Sum of vector of vectors of vectors ... of integers

user_6609908 0
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I'm trying to sum all the elements of a vector of vectors of vectors ... of integers: something like a std::vector<std::vector<std::vector<std::vector<int>>>> where there's no need to every layer have the same size.

I would like to accomplish it using template, so I did it:

namespace nn
{
    template < class T >
    int sumAllElements(std::vector<T> v)
    {
        int size = v.size();
        int output = 0;

        for ( int i = 0 ; i < size ; i++ )
        {
                                                //should call the function below 
            output += sumAllElements( v[ i ] ); //or this function, depending in 
                                                //which "layer" we are
        }

        return output;
    }

    int sumAllElements(std::vector<int> v)
    {
        int size = v.size();
        int output = 0;

        for ( int i = 0 ; i < size ; i++ )
        {
            output += v[ i ]; //we've reached the bottomest layer,
                              //so just sum everybory
        }

        return output;
    }
}

BUT, this is happening:

CMakeFiles\test.dir/objects.a(main.cpp.obj): In function `main':
D:/test/main.cpp:49: undefined reference to `int nn::sumAllElements<std::vector<int, std::allocator<int> > >(std::vector<std::vector<int, std::allocator<int> >, std::allocator<std::vector<int, std::allocator<int> > > >)'
collect2.exe: error: ld returned 1 exit status
mingw32-make.exe[3]: *** [CMakeFiles\test\build.make:141: test.exe] Error 1
mingw32-make.exe[2]: *** [CMakeFiles\Makefile2:67: CMakeFiles/test.dir/all] Error 2
mingw32-make.exe[1]: *** [CMakeFiles\Makefile2:79: CMakeFiles/test.dir/rule] Error 2
mingw32-make.exe: *** [Makefile:117: test] Error 2

I really don't know why...

Thanks in advance.

vector templates c++
3 Answers

You can't call a function that hasn't been declared yet. Templates can sometimes make that problem go away, but not always. And this is one of the cases where you simple need a declaration of int sumAllElements(std::vector<int> v) prior to template < class T > int sumAllElements(std::vector<T> v)

MSalters_154160

Reading your error message. It looks like your functions are in a separate compilation unit from main.cpp. If your functions are in a .h file, #include the header file in main.cpp.

I would suggest using the template specialization declaration:

template<>
int sumAllElements(std::vector<int> v)
{
 ...   
}

Another, unrelated suggestion, would be to pass the vectors by const reference. Currently, you are passing them by value which could be costly if the vectors are large.

user_17332910

You can use SFINAE to enable/disable the specialization that is needed:

template <class T, std::enable_if_t<std::is_arithmetic<T>::value, int> = 0>
auto sum_all(const std::vector<T>& v)
{
    T sum = 0;

    for (auto& e : v)
    {
        sum += e;
    }

    return sum;
}

template <class T, std::enable_if_t<!std::is_arithmetic<T>::value, int> = 0>
auto sum_all(const std::vector<T>& nested_v)
{
    decltype(sum_all(nested_v[0])) sum = 0;

    for (auto& e : nested_v)
    {
        sum += sum_all(e);
    }

    return sum;
}

See it on coliru

With C++17 you can have just one function (neat!):

template <class T>
auto sum_all(const std::vector<T>& nested_v)
{
    innermost_type_t<T> sum = 0;

    for (auto& e : nested_v)
    {
        if constexpr(std::is_arithmetic<T>::value)
            sum += e;
        else
            sum += sum_all(e);
    }

    return sum;
}

With innermost_type_t defined as:

template <class T> struct innermost_type
{
    using type = T;
};

template <class T> struct innermost_type<std::vector<T>>
{
    using type = typename innermost_type<T>::type;
};

template <class T>
using innermost_type_t = typename innermost_type<T>::type;
user_28053050
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