QTextStream versus iostream (esp. error-handling)

nicesw123

Hi there,

the error-handling in QTextStream is pretty weird.

Take this example, using only standard C++ library:
Here I can just query the stream using "operator bool":http://en.cppreference.com/w/cpp/io/basic_ios/operator_bool, and it makes for very nice code.

@
#include <iostream>
#include <sstream>
#include <string>

int main()
{
using namespace std;

istringstream iss{" { asdf } " };
char ch;
string s;
do {
    if (iss >> ch) {
        cout << "got " << ch << '\n';
        if (iss >> s) {
            cout << "got " << s << '\n';
            if (iss >> ch) {
                cout << "got " << ch << '\n';
                break;
            }
        }
    }

    cerr << "~~ got an error\n";
    
} while (0);
return 0;

}
@

Try writing this nicely with Qt.
Here's what I have come up with:

@
#include <QTextStream>
#include <QString>

template<typename T>
bool get(QTextStream& iss, T& var)
{
return (!iss.atEnd() &&
(iss >> ws, !iss.atEnd()) &&
(iss >> var, iss.status() == QTextStream::Ok));
}

QTextStream cout{stdout, QIODevice::WriteOnly};
QTextStream cerr{stderr, QIODevice::WriteOnly};

int main()
{
QString str{" { asdf } " };
QTextStream iss{&str};

char ch;
QString s;
do {
    if (get(iss, ch)) {
        cout << "got " << ch << endl;
        if (get(iss, s)) {
            cout << "got " << s << endl;
            if (get(iss, ch)) {
                cout << "got " << ch << endl;
                break;
            }
        }
    }

    cerr << "~~ got an error" << endl;
    
} while (0);
return 0;

}
@

So as far as I can see, I need to create my own function to do error handling. It's the function get(): It checks for eof and stream errors.

Is there any better way of doing this?

Thanks.

nicesw123

I really like the semantics and behaviour of the standard C++ library.

Here's how to get QTextStream to work the same way as C++ standard library (this is based on tests, not on detailed analysis: if you find some discrepancy please notify me!):

@
template<typename T>
bool get(QTextStream& iss, T& var)
{
return (!iss.atEnd() &&
(iss >> ws, !iss.atEnd()) &&
(iss >> var, iss.status() == QTextStream::Ok));
}

bool qt_getline(QTextStream& iss, QString& line)
{
return (!(iss.atEnd()) &&
(line = iss.readLine(), (!iss.atEnd() || line.size())));
}
@

Here's some Qt code using those functions:
@
#include <QTextStream>
#include <QTextStream>

QTextStream cout{stdout, QIODevice::WriteOnly};
QTextStream cin{stdin, QIODevice::ReadOnly};

template<typename T>
bool get(QTextStream& iss, T& var)
{
return (!iss.atEnd() &&
(iss >> ws, !iss.atEnd()) &&
(iss >> var, iss.status() == QTextStream::Ok));
}

bool qt_getline(QTextStream& iss, QString& line)
{
return (!(iss.atEnd()) &&
(line = iss.readLine(), (!iss.atEnd() || line.size())));
}

#define T1

int main()
{
#ifdef T1

QString str;
while (get(cin, str)) {
cout << "got: " << str << endl;
}
cout << "rem: " << str << endl;

#elif (defined T2)

char ch;
while (get(cin, ch)) {
cout << "got: " << ch << endl;
}
cout << "rem: " << ch << endl;

#elif (defined T3)

QString str;
while (qt_getline(cin, str)) {
cout << "got: " << str << endl;
}
cout << "rem: " << str << endl;

#endif

return 0;
}
@

Here's the equivalent code using standard C++ library:
@
#include <iostream>
#include <string>

#define T1

int main()
{
using namespace std;

#ifdef T1

string str;
while (cin >> str) {
cout << "got: " << str << endl;
}
cout << "rem: " << str << endl;

#elif (defined T2)

char ch;
while (cin >> ch) {
cout << "got: " << ch << endl;
}
cout << "rem: " << ch << endl;

#elif (defined T3)

string str;
while (getline(cin, str)) {
cout << "got: " << str << endl;
}
cout << "rem: " << str << endl;

#endif

return 0;
}
@