How QT counts the frequency of letters in a string

The problem background ： In a string , Find all the letters , Then count the frequency of each letter in the string . for instance ： character string abbccc, There are three letters in total （a、b、c）, The frequency of each letter is ：1、2、3.

1、 coded

#pragma once
#include <QMap>
#include <QDebug>
#include <QScopedPointer>

namespace
{
    
	bool isLetter(char c)
	{
    
		if ((c >= 'A' && c <= 'Z')
			|| c >= 'a' && c <= 'z')
		{
    
			return true;
		}
		return false;
	}
}

/*! * \brief LetterStatsPrivate class. */
class LetterStatsPrivate : public QMap<QChar, int>
{
    
public:
	LetterStatsPrivate() {
    }
	void StatisticResults()
	{
    
		qDebug() << QString("size(): %1").arg(size());
		QMap<QChar, int>::const_iterator itr;
		for (itr = constBegin(); itr != constEnd(); ++itr)
		{
    
			qDebug() << itr.key() << "\t" << itr.value();
		}
	}
};

/*! * \brief LetterStats class.Character stats */
class LetterStats
{
    
public:
	static QMap<QChar, int> GetStatisticResults(const QString& str)
	{
    
		QMap<QChar, int> result;
		LetterStats ls(str);
		ls.d->swap(result);
		return result;
	}

private:
	LetterStats(const QString& content)
		: d(new LetterStatsPrivate())
	{
    
		for (int i = 0; i < content.size(); ++i)
		{
    
			QChar ch = content.at(i);
			if (/*ch.isLetter()*/ isLetter(ch.toLatin1()))
			{
    
				QChar chLower = ch.toLower();
				int num = d->contains(chLower) ? d->value(chLower) + 1 : 1;
				d->insert(chLower, num);
			}
		}
		d->StatisticResults();
	};

	QScopedPointer<LetterStatsPrivate> d;
};

2、 How to use ？

#include <QtCore/QCoreApplication>
#include "LetterStats.hpp"
#include <QDebug>

int main(int argc, char *argv[])
{
    
    QCoreApplication a(argc, argv);

	const QString word("hello, [email protected]#$%^&* I'm Chinese \r\n123");

	QMap<QChar, int> resultMap = LetterStats::GetStatisticResults(word);
	qDebug() << "resultMap.size()=" << resultMap.size();

    return a.exec();
}

3、 Output results

"size(): 7"
'd'      1
'e'      1
'h'      1
'l'      3
'o'      2
'r'      1
'w'      1
resultMap.size()= 7