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The linker generates an error when it can’t resolve an external symbol. It means the linker couldn’t find a matching exported symbol definition in any of the linked files.In my case, set the cpp file to “C/C++ compiler” in “property”->”general”, resolve the LNK2019 error.The compiler can identify when a symbol isn’t declared, but it can’t tell when the symbol isn’t defined. That’s because the definition may be in a different source file or library. If a symbol is referred to but never defined, the linker generates an unresolved external symbol error.
How do I fix unresolved external symbol LNK2019?
In my case, set the cpp file to “C/C++ compiler” in “property”->”general”, resolve the LNK2019 error.
What is LNK2019 unresolved external symbol?
The compiler can identify when a symbol isn’t declared, but it can’t tell when the symbol isn’t defined. That’s because the definition may be in a different source file or library. If a symbol is referred to but never defined, the linker generates an unresolved external symbol error.
How do I fix LNK1120?
The LNK1120 message comes last, and shows the unresolved symbol error count. You don’t need to fix this error. This error goes away when you correct all of the LNK2001 and LNK2019 linker errors before it in the build output.
How do I fix error lnk2001?
If you are from IDE, right click on the project name from Solution Explore. Then choose Add, Existing Item…. Add one. cpp into your project.
What kind of an error is an unresolved external reference?
Answer. Unresolved external references occur when the symbol for a function or global variable is referenced in a program, but none of the object files or libraries specified in the link step contain a definition for that symbol.
What is linking error in Turbo C?
It means that you have not enabled the graphics library for linking. By default this setting is OFF when you install Turbo C++ version 3.0.
How do you fix undefined symbol in C?
To allow undefined symbols, as in the previous example, use the link-editor’s -z nodefs option to suppress the default error condition. Take care when using the -z nodefs option. If an unavailable symbol reference is required during the execution of a process, a fatal runtime relocation error occurs.
How do I fix my lnk2005?
This error can occur when a header file defines a function that isn’t inline . If you include this header file in more than one source file, you get multiple definitions of the function in the executable. To fix this issue, move the member function definitions inside the class.
How do I fix unresolved external symbol in C++?
So when we try to assign it a value in the main function, the linker doesn’t find the symbol and may result in an “unresolved external symbol” or “undefined reference”. The way to fix this error is to explicitly scope the variable using ‘::’ outside the main before using it.
How do I fix lnk1168?
To fix this issue, verify that the filename file handle is not locked, and that you have write permission for the file. If it is an executable, verify that it is not already running. You can use the Windows SysInternals utilities Handle or Process Explorer to determine which process has a file handle lock on filename .
How do I fix unresolved external symbol in C++?
So when we try to assign it a value in the main function, the linker doesn’t find the symbol and may result in an “unresolved external symbol” or “undefined reference”. The way to fix this error is to explicitly scope the variable using ‘::’ outside the main before using it.
How do I fix my lnk2005?
This error can occur when a header file defines a function that isn’t inline . If you include this header file in more than one source file, you get multiple definitions of the function in the executable. To fix this issue, move the member function definitions inside the class.
How do I fix error lnk1104?
To fix this issue, stop the program and unload it from the debugger before building it again. If the app is open in another program, such as a resource editor, close it. If your program is unresponsive, you may need to use Task Manager to end the process. You might also need to close and restart Visual Studio.
