Linking error with methods using class templates - c++

I need to use my class template inside one of the member functions (class methods), as shown below :
Declaration :
template <int N>
class Array
{
private :
int var[N];
public :
void increment ();
};
Definition :
template <int N>
void Array<N> :: increment ()
{
for (int i = 0; i < N; i++)
{
cout << i << endl;
}
}
Instantiation and method call in Main.cpp:
Array <5>var;
int main()
{
var.increment();
system ("pause");
}
I keep getting a linking error saying "1 unresolved externals" in Main.obj (Main.cpp).
Please consider all "cout" and "using namespace std" to be already done in the background. I'm just adding the relevant code here.
My main problem is that I don't know how to use the class template parameter (N) inside one of the class's methods.
UPDATE: Sorry I'm late but I've found a solution to this. Thanks for the help all.

Just to close this thread because I forgot about it long ago. The problem seems to be the fact that templates can only be implemented in a header file of .tpp file. The link given by #user4581301 helped in clearing this up.
This is the link

Related

function template won't compile in VS2019 express

I have a project with quite a lot of function templates that I wrote back in Visual Studio 2017, and it always worked fine. Now I have to build it in VS2019, because I need to include the lib in another project that is written in VS2019, and the thing won't build.
There is one function template it seems to take issue with, although it doesn't actually complain about the function itself. The compiler just says "identifier not found" when I invoke it in the code. The thing is there in the namespace, however, even InteliSense sees it and links to it without complaining. Just the compiler won't.
Here's the code in question:
// declaration
namespace Oparse
{
// lots of other functions, many of them templates
template <typename T> OpModel<T> *_ModelPtr(T *receiver) { return new OpModel<T>(receiver); };
}
// Invocation
namespace Oparse
{
template <class T, class U>
class OpModelFactory
: public OpNestable
{
public:
OpModelFactory<T, U>(vector<U*> &receiver) : OpNestable(OP_MODELFACTORY), receiver(receiver) {};
// other stuff
void Serialize(string key, stringstream &stream, unsigned int indents)
{
for (unsigned int i = 0; i < receiver.size(); ++i)
{
// check if the instances are really of the type of this OpModel, otherwise there may be duplicates between polymorphic factories populating the same receiver.
T *currentModel = dynamic_cast<T*>(receiver[i]);
if (currentModel != NULL)
{
OpModel<T> *parser = _ModelPtr<T>(currentModel); // <-- identifier not found
parser->Serialize(key, stream, indents);
delete parser;
}
}
};
private:
vector<U*> &receiver;
}
}
If I comment that invocation, the project builds, despite there being a whole lot more function templates declared right where this one is. I have no clue what to do to make the linker find it. Are there any Visual Studio wizzards who could give me a hint? I must honestly confess that I haven't used the IDE in years, and it's my first time in Visual Studio 2019...
Here's the complete output of the error. There's a second message to it, but I found it perfectly unhelpful:
1>D:\Orbiter_installs\Orbiter2016\Orbitersdk\Oparse\include\OpModel.h(138,27): error C3861: '_ModelPtr': identifier not found
1>D:\Orbiter_installs\Orbiter2016\Orbitersdk\Oparse\include\OpModel.h(152): message : see reference to class template instantiation 'Oparse::OpModelFactory<T,U>' being compiled
And no, there's not further message attached. I have seen similar messages that usually go on with "with ... $further information", but this is all I get.
There is a circular dependency problem.
In Oparse.h, you first include OpModel.h that requires_ModelPtr in implementation of Serialize , but_ModelPtr is only defined later in the header.
You need to forward declare the template method.
In OpModel.h, write this instead:
namespace Oparse
{
template<typename T> class OpModel;
template <typename T> OpModel<T>* _ModelPtr(T* receiver);
// Remaining of OpModel.h...
typedef map<string, pair<Oparse::OpValue*, vector<Oparse::OpValidator*>>> OpModelDef;
typedef vector<pair<Oparse::OpValue*, vector<Oparse::OpValidator*>>> OpValues;
...

Undefined reference when using pointers [duplicate]

