QT learning 11 string classes in QT

Qt Study 11 Qt String class in

an issue rooted in history

• C Language I won't support it In the real sense character string
• C Language use A character array and A set of functions Implement string operation
• C Language I won't support it Custom type , therefore Cannot get string type

Solution

• from C To C++ The evolutionary process of introduced Custom type

• stay C++ Medium can Complete the definition of string type through class

  C++ Whether the native type system in contains string types ？

Standard library STL

• STL yes In a sense Need and C++ Together with Standard library
• STL It's a set Completed by template technology C++ Class library
• STL Contains common Algorithm and data structure
• STL It contains String class

Qt VS STL

• STL The specific Implementation depends on the compiler manufacturer

• STL Of “ standard ” It's just that The interface is standard

  • Same global function
  • The same algorithm class and data structure class
  • The same class member function

• Different manufacturers The compiler of STL Differences exist

  Depend on STL Developed C++ The behavior of programs on different platforms may vary

Pre development choices

• Whether the project needs to use Current library support ？
  • STL？Qt？MFC？ Private libraries ？
• Whether the project needs to be in Migration between different platforms ？
  • Linux？Windows？Android？
• Whether the project needs Graphical user interface ？
  • GUI Applications ？ Command line application ？ Background services ？

Qt String class in

• Qt String class in
  • use Unicode code
  • Use Implicit sharing technology To save memory and unnecessary data copies
  • Cross platform use , There is no need to consider the platform compatibility of strings

QString VS string

• QString Direct support for The conversion between strings and numbers
• QString Direct support for String size comparison
• QString Direct support for Conversion between different character codes
• QString Direct support for std::string and std::wstring Mutual conversion of
• QString Direct support for Applications of regular expressions
• ...

Code example

#include <QDebug>

void Sample_1()
{
    
    QString s = "add";
    s.append(" ");    // "add "
    s.append("Qt");   // "add Qt"
    s.prepend(" ");   // " add Qt"
    s.prepend("C++"); // "C++ add Qt"
    qDebug() << s;
    s.replace("add", "&"); // "C++ & Qt"
    qDebug() << s;
}

void Sample_2()
{
    
    QString s = "";
    int index = 0;
    
    s.sprintf("%d. I'm %s, thank you!", 1, "Delphi Tang"); // "1. I'm Delphi Tang, thank you!"
    qDebug() << s;
    
    index = s.indexOf(",");
    s = s.mid(0, index);   // "1. I'm Delphi Tang"
    qDebug() << s;
    
    index = s.indexOf(".");
    s = s.mid(index + 1, s.length()); // " I'm Delphi Tang"
    s = s.trimmed();                  // "I'm Delphi Tang"
    qDebug() << s;

    index = s.indexOf(" ");
    s = s.mid(index + 1, s.length()); // "Delphi Tang"
    qDebug() << s;
}

void Sample_3(QString* a, int len)
{
    
    for(int i = 0; i < len; i++) {
    
        for(int j=i+1; j<len; j++) {
    
            if( a[j] < a[i] ) {
    
                QString tmp = a[i];
                a[i] = a[j];
                a[j] = tmp;
            }
        }
    }
}

int main()
{
    
    qDebug() << "Sample_1:";
    Sample_1();
    qDebug() << endl;
    qDebug() << "Sample_2:";

    Sample_2();
    qDebug() << endl;

    qDebug() << "Sample_3:";
    QString company[5] = {
    
        QString("Oracle"),
        QString("Borland"),
        QString("Microsoft"),
        QString("IBM"),
        QString("D.T.Software")
    };
    Sample_3(company, 5);
    for(int i = 0; i < 5; i++) {
    
        qDebug() << company[i];
    }

    return 0;
}

Qt Medium QString

• QString stay Qt The library is almost ubiquitous
• be-all Qt Graphical user components all depend on QString

Add the message corresponding to the calculator program

/* QCalculatorUI.cpp */
//  Content in previous code 
void QCalculatorUI::onButtonClicked()
{
    
    QPushButton* btn = (QPushButton*)sender();
    QString clickText = btn->text();

    if ( clickText == "<-" ) {
    
        QString text = m_edit->text();
        if ( text.length() > 0 ) {
    
            text.remove(text.length()-1, 1);
            m_edit->setText(text);
        }
    }
    else if ( clickText == "C" ) {
    
        m_edit->setText("");
    }
    else if ( clickText == "=" ) {
    
    }
    else {
    
        m_edit->setText(m_edit->text() + clickText);
    }
}

Summary

• application development Most of the time It's all going on string manipulation
• Qt Than STL It is more suitable for cross platform development scenarios
• Qt Medium QString Than STL in string It's bigger and easier to use
• Qt Graphical user components all depend on QString
• Required during project development Comprehensive selection of libraries to be used