c++


Error LINK2019 when I try to make a training for face detection [duplicate]


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].
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]
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]
The source file is decomposed into preprocessing tokens (2.5) and sequences of white-space characters (including comments). [SNIP]
Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed. [SNIP]
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]
Adjacent string literal tokens are concatenated.
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]
Translated translation units and instantiation units are combined as follows: [SNIP]
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.
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:
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 (MSVC)
Problems with template friends
Inconsistent 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 e.g. 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".
Declared but did not define a variable or function.
A typical variable declaration is
extern int x;
As this is only a declaration, a single definition is needed. A corresponding definition would be:
int x;
For example, the following would generate an error:
extern int x;
int main()
{
x = 0;
}
//int x; // uncomment this line for successful definition
Similar remarks apply to functions. Declaring a function without defining it leads to the error:
void foo(); // declaration only
int main()
{
foo();
}
//void foo() {} //uncomment this line for successful definition
Be careful that the function you implement exactly matches the one you declared. For example, you may have mismatched cv-qualifiers:
void foo(int& x);
int main()
{
int x;
foo(x);
}
void foo(const int& x) {} //different function, doesn't provide a definition
//for void foo(int& x)
Other examples of mismatches include
Function/variable declared in one namespace, defined in another.
Function/variable declared as class member, defined as global (or vice versa).
Function return type, parameter number and types, and calling convention do not all exactly agree.
The error message from the compiler will often give you the full declaration of the variable or function that was declared but never defined. Compare it closely to the definition you provided. Make sure every detail matches.
The order in which interdependent linked libraries are specified is wrong.
The order in which libraries are linked DOES matter if the libraries depend on each other. In general, if library A depends on library B, then libA MUST appear before libB in the linker flags.
For example:
// B.h
#ifndef B_H
#define B_H
struct B {
B(int);
int x;
};
#endif
// B.cpp
#include "B.h"
B::B(int xx) : x(xx) {}
// A.h
#include "B.h"
struct A {
A(int x);
B b;
};
// A.cpp
#include "A.h"
A::A(int x) : b(x) {}
// main.cpp
#include "A.h"
int main() {
A a(5);
return 0;
};
Create the libraries:
$ g++ -c A.cpp
$ g++ -c B.cpp
$ ar rvs libA.a A.o
ar: creating libA.a
a - A.o
$ ar rvs libB.a B.o
ar: creating libB.a
a - B.o
Compile:
$ g++ main.cpp -L. -lB -lA
./libA.a(A.o): In function `A::A(int)':
A.cpp:(.text+0x1c): undefined reference to `B::B(int)'
collect2: error: ld returned 1 exit status
$ g++ main.cpp -L. -lA -lB
$ ./a.out
So to repeat again, the order DOES matter!
Symbols were defined in a C program and used in C++ code.
The function (or variable) void foo() was defined in a C program and you attempt to use it in a C++ program:
void foo();
int main()
{
foo();
}
The C++ linker expects names to be mangled, so you have to declare the function as:
extern "C" void foo();
int main()
{
foo();
}
Equivalently, instead of being defined in a C program, the function (or variable) void foo() was defined in C++ but with C linkage:
extern "C" void foo();
and you attempt to use it in a C++ program with C++ linkage.
If an entire library is included in a header file (and was compiled as C code); the include will need to be as follows;
extern "C" {
#include "cheader.h"
}
If all else fails, recompile.
I was recently able to get rid of an unresolved external error in Visual Studio 2012 just by recompiling the offending file. When I re-built, the error went away.
This usually happens when two (or more) libraries have a cyclic dependency. Library A attempts to use symbols in B.lib and library B attempts to use symbols from A.lib. Neither exist to start off with. When you attempt to compile A, the link step will fail because it can't find B.lib. A.lib will be generated, but no dll. You then compile B, which will succeed and generate B.lib. Re-compiling A will now work because B.lib is now found.
Incorrectly importing/exporting methods/classes across modules/dll (compiler specific).
MSVS requires you to specify which symbols to export and import using __declspec(dllexport) and __declspec(dllimport).
