Template class constructor

Question

Ok guys... I have following class

#include <functional>

template <typename TValue, typename TPred = std::less<TValue>>
class BinarySearchTree {
    struct TNode {
        TValue value;
        TNode *pLeft;
        TNode *pRight;
    };
public:
    BinarySearchTree();
    ~BinarySearchTree();

    . . .
private:
    TNode *pRoot;

     . . .
};

then in my .cpp file i defined the ctor/dtor like this:

template <typename TValue, typename TPred>
BinarySearchTree<TValue, TPred>::BinarySearchTree() : pRoot(0) {}

template <typename TValue, typename TPred>
BinarySearchTree<TValue, TPred>::~BinarySearchTree() {
    Flush(pRoot);
}

my main function:

int main() {    
    BinarySearchTree<int> obj1;
    return 0;
}

and i get following linkage error:

public: __thiscall BinarySearchTree<int,struct std::less<int>>::BinarySearchTree<int,struct std::less<int> >(void)

i tried to put the constructor definition into the header file and i get no error. only if i try to define it in the cpp file.

The answer to your question is [here](http://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file) - sorry, wrong link in the close vote — Andy Prowl, Jun 03 '13 at 18:53

score 24 · Accepted Answer · answered Jun 03 '13 at 18:58

24

Don't define templates in the cpp file, but put the implementation of the functions in the header file and leave your main function as it is. Templates get inlined by default. Therefore they are not visible to the linker. And the file that contains main() cannot see the definition of the templates. Hence the error.

answered Jun 03 '13 at 18:58

Ralph Tandetzky

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I wish this was more visible on all the C++ tutorial websites. – Nathan Goings Feb 01 '17 at 04:02

score 0 · Answer 2 · answered Jun 03 '13 at 18:58

0

leave space at last and try. it might be taken as shift left operator!

template <typename TValue, typename TPred = std::less<TValue> >

answered Jun 03 '13 at 18:58

Dineshkumar

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It is OK to not add space between `>>` since C++11. – taocp Jun 03 '13 at 19:02

Template class constructor

2 Answers2