This question already has answers here:
Why can templates only be implemented in the header file?
(17 answers)
Closed 7 years ago.
I have no idea why this is happenning, since I think I have everything properly declared and defined.
I have the following program, designed with templates. It's a simple implementation of a queue, with the member functions "add", "substract" and "print".
I have defined the node for the queue in the fine "nodo_colaypila.h":
#ifndef NODO_COLAYPILA_H
#define NODO_COLAYPILA_H
#include <iostream>
template <class T> class cola;
template <class T> class nodo_colaypila
{
T elem;
nodo_colaypila<T>* sig;
friend class cola<T>;
public:
nodo_colaypila(T, nodo_colaypila<T>*);
};
Then the implementation in "nodo_colaypila.cpp"
#include "nodo_colaypila.h"
#include <iostream>
template <class T> nodo_colaypila<T>::nodo_colaypila(T a, nodo_colaypila<T>* siguiente = NULL)
{
elem = a;
sig = siguiente;//ctor
}
Afterwards, the definition and declaration of the queue template class and its functions:
"cola.h":
#ifndef COLA_H
#define COLA_H
#include "nodo_colaypila.h"
template <class T> class cola
{
nodo_colaypila<T>* ult, pri;
public:
cola<T>();
void anade(T&);
T saca();
void print() const;
virtual ~cola();
};
#endif // COLA_H
"cola.cpp":
#include "cola.h"
#include "nodo_colaypila.h"
#include <iostream>
using namespace std;
template <class T> cola<T>::cola()
{
pri = NULL;
ult = NULL;//ctor
}
template <class T> void cola<T>::anade(T& valor)
{
nodo_colaypila <T> * nuevo;
if (ult)
{
nuevo = new nodo_colaypila<T> (valor);
ult->sig = nuevo;
ult = nuevo;
}
if (!pri)
{
pri = nuevo;
}
}
template <class T> T cola<T>::saca()
{
nodo_colaypila <T> * aux;
T valor;
aux = pri;
if (!aux)
{
return 0;
}
pri = aux->sig;
valor = aux->elem;
delete aux;
if(!pri)
{
ult = NULL;
}
return valor;
}
template <class T> cola<T>::~cola()
{
while(pri)
{
saca();
}//dtor
}
template <class T> void cola<T>::print() const
{
nodo_colaypila <T> * aux;
aux = pri;
while(aux)
{
cout << aux->elem << endl;
aux = aux->sig;
}
}
Then, I have a program to test these functions as follows:
"main.cpp"
#include <iostream>
#include "cola.h"
#include "nodo_colaypila.h"
using namespace std;
int main()
{
float a, b, c;
string d, e, f;
cola<float> flo;
cola<string> str;
a = 3.14;
b = 2.71;
c = 6.02;
flo.anade(a);
flo.anade(b);
flo.anade(c);
flo.print();
cout << endl;
d = "John";
e = "Mark";
f = "Matthew";
str.anade(d);
str.anade(e);
str.anade(f);
cout << endl;
c = flo.saca();
cout << "First In First Out Float: " << c << endl;
cout << endl;
f = str.saca();
cout << "First In First Out String: " << f << endl;
cout << endl;
flo.print();
cout << endl;
str.print();
cout << "Hello world!" << endl;
return 0;
}
But when I build, the compiler throws errors in every instance of the template class:
undefined reference to `cola(float)::cola()'... (it's actually cola'<'float'>'::cola(), but this doesn't let me use it like that.)
And so on. Altogether, 17 warnings, counting the ones for the member functions being called in the program.
Why is this? Those functions and constructors WERE defined. I thought that the compiler could replace the "T" in the template with "float", "string" or whatever; that was the advantage of using templates.
I read somewhere here that I should put the declaration of each function in the header file for some reason. Is that right? And if so, why?
This is a common question in C++ programming. There are two valid answers to this. There are advantages and disadvantages to both answers and your choice will depend on context. The common answer is to put all the implementation in the header file, but there's another approach will will be suitable in some cases. The choice is yours.
The code in a template is merely a 'pattern' known to the compiler. The compiler won't compile the constructors cola<float>::cola(...) and cola<string>::cola(...) until it is forced to do so. And we must ensure that this compilation happens for the constructors at least once in the entire compilation process, or we will get the 'undefined reference' error. (This applies to the other methods of cola<T> also.)
Understanding the problem
The problem is caused by the fact that main.cpp and cola.cpp will be compiled separately first. In main.cpp, the compiler will implicitly instantiate the template classes cola<float> and cola<string> because those particular instantiations are used in main.cpp. The bad news is that the implementations of those member functions are not in main.cpp, nor in any header file included in main.cpp, and therefore the compiler can't include complete versions of those functions in main.o. When compiling cola.cpp, the compiler won't compile those instantiations either, because there are no implicit or explicit instantiations of cola<float> or cola<string>. Remember, when compiling cola.cpp, the compiler has no clue which instantiations will be needed; and we can't expect it to compile for every type in order to ensure this problem never happens! (cola<int>, cola<char>, cola<ostream>, cola< cola<int> > ... and so on ...)
The two answers are:
Tell the compiler, at the end of cola.cpp, which particular template classes will be required, forcing it to compile cola<float> and cola<string>.
Put the implementation of the member functions in a header file that will be included every time any other 'translation unit' (such as main.cpp) uses the template class.
Answer 1: Explicitly instantiate the template, and its member definitions
At the end of cola.cpp, you should add lines explicitly instantiating all the relevant templates, such as
template class cola<float>;
template class cola<string>;
and you add the following two lines at the end of nodo_colaypila.cpp:
template class nodo_colaypila<float>;
template class nodo_colaypila<std :: string>;
This will ensure that, when the compiler is compiling cola.cpp that it will explicitly compile all the code for the cola<float> and cola<string> classes. Similarly, nodo_colaypila.cpp contains the implementations of the nodo_colaypila<...> classes.
In this approach, you should ensure that all the of the implementation is placed into one .cpp file (i.e. one translation unit) and that the explicit instantation is placed after the definition of all the functions (i.e. at the end of the file).
Answer 2: Copy the code into the relevant header file
The common answer is to move all the code from the implementation files cola.cpp and nodo_colaypila.cpp into cola.h and nodo_colaypila.h. In the long run, this is more flexible as it means you can use extra instantiations (e.g. cola<char>) without any more work. But it could mean the same functions are compiled many times, once in each translation unit. This is not a big problem, as the linker will correctly ignore the duplicate implementations. But it might slow down the compilation a little.
Summary
The default answer, used by the STL for example and in most of the code that any of us will write, is to put all the implementations in the header files. But in a more private project, you will have more knowledge and control of which particular template classes will be instantiated. In fact, this 'bug' might be seen as a feature, as it stops users of your code from accidentally using instantiations you have not tested for or planned for ("I know this works for cola<float> and cola<string>, if you want to use something else, tell me first and will can verify it works before enabling it.").
Finally, there are three other minor typos in the code in your question:
You are missing an #endif at the end of nodo_colaypila.h
in cola.h nodo_colaypila<T>* ult, pri; should be nodo_colaypila<T> *ult, *pri; - both are pointers.
nodo_colaypila.cpp: The default parameter should be in the header file nodo_colaypila.h, not in this implementation file.
You will have to define the functions inside your header file.
You cannot separate definition of template functions in to the source file and declarations in to header file.
When a template is used in a way that triggers its intstantation, a compiler needs to see that particular templates definition. This is the reason templates are often defined in the header file in which they are declared.
Reference:
C++03 standard, ยง 14.7.2.4:
The definition of a non-exported function template, a non-exported member function template, or a non-exported member function or static data member of a class template shall be present in every translation unit in which it is explicitly instantiated.
EDIT:
To clarify the discussion on the comments:
Technically, there are three ways to get around this linking problem:
To move the definition to the .h file
Add explicit instantiations in the .cpp file.
#include the .cpp file defining the template at the .cpp file using the template.
Each of them have their pros and cons,
Moving the defintions to header files may increase the code size(modern day compilers can avoid this) but will increase the compilation time for sure.
Using the explicit instantiation approach is moving back on to traditional macro like approach.Another disadvantage is that it is necessary to know which template types are needed by the program. For a simple program this is easy but for complicated program this becomes difficult to determine in advance.
While including cpp files is confusing at the same time shares the problems of both above approaches.
I find first method the easiest to follow and implement and hence advocte using it.
This link explains where you're going wrong:
[35.12] Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file?
Place the definition of your constructors, destructors methods and whatnot in your header file, and that will correct the problem.
This offers another solution:
How can I avoid linker errors with my template functions?
However this requires you to anticipate how your template will be used and, as a general solution, is counter-intuitive. It does solve the corner case though where you develop a template to be used by some internal mechanism, and you want to police the manner in which it is used.

