c++ - 什么是 undefined reference /未解析的外部符号错误，我该如何解决？

Question

什么是 undefined reference /未解析的外部符号错误？常见原因是什么以及如何修复/预防它们？

Answer 1

编译 C++ 程序需要几个步骤，如 2.2 (credits to Keith Thompson for the reference) 所指定的那样:

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) ifnecessary. [SNIP]
2. Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines toform logical source lines. [SNIP]
3. The source file is decomposed into preprocessing tokens (2.5) and sequences of white-space characters (including comments). [SNIP]
4. Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed. [SNIP]
5. Each source character set member in a character literal or a string literal, as well as each escape sequence and universal-character-namein a character literal or a non-raw string literal, is converted tothe corresponding member of the execution character set; [SNIP]
6. Adjacent string literal tokens are concatenated.
7. White-space characters separating tokens are no longer significant. Each preprocessing token is converted into a token. (2.7). Theresulting tokens are syntactically and semantically analyzed andtranslated as a translation unit. [SNIP]
8. Translated translation units and instantiation units are combined as follows: [SNIP]
9. All external entity references are resolved. Library components are linked to satisfy external references to entities not defined in thecurrent translation. All such translator output is collected into aprogram image which contains information needed for execution in itsexecution environment. (emphasis mine)

[footnote] Implementations must behave as if these separate phases occur, although in practice different phases might be folded together.

指定的错误发生在编译的最后阶段，通常称为链接。它基本上意味着您将一堆实现文件编译成目标文件或库，现在您想让它们一起工作。

假设您在 a.cpp 中定义了符号 a。现在，b.cpp 声明了那个符号并使用了它。在链接之前，它只是假定该符号是在某处 定义的，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(好吧，实际上链接到使用它的对象或库)。

如果您使用的是 Microsoft Visual Studio，您会看到项目生成 .lib 文件。其中包含一个导出符号表和一个导入符号表。导入的符号根据您链接的库进行解析，导出的符号提供给使用该 .lib(如果有)的库。

其他编译器/平台也存在类似的机制。

常见的错误消息是 error LNK2001、error LNK1120、error LNK2019 for Microsoft Visual Studio 和 GCC 的 undefined reference symbolName。

代码:

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;
}

将在 GCC 中产生以下错误:

/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

以及 Microsoft Visual Studio 的类似错误:

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

常见原因包括:

• Failure to link against appropriate libraries/object files or compile implementation files
• Declared and undefined variable or function.
• Common issues with class-type members
• Template implementations not visible.
• Symbols were defined in a C program and used in C++ code.
• Incorrectly importing/exporting methods/classes across modules/dll. (MSVS specific)
• Circular library dependency
• undefined reference to `WinMain@16'
• Interdependent library order
• Multiple source files of the same name
• Mistyping or not including the .lib extension when using the #pragma (Microsoft Visual Studio)
• Problems with template friends
• Inconsistent UNICODE definitions
• Missing "extern" in const variable declarations/definitions (C++ only)
• Visual Studio Code not configured for a multiple file project