In my Qt GUI app I use libwebrtc. From one on callbacks I want to emit signal with data, which is std::string. As core app logic use QString, I want convert std::string to Qstring.
I try following:
QString::fromStdString(stdstr)
QString::fromLatin1(stdstr.data())
Both of this return broken text, something like this
Only working way for me was
QString qstr;
for(uint i =0; i< stdstr.length(); i++)
qstr.append(stdstr.at(i));
Here my thoughts about reason of problem:
Encoding problems.
Binary problems
About encoding, libwebrtc should return std::string in UTF-8 by default, same as QString.
About binary. As I understand Qt built with GCC and corresponding stdlib, but libwebrtc build with CLang and libc++. For building I also specify usinng QMAKE_CXXFLAGS += -stdlib=libc++
What is correct way to convert types here?
UPD
I compare length of converted string and source string, they are very different.
For std::string I get 64, and for converted QString I get 5.
UPD2
Here is full function and corresponding slot for SPDGen signal
void foo(const webrtc::IceCandidateInterface *candidate) override {
std::string str;
candidate->ToString(&str);
QString qstr = QString::fromStdString(str);
qDebug() << qstr;
Q_EMIT SPDGen(qstr);
}
connect(conductor, &Conductor::SPDGen, this, [=](QString value){
ui->textEdit->setText(value);
});
Related
Using console, I want to write the desired user's input into a file via getline function inside the wFile function and then read it. I face with logical error during Runtime; whatever I as user write doesn't type into the output terminal and it doesn't succeed more steps. Apparently fwrite function with this feature in the libraries exists, but I want to write my own code differently this way. I think I must have neglected a point. Here's the code:
#include <QCoreApplication>
#include <QDebug>
#include <QFile>
#include <QString>
#include <QTextStream>
#include <String>
#include <cstdlib>
using namespace std;
void wFile(QString Filename)
{
QFile mFile(Filename);
QTextStream str(&mFile);
qDebug() << "what do you want to write in the desired file: ";
istream& getline (istream& is, string& str);
if (!mFile.open(QFile::WriteOnly | QFile::Text))
{
qDebug() << "could not open the file";
return;
}
mFile.flush();
mFile.close();
}
void read (QString Filename){
QFile nFile(Filename);
if(!nFile.open(QFile::ReadOnly | QFile::Text))
{
qDebug() << "could not open file for reading";
return;
}
QTextStream in(&nFile);
QString nText = in.readAll();
qDebug() << nText;
nFile.close();
}
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QString nFilename ="P:/DocumentArminV.txt";
wFile(nFilename);
read(nFilename);
return a.exec();
}
Spoiler alarm: At the very end of this answer, there is a very simple recommendation for a fix.
What OP did
Concerning istream& getline (istream& is, string& str); in wFile(QString Filename):
This declares function getline() in function wFile().
This is a valid declaration concerning C++.
Concerning the sample code, I missed the respective headers. IMHO,
#include <istream> and
#include <string>
are required to make this compiling.
However, it is possible that the existing #includes include them indirectly. So, OP's code may even compile without them.
Declaring functions, which are not used as well as re-declaring functions which are already declared is somehow useless but not wrong.
To demonstrate this, I made a small sample:
#include <cstdio>
#include <istream>
#include <string>
void func()
{
puts("in func()\n");
std::istream& getline(std::istream&, std::string&);
// Even duplicated prototyping is accepted without complaints:
std::istream& getline(std::istream&, std::string&);
}
int main ()
{
func();
return 0;
}
compiles and runs perfectly.
Output:
in func()
Live Demo on coliru
What OP (probably) wanted
Using console, I want to write the desired user's input into a file via getline function inside the wFile function and then read it.
This sounds a bit confusing to me. std::getline(std::cin, ) can be used to read user input from console. May be, it's a bit bad wording only.
Assuming, the OP wanted to read input from console, obviously, declaring a function is not sufficient – it must be called to become effective:
#include <iostream>
void func()
{
std::cout << "Enter file name: ";
std::string fileName; std::getline(std::cin, fileName);
std::cout << "Got file name '" << fileName << "'\n");
}
int main ()
{
func();
return 0;
}
Output:
Enter file name: test.txt↵
Got file name 'test.txt'
Live Demo on coliru
C++ std vs. Qt
Qt is undoubtly built on top of the C++ std library. However, though it's possible it is not recommended to mix both APIs when it can be prevented (or there aren't specific reasons to do so).
