Why Do I Get "Unresolved External Symbol" Errors When Using Templates

Why do I get unresolved external symbol errors when using templates? [duplicate]

Templated classes and functions are not instantiated until they are used, typically in a separate .cpp file (e.g. the program source). When the template is used, the compiler needs the full code for that function to be able to build the correct function with the appropriate type. However, in this case the code for that function is detailed in the template's source file and hence unavailable.

As a result of all this the compiler just assumes that it's defined elsewhere and only inserts the call to the templated function. When it comes to compile the template's source file, the specific template type that is being used in the program source isn't used there so it still won't generate the code required for the function. This results in the unresolved external symbol.

The solutions available for this are to:

1. include the full definition of
   the member function in the
   template's header file and not have
   a source file for the template,
2. define all the member functions in
   the template's source file as
   "inline" (Update: [this does not work on Visual Studio 2017+]), or
3. define the member
   functions in the template's source
   with the "export" keyword.
   Unfortunately this isn't supported
   by a lot of compilers. (Update: this has been removed from the standard as of C++11.)

Both 1 and 2 basically address the problem by giving the compiler access to the full code for the templated function when it is attempting to build the typed function in the program source.

error LNK2019: unresolved external symbol when using templates [duplicate]

Templates should be implemented in the header.

Method.h

template<typename type>
void Method(Model<type>* sys){ /*Your code here*/ };

See also: https://stackoverflow.com/a/495056/868546

From Mark Ransom:

In the .cpp file, the compiler can't guess what the template parameter will be when you use the function in other files, so it doesn't generate any actual code. The linker notices the lack of code and complains.

The usual way is to define the entire function body in the header, much as an inline function, so the compiler can emit the code when it sees the function being used with the template parameter filled in.

error LNK2019: unresolved external symbol when using templates [duplicate]

Templates should be implemented in the header.

Method.h

template<typename type>
void Method(Model<type>* sys){ /*Your code here*/ };

See also: https://stackoverflow.com/a/495056/868546

From Mark Ransom:

In the .cpp file, the compiler can't guess what the template parameter will be when you use the function in other files, so it doesn't generate any actual code. The linker notices the lack of code and complains.

The usual way is to define the entire function body in the header, much as an inline function, so the compiler can emit the code when it sees the function being used with the template parameter filled in.

Unresolved external symbol with template function

C++ does not allow you to declare a template in a header file and define it in a .cpp file. The reason is that templates can only be created when the template parameters are known and so they can't be complied in advance.

To solve your problem, you will need to declare and define template <class T> T GetAsset(string assetName) in the AssetManager.h file

unresolved external symbol error when trying to compile C++ project [duplicate]

If you compile templates, the source needs to be available at any place where they are used, so include your .cpp file at the bottom of the .h file, or include the .cpp file instead of the .h file.

Unresolved external symbol error on variadic template c++ due to usage of unsigned int

It's a linker error, so (I suppose) you have declared the template function in a header file and defined it in a c++ (not header) file.

If you use the template function that receive the unsigned int in a different c++ file, the compiler doesn't know which versions of the function to implement.

Simple solution: declare and define the template functions/classes/structs in the headers.

If I'm wrong... please prepare a minimal example to replicate the error.

C++ keep getting error LNK2019: unresolved external symbol [duplicate]

You need to put all the template implementations that you have in your .cpp file in the header file, or in a file included by the header. And don't try to compile the implementation file. Some systems attempt to compile files with a .cpp suffix. The compiler needs to see the code in order to instantiate templates.

Unresolved External Symbol -- Template Class [duplicate]

Move the contents of SelectionSort.cpp to SelectionSort.h, just below the class declaration. Also ensure that you have a header guard around the contents of your entire .h file.

The problem comes from how C++ implements templates. Every time it sees a new type used with a template class (like Selection<int>), it recreates the entire class, replacing ItemType with int.

For this reason, it needs to know the full definition of the class (along with its methods) at compile time. It cannot just use the class definition and delay linking until later.

error LNK2019: unresolved external symbol template [duplicate]

You have to move the implementation of the functions which uses template out of .cpp and put it into header file(.h)

template functions and classes should be implemented in header file (.h).

Edit: Compiled in my machine - working fine. Why is the down vote ?
here is the code in header file

#ifndef Header_h
#define Header_h

#include<iostream>
using namespace std;
typedef int BOOL;
#define TRUE 1
#define FALSE 0

template <class t>
class searching
{
    private :
    t *ptr;
    int isize;
    BOOL sort;

    public:
    searching(int into){
        isize =into;
        ptr= new t [isize];
    }
    searching(searching &ref)
    {
        isize=ref.isize;
        ptr=new t[isize];

        for(int i=0;i<isize;i++)
        {
            ptr[i]=ref.ptr[i];
        }
    }
    ~searching(){
        delete []ptr;
    }


    BOOL linearsearch(t value){
        int i=0;
        for(i=0;i<isize;i++)
        {
            if(ptr[i]==value)
            break;
        }
        if(i==isize)
        {
            return FALSE;
        }
        else
        {
            return TRUE;
        }
    }


    void set_data(){
        cout<<"Enter "<<isize<<" element"<<endl;
        for(int i=0;i<isize;i++)
        {
            cin>>ptr[i];
        }
    }
    void get_data(){
        cout<<"Eelements are :"<<endl;
        for(int i=0;i<isize;i++)
        {
            cout<<ptr[i]<<endl;
        }
    }

};
#endif /* Header_h */

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