Linker Tools Error LNK2001 | Microsoft Docs
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What is an unresolved external symbol
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Use the decorated name to find the error
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Azure – Web Application Firewall
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- Summary of article content: Articles about Azure – Web Application Firewall Remove your setting of the entry point to mainWCRTStartup. This is the wrong name. For a well formed C++ Console application project you can … …
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visual c++ – error LNK2001: unresolved external symbol (C++) – Stack Overflow
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- Summary of article content: Articles about visual c++ – error LNK2001: unresolved external symbol (C++) – Stack Overflow That means that the definition of your function is not present in your program. You forgot to add that one.cpp to your program. …
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c++ – How can I solve the error LNK2019: unresolved external symbol – function? – Stack Overflow
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Linker Tools Error LNK2019 | Microsoft Docs
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Linker tools error LNK1120 | Microsoft Docs
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error LNK2001: unresolved external symbol
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- Summary of article content: Articles about error LNK2001: unresolved external symbol error LNK2001: unresolved external symbol. Khi lập trình bằng VC2008 chia ra 2 file header với cpp nhiều lúc nó hay hiện lên cái lỗi … …
- Most searched keywords: Whether you are looking for error LNK2001: unresolved external symbol error LNK2001: unresolved external symbol. Khi lập trình bằng VC2008 chia ra 2 file header với cpp nhiều lúc nó hay hiện lên cái lỗi … header, mình, unresolved, người, chút, kinh, lnk2001, nghiệm, code, chung, nhiều, bằng, vc2008, class, file, trình, symbol, external, hiện, error, chiaKhi lập trình bằng VC2008 class chia ra 2 file header với cpp nhiều lúc nó hay hiện lên cái lỗi này. Gộp lại, bỏ chung vô cái header nó lại hết. Mọi người có ai gặp lỗi này cho mình chút kinh nghiệm với. Code dài mà đem gộp lại, lúc đọc thì có mà oải…
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error LNK2001 unresolved external symbol
error LNK2001 unresolved external symbol
error LNK2001: unresolved external symbol _SCardEstablishContext@16 – CodeProject
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- Summary of article content: Articles about error LNK2001: unresolved external symbol _SCardEstablishContext@16 – CodeProject The error indicates that your project is not linked with the module containing the function SCardEstablishContext . …
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Windows build script gives error unresolved external symbol
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Why do I receive a “LINK : error LNK2001: unresolved external symbol mexFunction” error when I attempt to mex a C++… –
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LNK2001: unresolved external symbol maincrtstartup | ProgrammerAH
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Linker Tools Error LNK2001
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Article
10/25/2021
10 minutes to read
7 contributors
In this article
unresolved external symbol “symbol”
The compiled code makes a reference or call to symbol. The symbol isn’t defined in any libraries or object files searched by the linker.
This error message is followed by fatal error LNK1120. To fix error LNK1120, first fix all LNK2001 and LNK2019 errors.
There are many ways to get LNK2001 errors. All of them involve a reference to a function or variable that the linker can’t resolve, or find a definition for. The compiler can identify when your code doesn’t declare a symbol, but not when it doesn’t define one. That’s because the definition may be in a different source file or library. If your code refers to a symbol, but it’s never defined, the linker generates an error.
What is an unresolved external symbol?
A symbol is the internal name for a function or global variable. It’s the form of the name used or defined in a compiled object file or library. A global variable is defined in the object file where storage is allocated for it. A function is defined in the object file where the compiled code for the function body is placed. An external symbol is one referenced in one object file, but defined in a different library or object file. An exported symbol is one that’s made publicly available by the object file or library that defines it.
To create an application or DLL, every symbol used must have a definition. The linker must resolve, or find the matching definition for, every external symbol referenced by each object file. The linker generates an error when it can’t resolve an external symbol. It means the linker couldn’t find a matching exported symbol definition in any of the linked files.
Compilation and link issues
This error can occur:
When the project is missing a reference to a library (.LIB) or object (.OBJ) file. To fix this issue, add a reference to the required library or object file to your project. For more information, see lib Files as linker input.
When the project has a reference to a library (.LIB) or object (.OBJ) file that in turn requires symbols from another library. It may happen even if you don’t call functions that cause the dependency. To fix this issue, add a reference to the other library to your project. For more information, see Understanding the classical model for linking: Taking symbols along for the ride.
If you use the /NODEFAULTLIB or /Zl options. When you specify these options, libraries that contain required code aren’t linked into the project unless you’ve explicitly included them. To fix this issue, explicitly include all the libraries you use on the link command line. If you see many missing CRT or Standard Library function names when you use these options, explicitly include the CRT and Standard Library DLLs or library files in the link.
If you compile using the /clr option. There may be a missing reference to .cctor . For more information on how to fix this issue, see Initialization of mixed assemblies.
If you link to the release mode libraries when building a debug version of an application. Similarly, if you use options /MTd or /MDd or define _DEBUG and then link to the release libraries, you should expect many potential unresolved externals, among other problems. Linking a release mode build with the debug libraries also causes similar problems. To fix this issue, make sure you use the debug libraries in your debug builds, and retail libraries in your retail builds.
