How to separate a realisation of template constructor (if the class isn't template)

Question

I've read this page Why can templates only be implemented in the header file? but it's about a template class. And the constructor takes same type as class (if class is < int >, constructor also takes int, and so on). But I have simple (non-template) class. Sorry for duplication, please explain what should I do in this case?

I have a class 'Object' with field 'double d' and I want to make a template constructor. If I realize it inside the class, all works good, but if move the realisation outside of the class (from .h to .cpp), it doesn't work. Here's my code:

Object.h

class Object {
    double d;
public:
    template <class T> Object(T t);
};

Object.cpp

#include "Object.h"

template <class T> Object::Object(T t) {
    d = t;
}

main.cpp

#include "Object.h"

int main() {
    int a = 5;
    Object x(a);
    float b = 2.5;
    Object y(b);
    return 0;
}

Error appears in main.cpp in lines that creates x and y, there are:

undefined reference to `Object::Object<int>(int)'
undefined reference to `Object::Object<float>(float)'

100% duplicate of [Why can templates only be implemented in the header file?](http://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file) — ForceBru, Apr 10 '16 at 19:55

ForceBru · Accepted Answer · 2016-04-10T20:13:44.463

1

Get rid of Object.cpp and move its contents to a header file.

When the compiler deals with Object.cpp, it doesn't generate any byte code because there are only some templates. When it encounters a 'mention' of Object::Object<int>(int) in your code, it goes to the header file, deduces the template parameters and successfully compiles everything.

The linker, however, isn't happy. It sees a call to Object::Object<int>(int), but can't find byte code for this method and raises an error.

Otherwise, you can define template <class T> Object::Object(T t) in main, but that's not so good and may lead to confusion.

edited Apr 10 '16 at 20:13

answered Apr 10 '16 at 19:54

ForceBru

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About adding "template class Object; template class Object; " at the beginning of main.cpp - it doesn't work because I have non-template class (not "template Object", but simple "class Object"). – Pavel Apr 10 '16 at 20:14
So it's remaining one question - is it a good tone to write the realisation of constructor inside the interface? – Pavel Apr 10 '16 at 20:16
@Pavel, if this constructor isn't very long, then it's OK – ForceBru Apr 10 '16 at 20:17
If I understand right, interface needs to show user which methods he can use – Pavel Apr 10 '16 at 20:18
@Pavel, that's why they're called _interfaces_ – ForceBru Apr 10 '16 at 20:22

score 0 · Answer 2 · answered Apr 10 '16 at 20:04

0

You must implement all templates you want to use in the header file. If you only implement them in the .cpp file, only that .cpp file can see it.

answered Apr 10 '16 at 20:04

kmdreko

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How to separate a realisation of template constructor (if the class isn't template)

2 Answers2