This dual functionality is usually obtained through the use of a macro:
#ifdef THIS_MODULE
#define DLLIMPEXP __declspec(dllexport)
#else
#define DLLIMPEXP __declspec(dllimport)
#endif
The macro THIS_MODULE would only be defined in the module that exports the function. That way, the declaration:
DLLIMPEXP void foo();
expands to
__declspec(dllexport) void foo();
and tells the compiler to export the function, as the current module contains its definition. When including the declaration in a different module, it would expand to
__declspec(dllimport) void foo();
and tells the compiler that the definition is in one of the libraries you linked against (also see 1)).
You can similary import/export classes:
class DLLIMPEXP X
{
};
Template implementations not visible.
Unspecialized templates must have their definitions visible to all translation units that use them. That means you can't separate the definition of a template
to an implementation file. If you must separate the implementation, the usual workaround is to have an impl file which you include at the end of the header that
declares the template. A common situation is:
template<class T>
struct X
{
void foo();
};
int main()
{
X<int> x;
x.foo();
}
//differentImplementationFile.cpp
template<class T>
void X<T>::foo()
{
}
To fix this, you must move the definition of X::foo to the header file or some place visible to the translation unit that uses it.
Specialized templates can be implemented in an implementation file and the implementation doesn't have to be visible, but the specialization must be previously declared.
For further explanation and another possible solution (explicit instantiation) see this question and answer.
what is an "undefined reference/unresolved external symbol"
I'll try to explain what is an "undefined reference/unresolved external symbol".
note: i use g++ and Linux and all examples is for it
For example we have a code
// src1.cpp
void print();
static int local_var_name; // 'static' makes variable not visible for other modules
int global_var_name = 123;
int main()
{
print();
return 0;
}
and
// src2.cpp
extern "C" int printf (const char*, ...);
extern int global_var_name;
//extern int local_var_name;
void print ()
{
// printf("%d%d\n", global_var_name, local_var_name);
printf("%d\n", global_var_name);
}
Make object files
$ g++ -c src1.cpp -o src1.o
$ g++ -c src2.cpp -o src2.o
After assembler phase we have an object file, which contains any symbols to export.
Look at the symbols
$ readelf --symbols src1.o
Num: Value Size Type Bind Vis Ndx Name
5: 0000000000000000 4 OBJECT LOCAL DEFAULT 4 _ZL14local_var_name # [1]
9: 0000000000000000 4 OBJECT GLOBAL DEFAULT 3 global_var_name # [2]
I've rejected some lines from output, because they are not matter
So, we see follow symbols to export.
[1] - this is our static (local) variable (important - Bind has a type "LOCAL")
[2] - this is our global variable
src2.cpp exports nothing and we have seen no its symbols
Link our object files
$ g++ src1.o src2.o -o prog
and run it
$ ./prog
123
Linker sees exported sysmbols and links it. Now we try to uncomment lines in src2.cpp like here
// src2.cpp
extern "C" int printf (const char*, ...);
extern int global_var_name;
extern int local_var_name;
void print ()
{
printf("%d%d\n", global_var_name, local_var_name);
}
and rebuild an object file
$ g++ -c src2.cpp -o src2.o
OK (no errors), because we only build object file, linking is not done yet.
Try to link
$ g++ src1.o src2.o -o prog
src2.o: In function `print()':
src2.cpp:(.text+0x6): undefined reference to `local_var_name'
collect2: error: ld returned 1 exit status
It is happened because our local_var_name is static, i.e. it is not visible for other modules.
