I am trying to run this sudoku solver and a weird error appears

Question

My objective is to solve a sudoku game while using a backtracking algorithm. When I run it I get this error message:

obj/sudoku_solver.o: In function sudoku_backtracker(std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >&)': /home/user/Documents/project2v1/src/sudoku_solver.cpp:8: undefined reference tofind_empty_spot(std::__1::vector >, std::__1::allocator > > >)' clang: error: linker command failed with exit code 1 (use -v to see invocation) make: * [project2.out] Error 1

I don't understand what is going on and if somebody could explain what this means and what I did wrong it would be a great help. Thanks guys.

main.cpp

#include <iostream>
#include <string>
#include <sstream>
#include <vector>
#include "sudoku_solver.hpp"

int main()
{


    std::vector< std::vector<int> > board(9,std::vector<int>(9));

    int i =0,j=0;

    for(std::string line;std::getline(std::cin,line);)
    {
        if(i==9)
        {
            i=0;
            break;
        }

        std::stringstream line_stream(line);
        for(std::string token;std::getline(line_stream,token,' ');)
        {
            if(j==9)
            {
                j=0;
            }
            board[i][j] = std::stoi(token);
            j++;
        }
        i++;
    }

    if(sudoku_backtracker(board)==1)
    {
        for(int i = 0;i<9;i++)
        {
            for(int j = 0;j<9;j++)
            {
                std::cout<<board[i][j];
            }
            std::cout<<endl;
        }
    }
    return 0;
}

sudoku_solver.hpp

#ifndef __SUDOKU_SOLVER_HPP__
#define __SUDOKU_SOLVER_HPP__
#include <vector>

int sudoku_backtracker(std::vector< std::vector<int> > &board);

std::pair<int, int> find_empty_spot(std::vector< std::vector<int> > board);

bool checks_num_is_valid(std::vector< std::vector<int> > board,int row,int column,int number);

#endif //__SUDOKU_SOLVER_HPP__

sudoku_solver.cpp

#include "sudoku_solver.hpp"
#include <iostream>
#include <string>

int sudoku_backtracker(std::vector< std::vector<int> > &board)
{
    int test_num = 1;
    std::pair<int,int> empty_spot = find_empty_spot(board);
    if(empty_spot.first == -1)
    {
        return 1;
    }

    while(test_num != 10)
    {
        if(checks_num_is_valid(board,empty_spot.first,empty_spot.second,test_num))
        {
            board[empty_spot.first][empty_spot.second] = test_num;
            int recursive_sudoku = sudoku_backtracker(board);
            if(recursive_sudoku==1)
            {
                return 1;
            }
            board[empty_spot.first][empty_spot.second] = 0;
        }

        test_num++;
    }
    return 0;
}


std::pair<int, int> find_empty_spot(std::vector< std::vector<int> > board,int row,int column)
{
    for(int row=0;row<9;row++)
    {
        for(int column=0;column<9;column++)
        {
            if(board[row][column] == 0){return std::make_pair(row,column);}
        }
    }
    return std::make_pair(-1,-1);
}

bool checks_num_is_valid(std::vector< std::vector<int> > board, int row,int column, int number)
{
    bool num_not_in_column = true;
    bool num_not_in_row = true;
    bool num_not_in_box = true;
    //box_start_row as bsr and box_start_column as bsc
    //this is the starting point to check the numbers inside the box and make
    //sure the test number is valid
    int bsr = 0,bsc = 0;

    //checks the numbers in the same column but different rows
    for(int i =0;i<9;i++)
    {
        if(i==row){continue;}
        if(board[i][column] == number){num_not_in_column = false;break;}
    }
    //checks numbers in the same row but different columns
    for(int i = 0;i<9;i++)
    {
        if(i==column){continue;}
        if(board[row][i] == number){num_not_in_row = false;break;}
    }

    //checks wether the numer is int the same box
    if(row<=2){bsr =0;}
    if(row>=3 && row<=5){bsr = 3;}
    if(row>=6 && row<=8){bsr = 6;}
    if(column <=2){bsc =0;}
    if(column>=3 && column<=5){bsc=3;}
    if(column>=6 && column<=8){bsc=6;}
    //double for loop to check all the values inside the box
    for(bsr;bsr<bsr+3;bsr++)
    {
        for(bsc;bsc<bsc+3;bsc++)
        {
            if(bsr==row && bsc==column)
            {continue;}
            else
            {
                if(board[bsr][bsc] == number)
                {
                    num_not_in_box = false;
                }
            }
        }
    }
    bool result = num_not_in_row && num_not_in_column && num_not_in_box;
    return result;
}

Answer 1

This time this declaration:

std::pair<int, int> find_empty_spot(std::vector< std::vector<int> > board);

does not match definition:

std::pair<int, int> find_empty_spot(std::vector< std::vector<int> > board,int row,int column)

neither usage matches definition:

std::pair<int,int> empty_spot = find_empty_spot(board);