C++ - how to forward declare a class in the same file as main() - getting variable uses undefined class error

I'm aware of using function prototypes, and I was under the impression that forward class declarations could serve a similar purpose when main() and a class are in the same file. For example, I would have expected this would compile:
// main.cpp
#include <iostream>
// class prototypes
class MyClass;
int main(void)
{
MyClass myClass;
// do stuff with myClass here
return(0);
}
class MyClass
{
public:
int someInt;
double someDouble;
// more stuff here . . .
};
But on the MyClass myClass; line I'm getting the error 'myClass' uses undefined class 'MyClass'. What am I doing wrong?
P.S. I'm aware that I could cut/paste main() below all the classes it uses and that would fix the error, but I'd prefer to keep main() as the first function or class.
P.P.S. I'm aware that in any substantial size production program main(), .h content, and .cpp content would be in 3 separate files. In this case I'm attempting to write a small example or test program where main and a class(es) are in the same file.
Forward declarations can only be used via pointers or references.
Calling a constructor function doesn't fall into this category.
I'm aware that I could cut/paste main() below all the classes it uses and that would fix the error, but I'd prefer to keep main() as the first function or class.
That's why usually header files are used, instead of placing all the declarations and definitions in the main.cpp file.
I'm aware that in any substantial size production program main(), .h content, and .cpp content would be in 3 separate files. In this case I'm attempting to write a small example or test program where main and a class(es) are in the same file.
You should still stick to that idiom though, everything else would probably end up in a mess.
This doesn't use forward declarations but it partially addresses the spirit of a single main.cpp with your "main" at the top. I find this technique sometimes useful when you want to share something via an online C++ ide where a single file is much easier to deal with, and you want to focus on the action in main rather than implementation detail in helper structs/classes etc.
#include <iostream>
template<typename MyClass,typename MyOtherClass>
int main_()
{
MyClass a;
a.do_foo();
MyOtherClass b;
b.do_bar();
return 0;
}
struct MyClass
{
void do_foo() { std::cout << "MyClass: do_foo called\n"; }
};
struct MyOtherClass
{
void do_bar() { std::cout << "MyOtherClass: do_bar called\n"; }
};
int main()
{
return main_<MyClass,MyOtherClass>();
}