Now more deeply. Get translation phase output
$ g++ -S src1.cpp -o src1.s
// src1.s
look src1.s
.file "src1.cpp"
.local _ZL14local_var_name
.comm _ZL14local_var_name,4,4
.globl global_var_name
.data
.align 4
.type global_var_name, #object
.size global_var_name, 4
global_var_name:
.long 123
.text
.globl main
.type main, #function
main:
; assembler code, not interesting for us
.LFE0:
.size main, .-main
.ident "GCC: (Ubuntu 4.8.2-19ubuntu1) 4.8.2"
.section .note.GNU-stack,"",#progbits
So, we've seen there is no label for local_var_name, that's why linker not found it. But we are hackers :) and we can fix it. Open src1.s in your texteditor and change
.local _ZL14local_var_name
.comm _ZL14local_var_name,4,4
to
.globl local_var_name
.data
.align 4
.type local_var_name, #object
.size local_var_name, 4
local_var_name:
.long 456789
i.e. you should have like bellow
.file "src1.cpp"
.globl local_var_name
.data
.align 4
.type local_var_name, #object
.size local_var_name, 4
local_var_name:
.long 456789
.globl global_var_name
.align 4
.type global_var_name, #object
.size global_var_name, 4
global_var_name:
.long 123
.text
.globl main
.type main, #function
main:
; ...
we have changed visibility of local_var_name and set its value to 456789
Try to build an object file from it
$ g++ -c src1.s -o src2.o
ok, see readelf output (symbols)
$ readelf --symbols src1.o
8: 0000000000000000 4 OBJECT GLOBAL DEFAULT 3 local_var_name
now local_var_name has Bind GLOBAL (was LOCAL)
link
$ g++ src1.o src2.o -o prog
and run it
$ ./prog
123456789
ok, we hack it :)
So, as result - an "undefined reference/unresolved external symbol error" happens when linker cannot find global symbols in object files.
This is one of most confusion error messages that every VC++ programmers have seen time and time again. Let’s make things clarity first.
A. What is symbol?
In short, a symbol is a name. It can be a variable name, a function name, a class name, a typedef name, or anything except those names and signs that belong to C++ language. It is user defined or introduced by a dependency library (another user defined).
B. What is external?
In VC++, every source file (.cpp,.c,etc.) is considered as a translation unit, the compiler compiles one unit at a time, and generate one object file(.obj) for current translation unit. (Note that every header file that this source file included will be preprocessed and will be considered as part of this translation unit)Everything within a translation unit is considered as internal, everything else is considered as external. In C++, you may reference an external symbol by using keywords like: extern, __declspec (dllimport) and so on.
C. What is “resolve”?
Resolve is a linking-time term. In linking-time, linker attempts to find the external definition for every symbol in object files that cannot find its definition internally. The scope of this searching process including:
All object files that generated in compiling time
All libraries (.lib) that is either explicitly or implicitly
specified as additional dependencies of this building application.
This searching process is called resolve.
D. Finally, why Unresolved External Symbol?
If the linker cannot find the external definition for a symbol that has no definition internally, it reports an Unresolved External Symbol error.
E. Possible causes of LNK2019: Unresolved External Symbol error.
We already know that this error is due to the linker failed to find the definition of external symbols, the possible causes can be sorted as:
Definition exists
For example, if we have a function called foo defined in a.cpp:
int foo()
{
return 0;
}
In b.cpp we want to call function foo, so we add
void foo();
to declare function foo(), and call it in another function body, say bar():
void bar()
{
foo();
}
Now when you build this code you will get a LNK2019 error complaining that foo is unresolved symbol. In this case, we know that foo() has its definition in a.cpp, but different from the one we are calling(different return value). This is the case that definition exists.
Definition does not exist
If we want to call some functions in a library, but the import library is not added into the additional dependency list (set from: Project | Properties | Configuration Properties | Linker | Input | Additional Dependency) of your project setting. Now the linker will report a LNK2019 since the definition is not exist in current searching scope.
undefined reference to WinMain#16 or similar 'unusual' main() entry point reference (especially for visual-studio).
You may have missed to choose the right project type with your actual IDE. The IDE may want to bind e.g. Windows Application projects to such entry point function (as specified in the missing reference above), instead of the commonly used int main(int argc, char** argv); signature.
If your IDE supports Plain Console Projects you might want to choose this project type, instead of a windows application project.
