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I created two source files (main.cpp, cal.cpp) and one header file (cal.h).

In cal.cpp,I defined one template function 

template <class T>
T add_no(T a, T b){
    return (a + b);
}
template add_no<int>;

In cal.h, declared that.

template <class T>
T add_no(T a, T b);

From main.cpp, I called 

add_no(2, 5);

but getting the following error.

src\cal.cpp:30:10: error: ISO C++ forbids declaration of 'add_no' with no type [-fpermissive]
 template add_no<int>;
          ^
src\cal.cpp:30:10: error: template-id 'add_no<int>' used as a declarator
src\cal.cpp:30:10: error: 'add_no(T, T)' is not a variable template
* [.pio\build\esp32dev\src\cal.cpp.o] Error 1

Any idea, how I can solve it?

Jaideep Shekhar
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Vivek Roy
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  • Does this answer your question? [Why can templates only be implemented in the header file?](https://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file) – Richard Critten Jan 11 '20 at 13:42

1 Answers1

2

The correct syntax for explicit function template instantiation requires you to write out the full substituted signature:

template int add_no<int>(int, int);

instead of

template add_no<int>;

This syntax is necessary, because function templates can be overloaded and the compiler must be able to resolve the function template that you are referring to.

If the template argument is deducible from the function parameters, as is the case here, then you can drop the template argument list:

template int add_no(int, int);
walnut
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  • @VivekRoy Template argument deduction is done on the explicit template instantiation similar to how it works with function calls: `add_no(0, 0)` works the same way as `add_no(0, 0)` does, because the template argument `` can be deduced from the arguments `(0, 0)` of type `(int, int)`. – walnut Feb 11 '20 at 10:57