collect2: error: ld returned 1 exit status undefined references

This is my code for an online shopping cart containing two items. I'm getting the following linker error.

c:/mingw/bin/../lib/gcc/mingw32/9.2.0/../../../../mingw32/bin/ld.exe: C:\Users\chine\AppData\Local\Temp\ccQnOWnO.o:main.cpp:(.text+0x121): undefined reference to `ItemToPurchase::SetPrice(int)

I get similar errors to all my functions in my public class. I double-checked all my parameters and made sure they had the matching data types and made sure I'd defined my functions. Here is my code. I'm fairly new to C++ so I'm just looking to learn and make sure this doesn't happen again.

ItemToPurchase.cpp

#include "ItemToPurchase.h"

ItemToPurchase::ItemToPurchase() {
   string itemName = "none"; 
   int itemPrice = 0; 
   int itemQuantity = 0;
}

string ItemToPurchase::GetName() {
   return itemName; 
}

int ItemToPurchase::GetPrice() {
   return itemPrice; 
}

int ItemToPurchase::GetQuantity() {
   return itemQuantity; 
}

void ItemToPurchase::SetName(const char* itemName) {
   this->itemName = itemName;
}

void ItemToPurchase::SetPrice(int price) {
   itemPrice = price; 
}

void ItemToPurchase::SetQuantity(int quantity) {
   itemQuantity = quantity; 
}

main.c

#include "ItemToPurchase.h"

int main() {
    ItemToPurchase Item1;  
    ItemToPurchase Item2; 
    string item1name; 
    int item1price;
    int item1quantity; 
    string item2name; 
    int item2price; 
    int item2quantity; 

    cout << "Item 1"; 
    cout << "Enter the item name: "; 
    getline(cin, item1name);

    item1name = Item1.GetName(); 
    Item1.SetName(item1name.c_str()); 

    cout << "Enter the item price: ";
    cin >> item1price;

    item1price = Item1.GetPrice(); 
    Item1.SetPrice(item1price);

    cout << "Enter the item quantity: ";
    cin >> item1quantity; 

    item1quantity = Item1.GetQuantity();
    Item1.SetQuantity(item1quantity);

    cout << "Item 2";
    cout << "Enter the item name: ";
    getline(cin, item2name); 

    item2name = Item2.GetName();
    Item2.SetName(item2name.c_str()); 

    cout << "Enter the item price: ";
    cin >> item2price; 

    item2price = Item2.GetPrice();
    Item2.SetPrice(item2price); 

    cout << "Enter the item quantity: ";
    cin >> item2quantity; 

    item2quantity = Item2.GetQuantity();
    Item2.SetQuantity(item2quantity);


    cout << "TOTAL COST" << endl; 
    cout << item1name << item1quantity << "@ " << "$" << item1price << "= " << "$" << item1price * item1quantity << endl; 
    cout << item2name << item2quantity << "@ " << "$" << item2price << "= " << "$" << item2price * item2quantity << endl;

    cout << "TOTAL: " << "$" << (item1price * item1quantity) + (item2price * item2quantity) << endl;


    return 0; 

}

header file

#include <string>
#include <iostream> 
using namespace std;

class ItemToPurchase {
public: 
   ItemToPurchase();

   string GetName(); 
   int GetPrice();
   int GetQuantity(); 
   void SetName(const char* itemName);
   void SetPrice(int price);
   void SetQuantity(int quantity);

private:
   string itemName;
   int itemPrice;
   int itemQuantity;
};
#endif