Also if you're using 3rd party libraries make sure you have the correct 32/64 bit binaries
Microsoft offers a #pragma to reference the correct library at link time;
#pragma comment(lib, "libname.lib")
In addition to the library path including the directory of the library, this should be the full name of the library.
Visual Studio NuGet package needs to be updated for new toolset version
I just had this problem trying to link libpng with Visual Studio 2013. The problem is that the package file only had libraries for Visual Studio 2010 and 2012.
The correct solution is to hope the developer releases an updated package and then upgrade, but it worked for me by hacking in an extra setting for VS2013, pointing at the VS2012 library files.
I edited the package (in the packages folder inside the solution's directory) by finding packagename\build\native\packagename.targets and inside that file, copying all the v110 sections. I changed the v110 to v120 in the condition fields only being very careful to leave the filename paths all as v110. This simply allowed Visual Studio 2013 to link to the libraries for 2012, and in this case, it worked.
A bug in the compiler/IDE
I recently had this problem, and it turned out it was a bug in Visual Studio Express 2013. I had to remove a source file from the project and re-add it to overcome the bug.
Steps to try if you believe it could be a bug in compiler/IDE:
Clean the project (some IDEs have an option to do this, you can also
manually do it by deleting the object files)
Try start a new project,
copying all source code from the original one.
Suppose you have a big project written in c++ which has a thousand of .cpp files and a thousand of .h files.And let's says the project also depends on ten static libraries . Let's says we are on Windows and we build our project in Visual Studio 20xx . When you press Ctr + F7 Visual Studio start compiling the whole solution ( suppose we have just one project in the solution )
What's the meaning of compilation ?
Visual Studio search into file .vcxproj and start compiling each file which has the extansion .cpp . Order of compilation is undefined.So you must not assume that the file main.cpp is compiled first
If .cpp files depends on additional .h files in order to find symbols
that may or may not be defined in the file .cpp
If exists one .cpp file in which the compiler could not find one symbol,a compiler time error raises with the message Symbol x could not be found
For each file with extension .cpp is generated an object file .o and also Visual Studio writes the output in a file named ProjectName.Cpp.Clean.txt which contains all object files that must be processed by linker.
The Second step of compilation is done by Linker.Linker should merge all the object file and build finally the output ( which may be an executable or a library)
Steps In Linking a project
Parse all the object files and find the definition which was only declared in headers ( eg : The code of one method of a class as is mentioned in previous answers,or event the initialization of a static variable which is member inside a class)
If one symbol could not be found in object files he also is searched in Additional Libraries.For adding a new library to a project Configuration properties -> VC++ Directories -> Library Directories and here you specified additional folder for searching libraries and Configuration properties -> Linker -> Input for specifying the name of the library.
-If the Linker could not find the symbol which you write in one .cpp he raises an linker time error which may sounds like
error LNK2001: unresolved external symbol "void __cdecl foo(void)" (?foo##YAXXZ)
Observation
Once the Linker find one symbol he doesn't search in other libraries for it
The order of linking libraries does matter.
If Linker find an external symbol in one static library he includes the symbol in output of the project.However if the library is shared( dynamic ) he doesn't includes the code ( symbols ) in output, but Run-Time crashes may occur
How To Solve this kind of error
Compiler Time Error :
Make sure you write your c++ project syntactical correct.
Linker Time Error
Define all your symbol which you declare in your header files
Use #pragma once for allowing compiler not to include one header if it was already included by the current .cpp which is compiled
Make sure that your external library doesn't contains symbols that may enter in conflict with other symbols you defined in your header files
When you use template make sure you include the definition of each template function in header file for allowing the compiler to generate appropriate code for any instantiations .
Linked .lib file is associated to a .dll
I had the same issue. Say i have projects MyProject and TestProject. I had effectively linked the lib file for MyProject to the TestProject. However, this lib file was produced as the DLL for the MyProject was built. Also, I did not contain source code for all methods in the MyProject, but only access to the DLL's entry points.
To solve the issue, i built the MyProject as a LIB, and linked TestProject to this .lib file (i copy paste the generated .lib file into the TestProject folder). I can then build again MyProject as a DLL. It is compiling since the lib to which TestProject is linked does contain code for all methods in classes in MyProject.
