Question

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Why do I get “unresolved external symbol” errors when using templates?

I am using templates in my code while there is always an error LNK2019. Here is part of my code:

Method.h

template<typename type>
void Method(Model<type>* sys);

Method.cpp

template<typename type>
void Method(Model<type>* sys){ blablabla;}

Model.h

template<typename type>
class Model{ blablabla;}

class Model1:public Model<double>{ blablabla;}

Main.cpp

Model<double> *sys=new Model1();
Method(sys);

However, there always shows an error LNK2019: unresolved external symbol “void __cdec1 Method(class Model*)” referenced in function_main. Anyone knows where I am going wrong? Thanks a lot!

Answer 1

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.