Both, Qt and C++ std, are a possibility to write portable software.
Qt covers a lot of things which are provided in the std library as well but a lot of other things additionally which are not or not yet part of std. In some cases, the Qt is a bit less generic but more convenient though this is my personal opinion. IMHO, the following explains how I came to this:
std::string vs. QString
std::string stores a sequence of chars. The meaning of chars when exposed as glyph (e.g. printing on console or displaying in a window) depends on the encoding which is used in this exposing. There are lot of encodings which interprete the numbers in the chars in distinct ways.
Example:
std::string text = "\xc3\xbc";
Decoded/displayed with
Windows-1252: ü
UTF-8: ü
Based on character type of std::string, it is not possible to determine the encoding. Hence, an additional hint must be provided to decode this properly.
(AFAIK, it is similar for std::wstring and wchar_t.)
QString stores a sequence of Unicode characters. So, one universal encoding was chosen by design to mitigate the "encoding hell".
As long as the program operates on QString, no encoding issues should be expected. The same is true when combining QString with other functions of Qt. However, it becomes a bit more complicated when "leaving the Qt universe" – e.g. storing contents of a std::string to QString.
This is the point where the programmer has to provide the additional hint for the encoding of the contents in std::string. QString provides a lot of from...() and to...() methods which can be used to re-encode contents but the application programmer is still responsible to chose the right one.
Assuming that the intended contents of text should have been the ü (i.e. UTF-8 encoding), this can be converted to QString (and back) by:
// std::string (UTF-8) -> QString
std::string text = "\xc3\xbc";
QString qText = QString::fromUtf8(text.c_str());
// QString -> std::string (UTF-8)
std::string text2 = qText.toUtf8();
This has to be considered when input from std::cin shall be passed to QString:
std::cout << "Enter file name: ";
std::string input; std::getline(std::cin, input);
QString qFileName = QString::fromLocal8Bit(input);
And even now, the code contains a little flaw – the locale of std::cin might have changed with std::ios::imbue(). I must admit that I cannot say much more about this. (In daily work, I try to prevent this topic at all e.g. by not relying on Console input which I consider especially critical on Windows – the OS on which we usually deploy to customers.)
Instead, a last note about OP's code:
How to fix it
Remembering my above recommendation (not to mix std and Qt if not necessary), this can be done in Qt exclusively:
QTextStream qtin(stdin);
qtin.readline();
I must admit that I never did it by myself but found this in the Qt forum: Re: stdin reading.
The documentation says that QDataStream writes data in system independent way, but it says nothing about QBuffer. I develop a program that saves data in a file like this:
QByteArray a;
QBuffer b(&a);
b.open(QIODevide::WriteOnly);
quint32 x = 1;
b.write((char*)&x, sizeof(x));
b.close();
QFile f(...);
f.open(QIODevide::WriteOnly);
f.write(a.constData(), a.size());
f.close();
, and i want this file can be read in any other OS (win, linux, Mac OS). Will this code work or i must use QDataStream instead?
The QBuffer documentation says :
The QBuffer class provides a QIODevice interface for a QByteArray.
ie it is only a QByteArray underneath. On the other hand a QByteArray is portable because as long as you see the data as an array of byte and write one byte at a time you are fine. Your code will work:
When you say
I want this file to be read in any other OS
Is your file used by your program only or will it be used by other applications in the system? QDataStream provides nicer functions for I\O and you may be still able to take advantage of it.
