c++ template argument in vector return value

Question

I have a specific question that I can't really find an answer to in all the questions regarding template member functions. I want to write a function that gets me some data and return it as a vector of a specific type.

I have the following:

#include <vector>

class testClass
{
public:
    template <typename T> std::vector<T> getData(int column);
};


template <typename T> std::vector<T> testClass::getData(int column){
    std::vector<T> returnData;
    return returnData;
}

and to call the function:

int main()
{   
    testClass t;
    std::vector<int> data = t.getData(0);
    return 0;
}

When compiling this I get the error:

../templateTest/main.cpp:9:31: error: no matching member function for call to 'getData'
    std::vector<int> data = t.getData(0);
                            ~~^~~~~~~
../templateTest/testclass.h:8:42: note: candidate template ignored: couldn't infer template argument 'T'
    template <typename T> std::vector<T> getData(int column);
                                         ^

Ok, so it can't get the template argument from the template in the return type. To fix this I try to include the template argument in the call:

int main()
{   
    testClass t;
    std::vector<int> data = t.getData<int>(0);
    return 0;
}

This compiles but gives me a linker error:

Undefined symbols for architecture x86_64:
  "std::__1::vector<int, std::__1::allocator<int> > testClass::getData<int>(int)", referenced from:
      _main in main.o

One final try is to include the template argument also in the function definition:

class testClass
{
public:
    template <typename T> std::vector<T> getData<T>(int column);
};

This however doesn't compile... :

../templateTest/testclass.h:8:42: error: member 'getData' declared as a template
    template <typename T> std::vector<T> getData<T>(int column);

Is it possible what I try to do?

Thanks!!

----------EDIT---------

Putting the implementation in the header does work. If you however prefer to have your implementation in the .cpp. Add the the last line for each implementation that you plan on using.

#include "testclass.h"

template <typename T> std::vector<T> testClass::getData(int column){
    std::vector<T> returnData;
    return returnData;
}

template std::vector<int> testClass::getData(int column);

Side note: Are you sure that it's the function, not the class that should be templated? — UKMonkey
– UKMonkey, Commented Aug 23, 2017 at 9:36
Hmm... I use clang. And yes, it's the function. In the real implementation I need that function to return whatever I need. The actual class represents a whole bunch of different types of data. — YugT
– YugT, Commented Aug 23, 2017 at 9:37

WindyFields · Accepted Answer · 2017-08-23 09:38:52Z

1

Most likely your just defined your function template in a .cpp file instead of .h. That is why on t.getData<int>(0) you get a linker error. If that is the case, read this post.

answered Aug 23, 2017 at 9:38

WindyFields

2,9251 gold badge23 silver badges24 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

chasep255 · Accepted Answer · 2017-08-23 09:44:40Z

0

I just ran the following program and it compiles fine.

#include <vector>

class testClass
{
public:
    template <typename T> std::vector<T> getData(int column);
};


template <typename T> std::vector<T> testClass::getData(int column){
    std::vector<T> returnData;
    return returnData;
}

int main()
{
    testClass t;
    std::vector<int> data = t.getData<int>(0);
    return 0;
}

With templates you should not be using a separate cpp file for the functions. Make sure you put the definition in the header file. This is necessary because any type can be passed in the template and the compiler will need to make a new version of the function for each different type. When dealing with templates I often just define my function inside the class.

class testClass
{
public:
    template <typename T> std::vector<T> getData(int column){
        std::vector<T> returnData;
        return returnData;
    }
};

answered Aug 23, 2017 at 9:44

chasep255

12.2k10 gold badges70 silver badges121 bronze badges

3 Comments

YugT Over a year ago

Sweet, thanks!! I indeed define the function in a separate cpp file.

YugT Over a year ago

Just as some extra info. If you do want to put your code in the cpp, you can add the specific implementation to your cpp file. It will then link. So in case of using let's say and int and a double add:

YugT Over a year ago

template std::vector<int> testClass::getData(int column); template std::vector<double> testClass::getData(int column);

Collectives™ on Stack Overflow

c++ template argument in vector return value

----------EDIT---------

2 Answers 2

Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

----------EDIT---------

2 Answers 2

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related