C++ Variadic Template "multiple definitions"/"already defined" Issue [duplicate]

This question already has answers here:
Explicit specialization of function templates causes linker error
(2 answers)
Closed 8 years ago.
Having recently discovered templates, I've been trying to get a good grasp on them and came across a usage for them that I'd really like to implement; but having tried multiple things, I keep getting errors.
I'm trying to create a function that can take multiple types of parameters at once in any order.
(I'm using VS 2013.)
What I have right now is an "already defined" error (only when including tem.h in more than one file):
the exact first error (the rest are basically the same just for each test.. overload?):
Error 1 error LNK2005: "void __cdecl test<struct A>(struct A *)" (??$test#UA##$$$V##YAXPAUA###Z) already defined in main.obj c:\Users\User\documents\visual studio 2013\Projects\TemplateTest\TemplateTest\foo.obj TemplateTest
foo is just a .cpp file containing only: #include "tem.h"
tem.h:
#ifndef TEM_H
#define TEM_H
struct A {};
struct B {};
#include <iostream>
template<typename First, typename... Rest>
void test(First *t, Rest&&... args){
test(t);
test(std::forward<Rest>(args)...);
}
template<>
void test<A>(A *val){
std::cout << "Handled A" << std::endl;
}
template<>
void test<B>(B *val){
std::cout << "Handled B" << std::endl;
}
#endif
main.cpp:
#include <iostream>
#include "tem.h"
int main(int argc, char *argv[]){
std::cout << "running test..." << std::endl;
A a;
B b;
test(&a, &b);
return 0;
}
What am I missing that is causing this error?
Edit:
Making each instance of test inline prevents the error, but I don't think that's really the best solution
Function template specializations are not themselves templates, they are normal functions with fancy syntax. Like any other normal function, they should not be defined in header files unless you use the inline keyword.
Either add inline to both specializations, or leave just their declarations in the header and move the definitions to some .cpp file.
The main template definition is OK and should be left in the header intact.
A general advice though is to avoid function template specializations. You can use normal non-template overloads:
void test (A*);
void test (B*);

c++ friend function implemented in a class can't access privates

I'm trying to define 2 classes, declare a friend function in one of them and implement it in the other.
I am actually trying to get exactly what found in this post working:
How can friend function be declared for only one particular function and class?
I read the answer and thought it made sense.
then implemented it in my code and it gave me the errors:
Edited:
Error 1 error LNK2019: unresolved external symbol _main referenced in function ___tmainCRTStartup ...\MSVCRTD.lib(crtexe.obj) - (solved by adding main function)
Error 2 error LNK1120: 1 unresolved externals ...\Debug\Project.exe 1 1.. - (solved by adding main function)
Error 3 IntelliSense: member "one::data1" (declared at line 19) is inaccessible ...\Friends.cpp - (Exists even after cleaning and rebuilding project)
I thought it might be a problem with how I implemented it, so I did a copy-paste to the code in the answer:
class one;
class two
{
private:
int data2;
public:
two()
{
data2 = 200;
}
// this goes fine, because the function is not yet defined.
int accessboth(one a);
};
class one
{
private:
int data1;
public:
one()
{
data1 = 100;
}
friend int two::accessboth(one a);
};
// don't forget "inline" if the definition is in a header.
inline int two::accessboth(one a)
{
return (a.data1 + (*this).data2);
}
but I got the same error in the same place: a.data1 was inaccessible
I then searched and found more examples that suggest the same technique to solve the problem, but they all gave me the same inaccessible error in the same place...
Is there something I'm missing? should the provided example (suggested solution from previous post) work?
Something changed in the specification maybe?
thank you for any help.
D
P.S
I tried commenting on the previous post in order not to open a new question about this, but was unable to comment due to low reputation (I'm new here).
You are missing main function. Like this one for example:
int main() {
one a;
two b;
b.accessboth(a);
}
Put it on the bottom of your file and it should compile.