I want to have some data cache which contains some objects which I can update over an UpdateCache function. However, I'm getting problems with a LNK2001 followed by a LNK1120.
HeaderFile.h
#ifndef headerfile_included
#define headerfile_included
#include <vector>
struct MyObject {
unsigned int A;
unsigned int B;
};
class MyClass {
private:
static std::vector<MyObject> myObjectCache;
public:
static void UpdateCache ();
};
#endif
CodeFile.cpp
#include "HeaderFile.h"
void MyClass::UpdateCache () {
myObjectCache.clear();
/* Repopulate cache with new data */
}
The error message I get from the linked is
error LNK2001: unresolved external symbol ""private: static class std::vector > MyClass::myObjectCache" (?myObjectCache#MyClass##0V?$vector#UMyObject##V?$allocator#UMyObject###std###std##A)".
fatal error LNK1120: 1 unresolved externals
My opinion is that it is some problem with the partitioning into the header file and the code file as I have had similar problems with improper partitioning. If it is again such a problem, it would be nice if you could post some rule on what to put into the header file and what into the code file since it's pretty confusing.
You need to add this to a cpp file:
std::vector<MyObject> MyClass::myObjectCache;
The reason is that as a static exists without a class ever being instantiated it needs to exist whether an instance of the class is instantiated or not. The line above creates the instance of the static and thus it exists whether or not you ever create an instance of the class itself.
Since your vector is static, essentially a global entity as far as the compiler is concerned, you need to be sure to give it a home in a compilation unit. That's why you have to do what #Goz says and do
std::vector<MyObject> MyClass::myObjectCache;
Related
I am new to C++, so I am sorry if this a simple or obvious error. I have been reading a lot other questions and the documentation, but I haven't been able to find a solution yet.
I am trying to add a new class definition to a large existing project. Everything compiles perfectly fine without my additions. However, when I add my code below I get a LNK2019 error on the constructor method (and any additional method). I have noticed adding/referencing properties does not cause this linker error, only the methods. Below is the most simple example that produces the error:
Header:
namespace foo
{
class bar_interface
{
public:
//My code
#ifndef Point_H
#define Point_H
class Point
{
public:
Point(int x, int y);
};
#endif
//existing code
void onStartup();
}
}
Class:
//My code
#ifndef Point_H
#define Point_H
class foo:bar_interface::Point{
public:
Point(int x, int y)
{
};
};
#endif
//existing code
void bar_interface::onStartup()
{
foo::bar_interface::Point p( (int)8, (int)8 );
//continue existing code
}
Error 1 error LNK2019: unresolved external symbol "public: __thiscall
foo::bar_interface::Point::Point(int,int)"
(??0Point#bar_interface#foo##QAE#HH#Z)
referenced in function "public: void __thiscall
foo::bar_interface::onStartup(void)"
(?onStartup#bar_interface#foo##QAEXXZ)
I realize that probably do not need such explicit calls to Point or casting the numbers as ints, but I wanted to make sure I wasn't missing anything obvious (removing them doesnt change the error). I have tried moving the 'Point' class to its own file and defining it outside the 'bar_interface' but within the 'foo' namespace. Removing the #ifndef code creates a C2011 redefinition error. I am at a loss for how to proceed.
Unresolved external means that the definition is missing, that is that the linker cannot find an implementation of the named function.
Somewhere you need:
namespace foo
{
bar_interface::Point::Point(int,int)
{ ... }
}
Remove all your lines from the code above that start from # and the reason of the issue becomes cleaner.
I am new to c++. I started now playing with classes and I am having a noob problem with statics.
class Test
{
public:
Test(){};
~Test(){};
static void test();
static Helper* helper;
};
void Test::test()
{
Object obj = Test::helper->getObject();
//...
}
When I try to compile it gives the error:
main.obj : error LNK2001: unresolved external symbol "public: static class Helper* Test::helper" (?helper#Test##2PAVHelper##A)
What is wrong with my code?
The first answer is correct. The reason behind this is that you need to allocate memory for static objects outside the class definition. If you define the the class in a header file, and include it in several cpp files, the compiler doesn't know where and how you want to create the object that 'helper' points to.
you need to define Test::helper. Write something like this outside the class:
Helper* Test::helper = new Helper;
EDIT: I know there are similar questions, but I cannot find an answer to a following issue: Why the methods inside the class are working correctly and outside are not.
I've got a weird problem in my project which I'm developing in MSVC++ 2012. My project consists of different modules of code. The important modules from the problem's point of view is a library and the GUI. They exist as different projects in the same solution.