It will be platform specific. x representation in memory depend on the endianess.It doesn't occur in the QBuffer, but when you do :
b.write((char*)&x, sizeof(x));
If you are on machines of different endianess, you will obtain different values for the resulting array by doing
char* data = &x;
qDebug()<< data[0];
qDebug()<< data[1];
qDebug()<< data[2];
qDebug()<< data[3];
Take a look at the source code of QDataStream operator
QDataStream &QDataStream::operator<<(qint32 i){
CHECK_STREAM_WRITE_PRECOND(*this)
if (!noswap) {
i = qbswap(i);
}
if (dev->write((char *)&i, sizeof(qint32)) != sizeof(qint32))
q_status = WriteFailed;
return *this;
}
I am trying to compile a library written in Qt 4.6. On my current Linux machine I have only Qt 4.7 installed. The following code part:
/*file try.h*/
void fileOpen(QString s = NULL) ;
/*file try.cpp*/
void MainWindow::fileOpen(QString s) {
QString filename ;
if(s.isNull()) filename = QFileDialog::getOpenFileName(
this,
"Choose a file",
".",
"Source file (*.)");
else filename = s ;
}
compiles with the following error (I used cmake but the corresponding line code is the one listed above):
In member function ‘virtual int MainWindow::qt_metacall(QMetaObject::Call, int,
void**)’:
/homes/combi/hodorog/Developments/axelOld/build/axel/src/QGui/moc_MainWindow.cxx:141:26:
error: conversion from ‘long int’ to ‘QString’ is ambiguous
/homes/combi/hodorog/Developments/axelOld/build/axel/src/QGui/moc_MainWindow.cxx:141:26:
note: candidates are:
/usr/include/QtCore/qstring.h:426:43: note: QString::QString(const char*)
/usr/include/QtCore/qstring.h:105:14: note: QString::QString(const QChar*)
So I am guessing the problem is that in qt. 4.7. there are two QString constructors that can take a pointer as an argument (as listed in the compilation error), whereas in qt 4.6. there is only one QString constructor that can take a pointer as an argument. How can I force QString to call the constructor with const char * as an argument?
Thank a lot for your help in advance,
madalina
void fileOpen(QString s = NULL);
You are trying to construct a QString object with 0. It seems you are confusing the null of pointers with a null QString. A null QString is one which is created with the constructor QString(). Given how your function is implemented (referring to s.isNull()), you should change the function declaration to
void fileOpen(QString s = QString());
Is there any way to force gcc to put
char* str = "Hello";
not in the .rodata without change this statement in
char str[] = "Hello!";
?
Ok, so better way to do this is modify the statement to char str[]. Thanks to all.
Why? Trying to change string literals leads to undefined behavior. It's evil. Consider this program:
"hello"[0] = 'y'; // Welcome to Undefined Behavior Land. Enjoy yor stay!
std::cout << "hello" << std::endl; // Could very will print "yello" now!
static char strbuf[] = "Hello";
char *str = strbuf;
How about using strdup if your platform has it, or implementing it yourself if it doesn't?
char *str = strdup("hello world");
This will allocate memory (at runtime) and copy the string literal into an appropriately sized chunk of memory which you can quite legitimately write to and modify later.
Don't forget to free() after use though.
You might be able to force GCC to put somethings in specific sections of your choosing using the __attribute__ ((section ("my_section"))) attribute, but you'll still have to modify the original source to do that so you'd be much better off doing it a "normal" way.
The following code somehow fails to notice any files with non-ASCII characters in their names (Cyrillic characters, specifically):
for (int path = 1; path < argc; path++) {
QFileInfo fi(argv[path]);
if (fi.isDir()) {
QDir dir(argv[path], "", QDir::LocaleAware, QDir::AllEntries);
qDebug() << dir.entryList();
QDirIterator it(QString(argv[path]), QDirIterator::Subdirectories);
while (it.hasNext()) {
it.next();
qDebug() << it.fileInfo().absoluteFilePath();
/* Processing; irrelevant in the context of the question */
}
}
}
What exactly am I doing wrong here? How should I handle QDir and QDirIterator to make them aware of Cyrillic filenames?
The system locale is en_US.UTF-8.
Update: On Windows, everything works correctly.
Get cmd line parameters out of QApplication itself.
So
QApplication app(argc, argv);
QStringList args = app.arguments();
for(...)
Qt will handle encoding properly. But that will only fix problems with unicode on cmd line. Not sure if that is your main problem though.
EDIT:
fromLocal8Bit() probably doesn't work because it wasn't local encoding, but utf8. So fromUtf8() would work on linux and osx (but it won't work on windows). On *nuxes it depends on some environment variables (LS_LANG or something). I guess Qt takes everything into account and converts it properly. You can look at the constructor code for QApplication if you want to know exactly what they do.
Which part is failing? Reading the initial directory specified argv[path] or the iterator? If it's the former, you should convert byte strings to QString for file processing using QFile::decodeName. The default char* => QString conversion uses Latin-1, which is not what you want for file names.
Don't use argv[path] just like that when constructing the QStrings. This will treat the string as a latin1 string (which doesn't care about cyrillic characters). Try using
const QString dirName = QString::fromLocal8Bit( argv[path] );
at the top of your loop and then use dirName everywhere instead of argv[path].