Classes: Instantiating Object Confusion - c++

Question

Below is code for a simple book list with a class to store book names and isbn numbers into an overloaded function using a vector. This program runs fine and I can test it by returning a specific name (or isbn) using an accessor function from my class.
Question: I tried calling (instantiating?) a constructor with parameters from my class but it would not work, so I commented it out. Yet I was still able to run the program without error. From my main below - //BookData bkDataObj(bookName, isbn);
From watching tutorials, I thought I always had to make an object for a specific constructor from a class that I needed to call? My program definitely still uses my overloaded constructor and function declaration BookData(string, int); without making an object for it in main first.
Thanks for any help or input on this matter.
Main
#include <iostream>
#include <string>
#include <vector>
#include "BookData.h"
using namespace std;
int main()
{
string bookName[] = { "Neuromancer", "The Expanse", "Do Androids Dream of Electric Sheep?", "DUNE" };
int isbn[] = { 345404475, 441569595, 316129089, 441172717 };
//BookData bkDataObj(bookName, isbn); //how did program run without instantiating object for class?
vector <BookData> bookDataArr;
int arrayLength = sizeof(bookName) / sizeof(string);
for (int i = 0; i < arrayLength; i++) {
bookDataArr.push_back(BookData(bookName[i], isbn[i]));
}
cout << "Book 4 is: " << bookDataArr[3].getBookNameCl(); //test if works
return 0;
}
BookData.h
#include <iostream>
#include <string>
using namespace std;
class BookData
{
public:
BookData();
BookData(string, int); //wasn't I supposed to make an object for this constructor in my main?
string getBookNameCl();
int getIsbnCl();
private:
string bookNameCl;
int isbnCl;
};
BookData.cpp
#include "BookData.h"
BookData::BookData() {
bookNameCl = " ";
isbnCl = 0;
}
BookData::BookData(string bookNameOL, int isbnOL) { //how did I use this function
bookNameCl = bookNameOL; //definition without an object in main?
isbnCl = isbnOL;
}
string BookData::getBookNameCl() { //can still return a book name
return bookNameCl;
}
int BookData::getIsbnCl() {
return isbnCl;
}