Unresolved external symbol in object files

Question

During coding in Visual Studio I got an unresolved external symbol error and I've got no idea what to do. I don't know what's wrong. Could you please decipher me? Where should I be looking for what kind of errors?

1>Form.obj : error LNK2019: unresolved external symbol "public: class Field * __thiscall Field::addField(class Field *)" (?addField@Field@@QAEPAV1@PAV1@@Z) referenced in function "public: void __thiscall Form::parse(class std::basic_stringstream<char,struct std::char_traits<char>,class std::allocator<char> > &)" (?parse@Form@@QAEXAAV?$basic_stringstream@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z)
1>Form.obj : error LNK2019: unresolved external symbol "public: virtual void __thiscall Field::parse(class std::basic_stringstream<char,struct std::char_traits<char>,class std::allocator<char> > &)" (?parse@Field@@UAEXAAV?$basic_stringstream@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) referenced in function "public: __thiscall InputField::InputField(class std::basic_stringstream<char,struct std::char_traits<char>,class std::allocator<char> > &)" (??0InputField@@QAE@AAV?$basic_stringstream@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z)
1>Form.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Field::prompt(void)" (?prompt@Field@@UAEXXZ)
1>Form.obj : error LNK2001: unresolved external symbol "public: virtual class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __thiscall Field::getName(void)" (?getName@Field@@UAE?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ)
1>Form.obj : error LNK2001: unresolved external symbol "public: virtual class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __thiscall Field::getType(void)" (?getType@Field@@UAE?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ)
1>Form.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Field::describe(void)" (?describe@Field@@UAEXXZ)
1>C:\Users\tomy\Documents\Visual Studio 2010\Projects\zapoctovkac++\Debug\zapoctovkac++.exe : fatal error LNK1120: 6 unresolved externals

Answer 1

Check you are including all the source files within your solution that you are referencing.

If you are not including the source file (and thus the implementation) for the class Field in your project it won't be built and you will be unable to link during compilation.

Alternatively, perhaps you are using a static or dynamic library and have forgotten to tell the linker about the .libs?

Answer 2

Make sure that you are not trying to overload the insertion or extraction operators as inline functions. I had this problem and it only went away when i removed that keyword.

Answer 3

What had caused it in my case:

I had a huge file Foo.cpp without a Foo.h. Foo.cpp began like this:

// ... 100 LOC here ...
namespace NS {
// ... 100 more LOC here ...
static int var;

I removed the "static" keyword and added a Foo.h with this:

extern int var;

Do you see the mistake?

I totally missed that var was originally defined in a namespace, because the namespace declaration was buried in other code. The fix is to change the extern like this:

namespace NS {
     extern int var;
}

Answer 4

I believe most of the points regarding the causes and remedies have been covered by all contributors in this thread. I just want to point out for my 'unresolved external' problem, it was caused by a datatype defined as macro that gets substituted differently than expected, which results in that incorrect type being supplied to the function in question, and since the function with type is never defined, it couldn't have been resolved. In particular, under C/C++ -> Language, there is an attribute called 'Treat WChar_t As Built in Type, which should have been defined as 'No (/Zc:wchar_t-)' but did not in my case.

Answer 5

My issue was: I had to do forward declaration of the class whose ctor was "unresolved external".

In the file where I got the error, I had to put something like this:

#include "ClassB" 

class ClassB; // this solved the problem

class ClassA{
    void foo(){
        ClassB* tmp = new ClassB();
        // ...
    }
};

Of course, my project is much more complicated and this is just a snippet example. Also when using namespaces, declare them as well.

Answer 6

Yet another possible problem (that I just scratched my head about for some time):

If you define your functions as inline, they—of course!—have to be defined in the header (or an inline file), not a cpp.
In my case, they were in an inline file, but only because they were a platform specific implementation, and a cpp included this corresponding inl file… instead of a header. Yeah, s**t happens.

I thought I'd leave this here, too, maybe someone else runs into the same issue and finds it here.