If your code refers to a symbol from one library version, but you link a different version of the library. Generally, you can’t mix object files or libraries that are built for different versions of the compiler. The libraries that ship in one version may contain symbols that can’t be found in the libraries included with other versions. To fix this issue, build all the object files and libraries with the same version of the compiler before linking them together. For more information, see C++ binary compatibility between Visual Studio versions.
If library paths are out of date. The Tools > Options > Projects > VC++ Directories dialog, under the Library files selection, allows you to change the library search order. The Linker folder in the project’s Property Pages dialog box may also contain paths that could be out of date.
When a new Windows SDK is installed (perhaps to a different location). The library search order must be updated to point to the new location. Normally, you should put the path to new SDK include and lib directories in front of the default Visual C++ location. Also, a project containing embedded paths may still point to old paths that are valid, but out of date. Update the paths for new functionality added by the new version that’s installed to a different location.
If you build at the command line, and have created your own environment variables. Verify that the paths to tools, libraries, and header files go to a consistent version. For more information, see Use the MSVC toolset from the command line.
Coding issues
This error can be caused by:
Mismatched case in your source code or module-definition (.def) file. For example, if you name a variable var1 in one C++ source file and try to access it as VAR1 in another, this error is generated. To fix this issue, use consistently spelled and cased names.
A project that uses function inlining. It can occur when you define the functions as inline in a source file, rather than in a header file. Inlined functions can’t be seen outside the source file that defines them. To fix this issue, define the inlined functions in the headers where they’re declared.
Calling a C function from a C++ program without using an extern “C” declaration for the C function. The compiler uses different internal symbol naming conventions for C and C++ code. The internal symbol name is what the linker looks for when resolving symbols. To fix this issue, use an extern “C” wrapper around all declarations of C functions used in your C++ code, which causes the compiler to use the C internal naming convention for those symbols. Compiler options /Tp and /Tc cause the compiler to compile files as C++ or C, respectively, no matter what the filename extension is. These options can cause internal function names different from what you expect.
An attempt to reference functions or data that don’t have external linkage. In C++, inline functions and const data have internal linkage unless explicitly specified as extern . To fix this issue, use explicit extern declarations on symbols referred to outside the defining source file.
A missing function body or variable definition. This error is common when you declare, but don’t define, variables, functions, or classes in your code. The compiler only needs a function prototype or extern variable declaration to generate an object file without error, but the linker can’t resolve a call to the function or a reference to the variable because there’s no function code or variable space reserved. To fix this issue, make sure to define every referenced function and variable in a source file or library you link.
A function call that uses return and parameter types or calling conventions that don’t match the ones in the function definition. In C++ object files, Name decoration encodes the calling convention, class or namespace scope, and return and parameter types of a function. The encoded string becomes part of the final decorated function name. This name is used by the linker to resolve, or match, calls to the function from other object files. To fix this issue, make sure the function declaration, definition, and calls all use the same scopes, types, and calling conventions.
C++ code you call, when you include a function prototype in a class definition, but don’t include the implementation of the function. To fix this issue, be sure to provide a definition for all class members you call.
An attempt to call a pure virtual function from an abstract base class. A pure virtual function has no base class implementation. To fix this issue, make sure all called virtual functions are implemented.
Trying to use a variable declared within a function (a local variable) outside the scope of that function. To fix this issue, remove the reference to the variable that isn’t in scope, or move the variable to a higher scope.
When you build a Release version of an ATL project, producing a message that CRT startup code is required. To fix this issue, do one of the following, Remove _ATL_MIN_CRT from the list of preprocessor defines to allow CRT startup code to be included. For more information, see General property page (Project). If possible, remove calls to CRT functions that require CRT startup code. Instead, use their Win32 equivalents. For example, use lstrcmp instead of strcmp . Known functions that require CRT startup code are some of the string and floating point functions.
Consistency issues
There’s currently no standard for C++ name decoration between compiler vendors, or even between different versions of the same compiler. Object files compiled with different compilers may not use the same naming scheme. Linking them can cause error LNK2001.
Mixing inline and non-inline compile options on different modules can cause LNK2001. If a C++ library is created with function inlining turned on (/Ob1 or /Ob2) but the corresponding header file describing the functions has inlining turned off (no inline keyword), this error occurs. To fix this issue, define the functions inline in the header file you include in other source files.
If you use the #pragma inline_depth compiler directive, make sure you’ve set a value of 2 or greater, and make sure you also use the /Ob1 or /Ob2 compiler option.
