Not able to pass a variable as a parameter in the constructor (C++)

Question

I'm a bit new to Object Oriented Programming in C++.

I have created this class "Demo".

Demo.h :

    #pragma once

    #include<iostream>

    using namespace std;

    class Demo
    {
     public :
        Demo(int a);
        ~Demo();
    };

Demo.cpp :

#include "Demo.h"

Demo::Demo(int a)
{
    cout << a;
}

Demo::~Demo()
{

}

Now if I pass an integer value while calling the constructor ...... it works properly. Like this :

#include <iostream>
#include<vector>

#include "Demo.h"

using namespace std;

int main()
{
    Demo(1);
}

But when I pass an integer variable as a parameter like this :

#include <iostream>
#include<vector>

#include "Demo.h"

using namespace std;

int main()
{
    int a = 1;
    Demo(a);
}

It doesn't work and gives an error and an exception :

Error (active) E0291 no default constructor exists for class "Demo"
Warning C26444 Avoid unnamed objects with custom construction and destruction

I am curious about why am I getting this error and how can I fix it.

Thanks in advance for the halp.