Use the linker to help diagnose the error
Most modern linkers include a verbose option that prints out to varying degrees;
Link invocation (command line),
Data on what libraries are included in the link stage,
The location of the libraries,
Search paths used.
For gcc and clang; you would typically add -v -Wl,--verbose or -v -Wl,-v to the command line. More details can be found here;
Linux ld man page.
LLVM linker page.
"An introduction to GCC" chapter 9.
For MSVC, /VERBOSE (in particular /VERBOSE:LIB) is added to the link command line.
The MSDN page on the /VERBOSE linker option.
A wrapper around GNU ld that doesn't support linker scripts
Some .so files are actually GNU ld linker scripts, e.g. libtbb.so file is an ASCII text file with this contents:
INPUT (libtbb.so.2)
Some more complex builds may not support this. For example, if you include -v in the compiler options, you can see that the mainwin gcc wrapper mwdip discards linker script command files in the verbose output list of libraries to link in. A simple work around is to replace the linker script input command file with a copy of the file instead (or a symlink), e.g.
cp libtbb.so.2 libtbb.so
Or you could replace the -l argument with the full path of the .so, e.g. instead of -ltbb do /home/foo/tbb-4.3/linux/lib/intel64/gcc4.4/libtbb.so.2
Since people seem to be directed to this question when it comes to linker errors I am going to add this here.
One possible reason for linker errors with GCC 5.2.0 is that a new libstdc++ library ABI is now chosen by default.
If you get linker errors about undefined references to symbols that involve types in the std::__cxx11 namespace or the tag [abi:cxx11] then it probably indicates that you are trying to link together object files that were compiled with different values for the _GLIBCXX_USE_CXX11_ABI macro. This commonly happens when linking to a third-party library that was compiled with an older version of GCC. If the third-party library cannot be rebuilt with the new ABI then you will need to recompile your code with the old ABI.
So if you suddenly get linker errors when switching to a GCC after 5.1.0 this would be a thing to check out.
Befriending templates...
Given the code snippet of a template type with a friend operator (or function);
template <typename T>
class Foo {
friend std::ostream& operator<< (std::ostream& os, const Foo<T>& a);
};
The operator<< is being declared as a non-template function. For every type T used with Foo, there needs to be a non-templated operator<<. For example, if there is a type Foo<int> declared, then there must be an operator implementation as follows;
std::ostream& operator<< (std::ostream& os, const Foo<int>& a) {/*...*/}
Since it is not implemented, the linker fails to find it and results in the error.
To correct this, you can declare a template operator before the Foo type and then declare as a friend, the appropriate instantiation. The syntax is a little awkward, but is looks as follows;
// forward declare the Foo
template <typename>
class Foo;
// forward declare the operator <<
template <typename T>
std::ostream& operator<<(std::ostream&, const Foo<T>&);
template <typename T>
class Foo {
friend std::ostream& operator<< <>(std::ostream& os, const Foo<T>& a);
// note the required <> ^^^^
// ...
};
template <typename T>
std::ostream& operator<<(std::ostream&, const Foo<T>&)
{
// ... implement the operator
}
The above code limits the friendship of the operator to the corresponding instantiation of Foo, i.e. the operator<< <int> instantiation is limited to access the private members of the instantiation of Foo<int>.
Alternatives include;
Allowing the friendship to extend to all instantiations of the templates, as follows;
template <typename T>
class Foo {
template <typename T1>
friend std::ostream& operator<<(std::ostream& os, const Foo<T1>& a);
// ...
};
Or, the implementation for the operator<< can be done inline inside the class definition;
template <typename T>
class Foo {
friend std::ostream& operator<<(std::ostream& os, const Foo& a)
{ /*...*/ }
// ...
};
Note, when the declaration of the operator (or function) only appears in the class, the name is not available for "normal" lookup, only for argument dependent lookup, from cppreference;
A name first declared in a friend declaration within class or class template X becomes a member of the innermost enclos

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