This error can occur if you omit the LINK option /NOENTRY when you create a resource-only DLL. To fix this issue, add the /NOENTRY option to the link command.
This error can occur if you use incorrect /SUBSYSTEM or /ENTRY settings in your project. For example, if you write a console application and specify /SUBSYSTEM:WINDOWS, an unresolved external error is generated for WinMain . To fix this issue, make sure you match the options to the project type. For more information on these options and entry points, see the /SUBSYSTEM and /ENTRY linker options.
Exported .def file symbol issues
This error occurs when an export listed in a .def file isn’t found. It could be because the export doesn’t exist, is spelled incorrectly, or uses C++ decorated names. A .def file doesn’t take decorated names. To fix this issue, remove unneeded exports, and use extern “C” declarations for exported symbols.
Use the decorated name to find the error
The C++ compiler and linker use Name Decoration, also known as name-mangling. Name decoration encodes extra information about the type of a variable in its symbol name. The symbol name for a function encodes its return type, parameter types, scope, and calling convention. This decorated name is the symbol name the linker searches for to resolve external symbols.
A link error can result if the declaration of a function or variable doesn’t exactly match the definition of the function or variable. That’s because any difference becomes part of the symbol name to match. The error can happen even if the same header file is used in both the calling code and the defining code. One way it may occur is if you compile the source files by using different compiler flags. For example, if your code is compiled to use the __vectorcall calling convention, but you link to a library that expects clients to call it using the default __cdecl or __fastcall calling convention. In this case, the symbols don’t match because the calling conventions are different.
To help you find the cause, the error message shows you two versions of the name. It displays both the “friendly name,” the name used in source code, and the decorated name (in parentheses). You don’t need to know how to interpret the decorated name. You can still search for and compare it with other decorated names. Command-line tools can help to find and compare the expected symbol name and the actual symbol name:
The /EXPORTS and /SYMBOLS options of the DUMPBIN command-line tool are useful here. They can help you discover which symbols are defined in your .dll and object or library files. You can use the symbols list to verify that the exported decorated names match the decorated names the linker searches for.
In some cases, the linker can only report the decorated name for a symbol. You can use the UNDNAME command-line tool to get the undecorated form of a decorated name.
Additional resources
For more information, see the Stack Overflow question “What is an undefined reference/unresolved external symbol error and how do I fix it?”.
How can I solve the error LNK2019: unresolved external symbol – function?
One option would be to include function.cpp in your UnitTest1 project, but that may not be the most ideal solution structure. The short answer to your problem is that when building your UnitTest1 project, the compiler and linker have no idea that function.cpp exists, and also have nothing to link that contains a definition of multiple . A way to fix this is making use of linking libraries.
Since your unit tests are in a different project, I’m assuming your intention is to make that project a standalone unit-testing program. With the functions you are testing located in another project, it’s possible to build that project to either a dynamically or statically linked library. Static libraries are linked to other programs at build time, and have the extension .lib , and dynamic libraries are linked at runtime, and have the extension .dll . For my answer I’ll prefer static libraries.
You can turn your first program into a static library by changing it in the projects properties. There should be an option under the General tab where the project is set to build to an executable ( .exe ). You can change this to .lib . The .lib file will build to the same place as the .exe .
In your UnitTest1 project, you can go to its properties, and under the Linker tab in the category Additional Library Directories, add the path to which MyProjectTest builds. Then, for Additional Dependencies under the Linker – Input tab, add the name of your static library, most likely MyProjectTest.lib .
Linker Tools Error LNK2019
Table of contents
Article
08/03/2021
9 minutes to read
8 contributors
In this article
unresolved external symbol ‘symbol’ referenced in function ‘function’
The compiled code for function makes a reference or call to symbol, but the linker can’t find the symbol definition in any of the libraries or object files to link.
This error message is followed by fatal error LNK1120. To fix error LNK1120, you must fix all LNK2001 and LNK2019 errors first.
Possible causes
There are many ways to get this error. All of them involve a reference to a function or variable that the linker couldn’t resolve, or find a definition for. The compiler can identify when a symbol isn’t declared, but it can’t tell when the symbol isn’t defined. That’s because the definition may be in a different source file or library. If a symbol is referred to but never defined, the linker generates an unresolved external symbol error.
