LNK2019 error for Visual Studio - unresolved external symbol - c++

Question

I am a beginner and running into the above-said error. The following is the complete code from three files:
ball.h:
#ifndef BALL_H
#define BALL_H
namespace
{
inline constexpr double gravity{ 9.81 };
}
double getInitialHeight(void);
double calculateHeight(double, int);
void printHeight(double, int);
void calculateAndPrintHeight(double, int);
void solve(void);
#endif
ball.cpp:
#include "ball.h"
#include <iostream>
double getInitialHeight()
{
std::cout << "Enter the height of the tower in meters ";
double initialHeight{};
std::cin >> initialHeight;
return initialHeight;
}
double calculateHeight(double initialHeight, int secondsPassed)
{
double distanceFallen{ BALL_H::gravity * secondsPassed * secondsPassed / 2.0 };
double currentHeight{ initialHeight - distanceFallen };
return currentHeight;
}
void printHeight(double height, int secondsPassed)
{
if (height > 0.0)
{
std::cout << "At " << secondsPassed << " seconds, the ball is at height\t" << height << " meters.\n";
}
else
{
std::cout << "At " << secondsPassed << " seconds, the ball is on the ground.\n";
std::exit(0);
}
}
void calculateAndPrintHeight(double initialHeight, int secondsPassed)
{
double height{ calculateHeight(initialHeight, secondsPassed) };
printHeight(height, secondsPassed);
}
void solve()
{
const double initialHeight{ getInitialHeight() };
int secondsPassed{ 0 };
while (true)
{
calculateAndPrintHeight(initialHeight, secondsPassed);
secondsPassed++;
}
}
Solution.cpp(the main project file in the solution):
#include <iostream>
#include "ball.h"
int main()
{
solve();
return 0;
}
I understand that this problem is caused because linker cannot find a reference to solve(). However, I am not sure how to solve the issue. One easy solution is to simply include ball.cpp rather than ball.h:
#include <iostream>
#include "ball.cpp"
int main()
{
solve();
return 0;
}
This code works but I'd like to know how to use headers instead since I am not sure if this a good practice.
EDIT:
Here's the error list:

Answer 1

When you add header file, you could right click Header Files and select Add->New Item.
When you add .cpp file, you could right click Source Files and select Add->New Item.
Then, add the following code in Source.cpp.
#include <iostream>
#include "ball.h"
int main()
{
solve();
return 0;
}
Finally, it works fine.