Template Linking Error

[Ralph Wiggum]

First post here. I think I am missing a "T" somewhere. Anyway, this is just a trivial container task that holds one element. All I have written is a constructor. It works fine when the implementation is in the .h file, but when I try to place the constructor code into a separate .cpp, I get the following errors.

C:\Users\Study\Documents\Study\study2\mycontainer. cpp|5|error: `template<class T> class mycontainer' used without template parameters|
C:\Users\Study\Documents\Study\study2\mycontainer. cpp|6|error: ISO C++ forbids declaration of `mycontainer' with no type|
C:\Users\Study\Documents\Study\study2\mycontainer. cpp|6|error: declaration of template `template<class T> int mycontainer(T)'|
C:\Users\Study\Documents\Study\study2\mycontainer. h|8|error: conflicts with previous declaration `template<class T> class mycontainer'|
C:\Users\Study\Documents\Study\study2\mycontainer. h|8|error: previous non-function declaration `template<class T> class mycontainer'|
C:\Users\Study\Documents\Study\study2\mycontainer. cpp|6|error: conflicts with function declaration `template<class T> int mycontainer(T)'|
C:\Users\Study\Documents\Study\study2\mycontainer. cpp||In function `int mycontainer(T)':|
C:\Users\Study\Documents\Study\study2\mycontainer. cpp|7|error: `element' was not declared in this scope|
C:\Users\Study\Documents\Study\study2\mycontainer. cpp|8|warning: no return statement in function returning non-void|
||=== Build finished: 7 errors, 1 warnings ===|

Code is as follows...

// mycontainer.h
#ifndef _MYCONTAINER_H
#define _MYCONTAINER_H




template <class T>
class mycontainer
{
    private:
        T element;
    public:
        mycontainer (T arg);
};
#endif

//  mycontainer.cpp
#include "mycontainer.h"


template <class T>
mycontainer::mycontainer (T arg)
{
    element = arg;
}

// main.cpp
#include "mycontainer.h"




int main ()
{
    mycontainer<int> myint (10);
    return 0;
}

[jimblumberg]

When dealing with templates both the definition and the implementation must be in the same compilation unit (usually the same file).

Jim

[King Mir]

I suggest doing this:

// mycontainer.h
#ifndef _MYCONTAINER_H
#define _MYCONTAINER_H

template <class T>
class mycontainer
{
    private:
        T element;
    public:
        mycontainer (T arg);
};
#include "mycontainer.hpp"
#endif

//  mycontainer.hpp

template <class T>
mycontainer::mycontainer (T arg)
{
    element = arg;
}

// main.cpp unchanged
#include "mycontainer.h"

int main ()
{
    mycontainer<int> myint (10);
    return 0;
}

[laserlight]

By the way, change the _MYCONTAINER_H to say, MYCONTAINER_H. Names that begin with an underscore followed by an uppercase letter are reserved to the implementation for any use.

[phantomotap]

O_o

At least a part of the problem is that all the code posted so far is wrong.

template <class T>
mycontainer::mycontainer (T arg)
{
    element = arg;
}

-->

template <class T>
mycontainer<T>::mycontainer (T arg)
{
    element = arg;
}

Soma

[Ralph Wiggum]

These two changes made it work. Thanks.

I suggest doing this...

// ...
#include "mycontainer.hpp"
// ...

O_o
At least a part of the problem is that all the code posted so far is wrong.

template <class T>
mycontainer<T>::mycontainer (T arg)

[Ralph Wiggum]

By the way, change the _MYCONTAINER_H to say, MYCONTAINER_H. Names that begin with an underscore followed by an uppercase letter are reserved to the implementation for any use.

Thank you. I will do that from now on. I've seen reserved terms like __cplusplus which are proceeded by two underscores and a lower case letter. You're saying that I, the programmer, should never define something with an underscore followed by an upper case letter? Those symbols are reserved by the compiler/linker (is that you mean by "implementation"?)?

[King Mir]

Right, as a programmer (and not a compiler writer), you shouldn't use names that start with a underscore followed by an upper-case letter, or contain a double underscore.

This page has a more complete answer:
standards - What are the rules about using an underscore in a C++ identifier? - Stack Overflow