Full source code is available on github.

There are several major source files for the C++ version of QQWing:

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/*
 * qqwing - A Sudoku solver and generator
 * Copyright (C) 2006-2014 Stephen Ostermiller http://ostermiller.org/
 * Copyright (C) 2007 Jacques Bensimon (jacques@ipm.com)
 * Copyright (C) 2011 Jean Guillerez (j.guillerez - orange.fr)
 * Copyright (C) 2014 Michael Catanzaro (mcatanzaro@gnome.org)
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
#ifndef QQWING_HPP
    #define QQWING_HPP

    #include <string>
    #include <vector>

    namespace qqwing {

        using namespace std;

        class LogItem;

        const int GRID_SIZE = 3;
        const int ROW_COL_SEC_SIZE = GRID_SIZE*GRID_SIZE;
        const int SEC_GROUP_SIZE = ROW_COL_SEC_SIZE*GRID_SIZE;
        const int BOARD_SIZE = ROW_COL_SEC_SIZE*ROW_COL_SEC_SIZE;
        const int POSSIBILITY_SIZE = BOARD_SIZE*ROW_COL_SEC_SIZE;

        /**
         * The version of QQwing, e.g. 1.2.3
         */
        string getVersion();

        /**
         * The board containing all the memory structures and
         * methods for solving or generating sudoku puzzles.
         */
        class SudokuBoard {
            public:
                enum PrintStyle {
                    ONE_LINE,
                    COMPACT,
                    READABLE,
                    CSV
                };
                enum Difficulty {
                    UNKNOWN,
                    SIMPLE,
                    EASY,
                    INTERMEDIATE,
                    EXPERT
                };
                enum Symmetry {
                    NONE,
                    ROTATE90,
                    ROTATE180,
                    MIRROR,
                    FLIP,
                    RANDOM
                };
                SudokuBoard();
                bool setPuzzle(int* initPuzzle);
                const int* getPuzzle();
                const int* getSolution();
                void printPuzzle();
                void printSolution();
                bool solve();

                /**
                 * Count the number of solutions to the puzzle
                 */
                int countSolutions();

                /**
                 * Count the number of solutions to the puzzle
                 * but return two any time there are two or
                 * more solutions.  This method will run much
                 * falter than countSolutions() when there
                 * are many possible solutions and can be used
                 * when you are interested in knowing if the
                 * puzzle has zero, one, or multiple solutions.
                 */
                int countSolutionsLimited();

                /**
                 * return true if the puzzle has no solutions at all
                 */
                bool hasNoSolution();

                /**
                 * return true if the puzzle has a solution
                 * and only a single solution
                 */
                bool hasUniqueSolution();

                /**
                 * return true if the puzzle has more than one solution
                 */
                bool hasMultipleSolutions();
                bool isSolved();
                void printSolveHistory();
                void setRecordHistory(bool recHistory);
                void setLogHistory(bool logHist);
                void setPrintStyle(PrintStyle ps);
                bool generatePuzzle();
                bool generatePuzzleSymmetry(SudokuBoard::Symmetry symmetry);
                int getGivenCount();
                int getSingleCount();
                int getHiddenSingleCount();
                int getNakedPairCount();
                int getHiddenPairCount();
                int getBoxLineReductionCount();
                int getPointingPairTripleCount();
                int getGuessCount();
                int getBacktrackCount();
                void printSolveInstructions();
                SudokuBoard::Difficulty getDifficulty();
                string getDifficultyAsString();
                ~SudokuBoard();

            private:
                /**
                 * The 81 integers that make up a sudoku puzzle.
                 * Givens are 1-9, unknowns are 0.
                 * Once initialized, this puzzle remains as is.
                 * The answer is worked out in "solution".
                 */
                int* puzzle;

                /**
                 * The 81 integers that make up a sudoku puzzle.
                 * The solution is built here, after completion
                 * all will be 1-9.
                 */
                int* solution;

                /**
                 * Recursion depth at which each of the numbers
                 * in the solution were placed.  Useful for backing
                 * out solve branches that don't lead to a solution.
                 */
                int* solutionRound;

                /**
                 * The 729 integers that make up a the possible
                 * values for a Sudoku puzzle. (9 possibilities
                 * for each of 81 squares).  If possibilities[i]
                 * is zero, then the possibility could still be
                 * filled in according to the Sudoku rules.  When
                 * a possibility is eliminated, possibilities[i]
                 * is assigned the round (recursion level) at
                 * which it was determined that it could not be
                 * a possibility.
                 */
                int* possibilities;

                /**
                 * An array the size of the board (81) containing each
                 * of the numbers 0-n exactly once.  This array may
                 * be shuffled so that operations that need to
                 * look at each cell can do so in a random order.
                 */
                int* randomBoardArray;

                /**
                 * An array with one element for each position (9), in
                 * some random order to be used when trying each
                 * position in turn during guesses.
                 */
                int* randomPossibilityArray;

                /**
                 * Whether or not to record history
                 */
                bool recordHistory;

                /**
                 * Whether or not to print history as it happens
                 */
                bool logHistory;

                /**
                 * A list of moves used to solve the puzzle.
                 * This list contains all moves, even on solve
                 * branches that did not lead to a solution.
                 */
                vector<LogItem*>* solveHistory;

                /**
                 * A list of moves used to solve the puzzle.
                 * This list contains only the moves needed
                 * to solve the puzzle, but doesn't contain
                 * information about bad guesses.
                 */
                vector<LogItem*>* solveInstructions;

                /**
                 * The style with which to print puzzles and solutions
                 */
                PrintStyle printStyle;

                /**
                 * The last round of solving
                 */
                int lastSolveRound;
                bool reset();
                bool singleSolveMove(int round);
                bool onlyPossibilityForCell(int round);
                bool onlyValueInRow(int round);
                bool onlyValueInColumn(int round);
                bool onlyValueInSection(int round);
                bool solve(int round);
                int countSolutions(bool limitToTwo);
                int countSolutions(int round, bool limitToTwo);
                bool guess(int round, int guessNumber);
                bool isImpossible();
                void rollbackRound(int round);
                bool pointingRowReduction(int round);
                bool rowBoxReduction(int round);
                bool colBoxReduction(int round);
                bool pointingColumnReduction(int round);
                bool hiddenPairInRow(int round);
                bool hiddenPairInColumn(int round);
                bool hiddenPairInSection(int round);
                void mark(int position, int round, int value);
                int findPositionWithFewestPossibilities();
                bool handleNakedPairs(int round);
                int countPossibilities(int position);
                bool arePossibilitiesSame(int position1, int position2);
                void addHistoryItem(LogItem* l);
                void shuffleRandomArrays();
                void print(int* sudoku);
                void rollbackNonGuesses();
                void clearPuzzle();
                void printHistory(vector<LogItem*>* v);
                bool removePossibilitiesInOneFromTwo(int position1, int position2, int round);
        };
    }
#endif