Everybody loves
QString("Put something here %1 and here %2")
.arg(replacement1)
.arg(replacement2);
but things get itchy as soon as you have the faintest chance that replacement1 actually contains %1 or even %2 anywhere. Then, the second QString::arg() will replace only the re-introduced %1 or both %2 occurrences. Anyway, you won't get the literal "%1" that you probably intended.
Is there any standard trick to overcome this?
If you need an example to play with, take this
#include <QCoreApplication>
#include <QDebug>
int main()
{
qDebug() << QString("%1-%2").arg("%1").arg("foo");
return 0;
}
This will output
"foo-%2"
instead of
"%1-foo"
as might be expected (not).
qDebug() << QString("%1-%2").arg("%2").arg("foo");
gives
"foo-foo"
and
qDebug() << QString("%1-%2").arg("%3").arg("foo");
gives
"%3-foo"
See the Qt docs about QString::arg():
QString str;
str = "%1 %2";
str.arg("%1f", "Hello"); // returns "%1f Hello"
Note that the arg() overload for multiple arguments only takes QString. In case not all the arguments are QStrings, you could change the order of the placeholders in the format string:
QString("1%1 2%2 3%3 4%4").arg(int1).arg(string2).arg(string3).arg(int4);
becomes
QString("1%1 2%3 3%4 4%2").arg(int1).arg(int4).arg(string2, string3);
That way, everything that is not a string is replaced first, and then all the strings are replaced at the same time.
You should try using
QString("%1-%2").arg("%2","foo");
Related
Let's have a look at this little application:
#include <QString>
#include <QDebug>
int main(int argc, char *argv[]) {
const auto test_string =
QString{"Some string \n \x01 \u0002 with some \r special chars"};
qDebug() << test_string;
qDebug(qPrintable(test_string));
}
It gives the following output:
"Some string \n \u0001 \u0002 with some \r special chars"
Some string
special chars
Press <RETURN> to close this window...
This demonstrates how the qDebug << operator comes with some functionality that converts all the special characters of a QString to some readable string, which can easily be put in a string declaration in C++.
I would like to use this functionality to feed strings into a custom logging framework. Is there a possibility to use the same conversion function directly?
Effectively, this would mean to convert test_string to a QString instance that gives the same output on both the above qDebug statements.
I had the same question and I did not found the complete answer (yet). However, I found QVariant which allows you to call toString() on the most basic C and Qt types:
QVariant("foo").toString(); // "foo"
QVariant(true).toString(); // "true"
QVariant(QDateTime("2020-11-28")).toString(); // "2020-11-28"
Then you could wrap this into one method:
QString variantToString(const QVariant variant) {
return (variant.userType() != QMetaType::QString
&& variant.canConvert(QMetaType::QStringList))
? "(" + variant.toStringList().join(", ") + ")"
: variant.toString();
}
variantToString(42); // "42" // works due to implicit cast
You could do a few extra checks for non-stringifiable types (see also canConvert() and userType(), e.g. lists, regular expressions or whatever you need, but I totally agree that it would be nicer to reuse Qt's own logging functions instead ...
I had a similar issue but I wanted to use my custom operator<< overload that was defined for QDebug, therefore, I did the following:
// This could be anything that provides an 'operator<<' overload.
QString value = "Hello, world!";
QString result;
QDebug{ &result } << value;
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.
I have no idea why, but i can´t get the simplest example of QTemporaryFile to run... My real intent is to write data from QAudioInput into a temporary file before it is processed later.
After trying several times I realized that neither .read(), .readLine(), .readAll() or .write() would have any effect... The error string is always "Unknown Error" and it neither works for QFile or QTemporaryFile.
#include <QCoreApplication>
#include <QTemporaryFile>
#include <QDebug>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QTemporaryFile tf;
tf.open();
tf.write("Test");
QTextStream in(&tf);
qDebug() << "Testprogramm";
qDebug() << tf.isOpen();
qDebug() << tf.errorString();
qDebug() << in.readLine();
qDebug() << tf.readAll();
tf.close();
return a.exec();
}
The debug posts:
Testprogramm
true
"Unknown error"
""
""
Thank you in advance!
You need to move the file pointer back to the beginning of the file. This has to be done on the file itself when there's no stream on the file, or using the stream when one exists. Also - QFile is a proper C++ class that manages the file resource. There's no need to manually close the file. QFile::~QFile does that job.
The following works just fine:
#include <QtCore>
int main() {
auto line = QLatin1String("Test");
QTemporaryFile tf;
tf.open();
Q_ASSERT(tf.isOpen());
tf.write(line.data());
tf.reset(); // or tf.seek(0)
QTextStream in(&tf);
Q_ASSERT(in.readLine() == line);
in.seek(0); // not in.reset() nor tf.reset()!
Q_ASSERT(in.readLine() == line);
}
The above also demonstrates the following techniques applicable to sscce-style code:
Inclusion of entire Qt module(s). Remember that modules include their dependencies, i.e. #include <QtWidgets> is sufficient by itself.
Absence of main() arguments where unnecessary.
Absence of QCoreApplication instance where unnecessary. You will get clear runtime errors if you need the application instance but don't have one.
Use of asserts to indicate conditions that are expected to be true - that way you don't need to look at the output to verify that it is correct.
Use of QLatin1String over QStringLiteral where ASCII strings need to be compared to both C strings and QStrings. Implicit ASCII casts can be a source of bugs and are discouraged.
QLatin1String is a constant (read-only) wrapper, designed to wrap C string literals - thus there's no need to make line additionally const, although in real projects you'd want to follow the project's style guide here.
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.
There is a magic number associated with each binary file , does anyone know how to retrieve this information from the file?
file <file_name>
magic numbers are usually stored in (linux):
/usr/share/file/magic
also check this link, someone was trying to use libmagic to get the information in C program, might be useful if you're writing something yourself.
Use libmagic from the file package to try and sniff out the type of file if that's your goal.
There are no general "magic" numbers in binary files on unix, though different formats might define their own. The above library knows about many of those and also use various other heuristics to try and figure out the format/type of file.
The unix file command uses magic number. see the file man page for more.(and where to find the magic file )
Read this: http://linux.die.net/man/5/magic
It's complex, and depends on the specific file type you're looking for.
There is a file command which in turn uses a magic library, the magic library reads from a file found in /etc called magic (this is installation dependant and may vary), which details what are the first few bytes of the file and tells the file what kind of a file it is, be it, jpg, binary, text, shell script. There is an old version of libmagic found on sourceforge. Incidentally, there is a related answer to this here.
Hope this helps,
Best regards,
Tom.
Expounding on #nos's answer:
Example below uses the default magic database to query the file passed on the command line. (Essentially an implementation of the file command. See man libmagic for more details/functions.
#include <iostream>
#include <magic.h>
#include <cassert>
int main(int argc, char **argv) {
if (argc == 1) {
std::cerr << "Usage " << argv[0] << " [filename]" << std::endl;
return -1;
}
const char * fname = argv[1];
magic_t cookie = magic_open(0);
assert (cookie !=nullptr);
int rc = magic_load(cookie, nullptr);
assert(rc == 0);
auto f= magic_file(cookie, fname);
if (f ==nullptr) {
std::cerr << magic_error(cookie) << std::endl;
} else {
std::cout << fname << ' ' << f << std::endl;
}
}