I have some methods in the library which are part of the classes (in this case Calibration3D):
void Calibration3D::load(const std::string &path)
I use it without problems when I need it in the GUI, however I need to use a following method (outside the class):
void xxxyyy()
But when I'm trying to use that function (outside the class but in the same namespace) I get a following error:
1>project_xml.obj : error LNK2001: unresolved external symbol "void __cdecl cci::xxxyyy(void)" (?xxxyyy#cci##YAXXZ) 1>D:\praca_pw\cci\build-msvc2012\x64\Release\\ccigui.exe : fatal error LNK1120: 1 unresolved externals
Anybody knows how to solve it?
When I have a header file like this:
namespace xyz {
void foo();
class bar { ... };
}
then I write the cpp file like this:
#include "xyz.h"
namespace xyz {
void foo() { ... }
bar::bar() { ... }
}
This means I have to type a lot less and make fewer mistakes with regard to namespaces.
OK, solved, it seems that when a method is defined inside the namespace in header file, it should also be defined explicitly as part of namespace in implementation file, in this case:
cci::xxxyyy()
{
...
}
will work and
xxxyyy()
{
...
}
will not.
I am trying to create a new instance of a class, however I am receiving a LNK2001 unresolved external symbol error when I attempt to compile my code.
As far as I can tell I have written and included the class in exactly the same manner as I included another class, in both cases -
#include "class.h" // In main.cpp
class Class { // In class.h
private:
// etc.
public:
Class();
~Class();
// etc.
};
#include "class.h" // In class.cpp
Is there a common / likely cause of these errors, or a good way I might go about finding the source of the issue?
Edit: The error is
"Error 1 error LNK2019: unresolved external symbol "class Max
__cdecl max(void)" (?max##YA?AVMax##XZ) referenced in function _main main.obj Racing "
Edit: In both cases, a class is implemented across a .h and a .cpp file included in a project. The error is only appearing with one class.
Somewhere you have written this:
Max max();
What you intended was to declare a variable max of type Max.
C++ thinks you intend to declare a function max which returns an object of type Max. This is what it is looking for.
If you just say this:
Max max;
The issue will go away.
Edit: This only occurs with constructors which take no arguments. If the constructor takes arguments, C++ can see from the parameters (which will be rvalues, e.g. constants or expressions) that it is an instantiation of the class not a function declaration.
Max max(5); // Clearly cannot be a function, because 5 is an rvalue
Or
Max max(int); // Clearly cannot be an instantiation, because int is a type
But if the constructor takes no arguments, to distinguish between them, you have to drop the brackets if you are instantiating.
I have a little class called Stuff that I want to store things in. These things are a list of type int. Throughout my code in whatever classes I use I want to be able to access these things inside the Stuff class.
Main.cpp:
#include "Stuff.h"
int main()
{
Stuff::things.push_back(123);
return 0;
}
Stuff.h:
#include <list>
class Stuff
{
public:
static list<int> things;
};
but I get some build errors with this code:
error LNK2001: unresolved external symbol "public: static class std::list<int,class std::allocator<int> > Stuff::things" (?things#Stuff##2V?$list#HV?$allocator#H#std###std##A) Main.obj CSandbox
fatal error LNK1120: 1 unresolved externals C:\Stuff\Projects\CSandbox\Debug\CSandbox.exe CSandbox
I am a C# guy, and I am trying to learn C++ for a side project. I think that I don't understand how C++ treats static members. So please explain what I have got wrong here.
Mentioning a static member in a class declaration is a declaration only. You must include one definition of the static member for the linker to hook everything up properly. Normally you would include something like the following in a Stuff.cpp file:
#include "Stuff.h"
list<int> Stuff::things;
Be sure to include Stuff.cpp in your program along with Main.cpp.
Static data members have to be defined outside class declarations, much like methods.
For example:
class X {
public:
static int i;
};
Must also have the following:
int X::i = 0; // definition outside class declaration
Stuff::things is only declared, but it is not defined.
please use:
// Stuff.cpp
#include "Stuff.h"
std::list<int> Stuff::things;
Added: it is also a good practice to protect your header files against multiple inclusion:
// Stuff.h
#ifndef STUFF_H_
#define STUFF_H_
#include <list>
class Stuff {
public:
static std::list<int> things;
};
#endif
Just For your Information, why this works is that in C++ all global variables (including static global) are created before the execution of the main function begins.
Static member have to be declared in the class but defined in the unit (cpp file) where it is really located.
The only exception is in case the class is template: in this case you have to define the member outside the class, but you have to provide it too with the class declaration, in the header file.