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？

最佳答案

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) if necessary. [SNIP]
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. 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-name in a character literal or a non-raw string literal, is converted to the 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). The resulting tokens are syntactically and semantically analyzed and translated 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 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)

[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

关于c++ - 什么是 undefined reference /未解析的外部符号错误，我该如何解决？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/47835085/