Here are some common problems that cause LNK2019:
The source file that contains the definition of the symbol isn’t compiled
In Visual Studio, make sure the source file that defines the symbol gets compiled as part of your project. Check the intermediate build output directory for a matching .obj file. If the source file isn’t compiled, right-click on the file in Solution Explorer and choose Properties to check the properties of the file. The Configuration Properties > General page should show an Item Type of C/C++ Compiler. On the command line, make sure the source file that contains the definition is compiled.
The object file or library that contains the definition of the symbol isn’t linked
In Visual Studio, make sure the object file or library that contains the symbol definition is linked as part of your project. On the command line, make sure the list of files to link includes the object file or library.
The declaration of the symbol isn’t spelled the same as the definition of the symbol
Verify you use the correct spelling and capitalization in both the declaration and the definition, and wherever the symbol is used or called.
A function is used but the type or number of the parameters don’t match the function definition
The function declaration must match the definition. Make sure the function call matches the declaration, and that the declaration matches the definition. Code that invokes template functions must also have matching template function declarations that include the same template parameters as the definition. For an example of a template declaration mismatch, see sample LNK2019e.cpp in the Examples section.
A function or variable is declared but not defined
LNK2019 can occur when a declaration exists in a header file, but no matching definition is implemented. For member functions or static data members, the implementation must include the class scope selector. For an example, see Missing Function Body or Variable.
The calling convention is different between the function declaration and the function definition
Calling conventions (__cdecl, __stdcall, __fastcall, or __vectorcall) are encoded as part of the decorated name. Make sure the calling convention is the same.
A symbol is defined in a C file, but declared without using extern “C” in a C++ file
Symbols defined in a file that is compiled as C have different decorated names than symbols declared in a C++ file unless you use an extern “C” modifier. Make sure the declaration matches the compilation linkage for each symbol. Similarly, if you define a symbol in a C++ file that will be used by a C program, use extern “C” in the definition.
A symbol is defined as static and then later referenced outside the file
In C++, unlike C, global constants have static linkage. To get around this limitation, you can include the const initializations in a header file and include that header in your .cpp files, or you can make the variable non-constant and use a constant reference to access it.
A static member of a class isn’t defined
A static class member must have a unique definition, or it will violate the one-definition rule. A static class member that can’t be defined inline must be defined in one source file by using its fully qualified name. If it isn’t defined at all, the linker generates LNK2019.
A build dependency is only defined as a project dependency in the solution
In earlier versions of Visual Studio, this level of dependency was sufficient. However, starting with Visual Studio 2010, Visual Studio requires a project-to-project reference. If your project doesn’t have a project-to-project reference, you may receive this linker error. Add a project-to-project reference to fix it.
An entry point isn’t defined
The application code must define an appropriate entry point: main or wmain for console applications, and WinMain or wWinMain for Windows applications. For more information, see main function and command-line arguments or WinMain function. To use a custom entry point, specify the /ENTRY (Entry-Point Symbol) linker option.
You build a console application by using settings for a Windows application
If the error message is similar to unresolved external symbol WinMain referenced in function function_name, link by using /SUBSYSTEM:CONSOLE instead of /SUBSYSTEM:WINDOWS. For more information about this setting, and for instructions on how to set this property in Visual Studio, see /SUBSYSTEM (Specify Subsystem).
You attempt to link 64-bit libraries to 32-bit code, or 32-bit libraries to 64-bit code
Libraries and object files linked to your code must be compiled for the same architecture as your code. Make sure the libraries your project references are compiled for the same architecture as your project. Make sure the /LIBPATH or Additional Library Directories property points to libraries built for the correct architecture.
You use different compiler options for function inlining in different source files
Using inlined functions defined in .cpp files and mixing function inlining compiler options in different source files can cause LNK2019. For more information, see Function Inlining Problems.
You use automatic variables outside their scope
Automatic (function scope) variables can only be used in the scope of that function. These variables can’t be declared extern and used in other source files. For an example, see Automatic (Function Scope) Variables.
You call intrinsic functions or pass argument types to intrinsic functions that aren’t supported on your target architecture
For example, if you use an AVX2 intrinsic, but don’t specify the /ARCH:AVX2 compiler option, the compiler assumes that the intrinsic is an external function. Instead of generating an inline instruction, the compiler generates a call to an external symbol with the same name as the intrinsic. When the linker tries to find the definition of this missing function, it generates LNK2019. Make sure you only use intrinsics and types supported by your target architecture.
