Error in making Template with Forward Declaration of functions in C++

Question

I'm having the below class code and it is not giving Error when functions' work is defined fully in class itself

#include <iostream>

using namespace std;

template <class user_defined_variable>
class vector
{
    int size;

public:
    user_defined_variable *arr;
    vector(int si = 1, bool choice = false)
    {
        arr = new user_defined_variable[size];
        size = si;
        if (choice)
        {
            cout << "Constructor is called! and size of array is " << size << endl;
        }
    }
    user_defined_variable sum_vector()
    {
        user_defined_variable sum = 0;

        for (int i = 0; i < size; i++)
        {
            sum += this->arr[i];
        }

        return sum;
    }
};
int main()
{

    vector<float> vec_1(3);
    vec_1.arr[0] = 5.6;
    vec_1.arr[1] = 2.12;
    vec_1.arr[2] = 3.004;
    cout << vec_1.sum_vector() << endl;

    return 0;
}

But, when I'm defining functions outside the class and making a Forward Declaration of the functions then it is giving Error The code which is giving Error is as follows

#include <iostream>

using namespace std;

template <class user_defined_variable>
class vector
{
    int size;

public:
    user_defined_variable *arr;
    vector(int, bool);
    user_defined_variable sum_vector();
};

vector::vector(int si = 1, bool choice = false)
{
    arr = new user_defined_variable[size];
    size = si;
    if (choice)
    {
        cout << "Constructor is called! and size of array is " << size << endl;
    }
}
user_defined_variable vector::sum_vector()
{
    user_defined_variable sum = 0;

    for (int i = 0; i < size; i++)
    {
        sum += this->arr[i];
    }

    return sum;
}
int main()
{

    vector<float> vec_1(3);
    vec_1.arr[0] = 5.6;
    vec_1.arr[1] = 2.12;
    vec_1.arr[2] = 3.004;
    cout << vec_1.sum_vector() << endl;

    return 0;
}

And the Error is

class_with_error.cpp:16:1: error: invalid use of template-name 'vector' without an argument list
 vector::vector(int si = 1, bool choice = false)
 ^~~~~~
class_with_error.cpp:16:1: note: class template argument deduction is only available with -std=c++17 or -std=gnu++17
class_with_error.cpp:6:7: note: 'template<class user_defined_variable> class vector' declared here
 class vector
       ^~~~~~
class_with_error.cpp:25:1: error: 'user_defined_variable' does not name a type
 user_defined_variable vector::sum_vector()
 ^~~~~~~~~~~~~~~~~~~~~
class_with_error.cpp: In function 'int main()':
class_with_error.cpp:39:26: error: no matching function for call to 'vector<float>::vector(int)'
     vector<float> vec_1(3);
                          ^
class_with_error.cpp:12:5: note: candidate: 'vector<user_defined_variable>::vector(int, bool) [with user_defined_variable = float]'
     vector(int, bool);
     ^~~~~~
class_with_error.cpp:12:5: note:   candidate expects 2 arguments, 1 provided
class_with_error.cpp:6:7: note: candidate: 'constexpr vector<float>::vector(const vector<float>&)'
 class vector
       ^~~~~~
class_with_error.cpp:6:7: note:   no known conversion for argument 1 from 'int' to 'const vector<float>&'        
class_with_error.cpp:6:7: note: candidate: 'constexpr vector<float>::vector(vector<float>&&)'
class_with_error.cpp:6:7: note:   no known conversion for argument 1 from 'int' to 'vector<float>&&'

Please help if you know the answer Some of the reference StackOverflow articles are

Your class and `std::vector` are going to conflict, especially when `using namespace std;`. Don't do that. That said, please reduce this to a [mcve]! — Ulrich Eckhardt, Oct 10 '21 at 06:15
There are lots of code here that aren't necessary to reproduce the problem. — Ulrich Eckhardt, Oct 10 '21 at 06:18

score 2 · Accepted Answer · answered Oct 10 '21 at 05:49

When you are defining your constructor this way:

vector::vector(int si = 1, bool choice = false)
{
    arr = new user_defined_variable[size];
    size = si;
    if (choice)
    {
        cout << "Constructor is called! and size of array is " << size << endl;
    }
}

You need to provide the template and it's arguments:

template <class user_defined_variable>
vector<user_define_variable>::vector(int si = 1, bool choice = false)
{
    arr = new user_defined_variable[size];
    size = si;
    if (choice)
    {
        cout << "Constructor is called! and size of array is " << size << endl;
    }
}

There are other issues with your code, as you can see from the errors you're getting, but we'll focus on this one.

Please provide the full code as I'm a beginner I'm facing problems — Aman Rathore, Oct 10 '21 at 06:16
The question as asked has been effectively answered. Adding further solutions for the other mistakes in the rest of the code runs the risk of cluttering the answer and reducing its usefulness to future askers. — user4581301, Oct 10 '21 at 06:34

Error in making Template with Forward Declaration of functions in C++

1 Answers1