B]+0x8): undefined reference to `typeinfo for A'
collect2: ld returned 1 exit status

and similar errors with MSVS:
1>test2.obj : error LNK2001: unresolved external symbol "void __cdecl foo(void)" (?foo@@YAXXZ)
1>test2.obj : error LNK2001: unresolved external symbol "int x" (?x@@3HA)
1>test2.obj : error LNK2001: unresolved external symbol "public: virtual __thiscall A::~A(void)" (??1A@@UAE@XZ)
1>test2.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall X::foo(void)" (?foo@X@@UAEXXZ)
1>...\test2.exe : fatal error LNK1120: 4 unresolved externals

Common causes include:




                        
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