You mix code that uses native wchar_t with code that doesn’t
C++ language conformance work that was done in Visual Studio 2005 made wchar_t a native type by default. If not all files have been compiled by using the same /Zc:wchar_t settings, type references may not resolve to compatible types. Make sure wchar_t types in all library and object files are compatible. Either update from a wchar_t typedef, or use consistent /Zc:wchar_t settings when you compile.
Third-party library issues and vcpkg
If you see this error when you’re trying to configure a third-party library as part of your build, consider using vcpkg, a C++ package manager, to install and build the library. vcpkg supports a large and growing list of third-party libraries. It sets all the configuration properties and dependencies required for successful builds as part of your project.
Sometimes it’s difficult to tell why the linker can’t find a particular symbol definition. Often the problem is that you haven’t included the code that contains the definition in your build. Or, build options have created different decorated names for external symbols. There are several tools and options that can help you diagnose LNK2019 errors.
The /VERBOSE linker option can help you determine which files the linker references. This option can help you verify whether the file that contains the definition of the symbol is included in your build.
The /EXPORTS and /SYMBOLS options of the DUMPBIN utility can help you discover which symbols are defined in your .dll and object or library files. Make sure the exported decorated names match the decorated names the linker searches for.
The UNDNAME utility can show you the equivalent undecorated external symbol for a decorated name.
Examples
Here are several examples of code that causes a LNK2019 error, together with information about how to fix the error.
A symbol is declared but not defined
In this example, an external variable is declared but not defined:
// LNK2019.cpp // Compile by using: cl /EHsc /W4 LNK2019.cpp // LNK2019 expected extern char B[100]; // B isn’t available to the linker int main() { B[0] = ‘ ‘; // LNK2019 }
Here is another example where a variable and function are declared as extern but no definition is provided:
// LNK2019c.cpp // Compile by using: cl /EHsc LNK2019c.cpp // LNK2019 expected extern int i; extern void g(); void f() { i++; g(); } int main() {}
Unless i and g are defined in one of the files included in the build, the linker generates LNK2019. You can fix the errors by including the source code file that contains the definitions as part of the compilation. Alternatively, you can pass .obj files or .lib files that contain the definitions to the linker.
A static data member is declared but not defined
LNK2019 can also occur when a static data member is declared but not defined. The following sample generates LNK2019, and shows how to fix it.
// LNK2019b.cpp // Compile by using: cl /EHsc LNK2019b.cpp // LNK2019 expected struct C { static int s; }; // Uncomment the following line to fix the error. // int C::s; int main() { C c; C::s = 1; }
Declaration parameters don’t match the definition
Code that invokes template functions must have matching template function declarations. Declarations must include the same template parameters as the definition. The following sample generates LNK2019 on a user-defined operator, and shows how to fix it.
// LNK2019e.cpp // compile by using: cl /EHsc LNK2019e.cpp // LNK2019 expected #include
using namespace std; template class Test { // The operator<< declaration doesn't match the definition below: friend ostream& operator<<(ostream&, Test&); // To fix, replace the line above with the following: // template friend ostream& operator<<(ostream&, Test &); }; template ostream& operator<<(ostream& os, Test & tt) { return os; } int main() { Test t; cout << "Test: " << t << endl; // LNK2019 unresolved external } Inconsistent wchar_t type definitions This sample creates a DLL that has an export that uses WCHAR , which resolves to wchar_t . // LNK2019g.cpp // compile with: cl /EHsc /LD LNK2019g.cpp #include "windows.h" // WCHAR resolves to wchar_t __declspec(dllexport) void func(WCHAR*) {} The next sample uses the DLL in the previous sample, and generates LNK2019 because the types unsigned short* and WCHAR* aren't the same. // LNK2019h.cpp // compile by using: cl /EHsc LNK2019h LNK2019g.lib // LNK2019 expected __declspec(dllimport) void func(unsigned short*); int main() { func(0); } To fix this error, change unsigned short to wchar_t or WCHAR , or compile LNK2019g.cpp by using /Zc:wchar_t-. Additional resources For more information about possible causes and solutions for LNK2001, see the Stack Overflow question What is an undefined reference/unresolved external symbol error and how do I fix it?.
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