What is an undefined reference/unresolved external symbol error and how do I fix it?
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.
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].
[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 Microsoft Visual Studio, 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 mechanisms exist for other compilers/ platforms.
Common error messages are error LNK2001 , error LNK1120 , error LNK2019 for Microsoft Visual Studio 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;
}
will 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 Microsoft Visual Studio :
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:
#pragma (Microsoft Visual Studio) UNICODE definitions Class members:
A pure virtual destructor needs an implementation.
Declaring a destructor pure still requires you to define it (unlike a regular function):
struct X
{
virtual ~X() = 0;
};
struct Y : X
{
~Y() {}
};
int main()
{
Y y;
}
//X::~X(){} //uncomment this line for successful definition
This happens because base class destructors are called when the object is destroyed implicitly, so a definition is required.
virtual methods must either be implemented or defined as pure.
This is similar to non- virtual methods with no definition, with the added reasoning that the pure declaration generates a dummy vtable and you might get the linker error without using the function:
struct X
{
virtual void foo();
};
struct Y : X
{
void foo() {}
};
int main()
{
Y y; //linker error although there was no call to X::foo
}
For this to work, declare X::foo() as pure:
struct X
{
virtual void foo() = 0;
};
Non- virtual class members
Some members need to be defined even if not used explicitly:
struct A
{
~A();
};
The following would yield the error:
A a; //destructor undefined
The implementation can be inline, in the class definition itself:
struct A
{
~A() {}
};
or outside:
A::~A() {}
If the implementation is outside the class definition, but in a header, the methods have to be marked as inline to prevent a multiple definition.
All used member methods need to be defined if used.
A common mistake is forgetting to qualify the name:
struct A
{
void foo();
};
void foo() {}
int main()
{
A a;
a.foo();
}
The definition should be
void A::foo() {}
static data members must be defined outside the class in a single translation unit :
struct X
{
static int x;
};
int main()
{
int x = X::x;
}
//int X::x; //uncomment this line to define X::x
An initializer can be provided for a static const data member of integral or enumeration type within the class definition; however, odr-use of this member will still require a namespace scope definition as described above. C++11 allows initialization inside the class for all static const data members.
Failure to link against appropriate libraries/object files or compile implementation files
Commonly, each translation unit will generate an object file that contains the definitions of the symbols defined in that translation unit. To use those symbols, you have to link against those object files.
Under gcc you would specify all object files that are to be linked together in the command line, or compile the implementation files together.
g++ -o test objectFile1.o objectFile2.o -lLibraryName
The libraryName here is just the bare name of the library, without platform-specific additions. So eg on Linux library files are usually called libfoo.so but you'd only write -lfoo . On Windows that same file might be called foo.lib , but you'd use the same argument. You might have to add the directory where those files can be found using -L‹directory› . Make sure to not write a space after -l or -L .
For XCode : Add the User Header Search Paths -> add the Library Search Path -> drag and drop the actual library reference into the project folder.
Under MSVS , files added to a project automatically have their object files linked together and a lib file would be generated (in common usage). To use the symbols in a separate project, you'd need to include the lib files in the project settings. This is done in the Linker section of the project properties, in Input -> Additional Dependencies . (the path to the lib file should be added in Linker -> General -> Additional Library Directories ) When using a third-party library that is provided with a lib file, failure to do so usually results in the error.
It can also happen that you forget to add the file to the compilation, in which case the object file won't be generated. In gcc you'd add the files to the command line. In MSVS adding the file to the project will make it compile it automatically (albeit files can, manually, be individually excluded from the build).
In Windows programming, the tell-tale sign that you did not link a necessary library is that the name of the unresolved symbol begins with __imp_ . Look up the name of the function in the documentation, and it should say which library you need to use. For example, MSDN puts the information in a box at the bottom of each function in a section called "Library".
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