Concatenating two QStrings with an integer

Question

I want to do something like this in C++ using Qt:

int i = 5;
QString directory = ":/karim/pic" + i + ".jpg";

where + means I want to concatenate the strings and the integer (that is, directory should be :/karim/pic5.jpg). How can I do this?

C++ concatenate string and int http://stackoverflow.com/questions/191757/c-concatenate-string-and-int There is an answer with 8 ways to do it in there. — dee-see, Aug 10 '11 at 13:22
The tags indicate he wants an answer for `qt`, the answers in the "duplicate" don't provide that. — JoeG, Aug 10 '11 at 13:47
Ok, I missed it being a Qt question, voted for reopen. The solution is: `QString dir = ":/karim/pic" + QString::number(i) + ".jpg";` — Vinicius Kamakura, Aug 10 '11 at 13:53
This is not the optimal solution, but since all the solutions tell how to do it with `std::string`, if you ever need a `QString` and you have an `std::string`, use [this function](http://doc.qt.nokia.com/latest/qstring.html#fromStdString) — Benjamin Lindley, Aug 10 '11 at 14:09

score 39 · Accepted Answer · edited May 02 '18 at 12:34

39

Qt's idiom for things like this is the arg()function of QString.

QString directory = QString(":/karim/pic%1.jpg").arg(i);

edited May 02 '18 at 12:34

Campa

answered Aug 10 '11 at 17:03

3

Slightly better is `QStringLiteral(":/karim/pic%1.jpg").arg(i);`, which is generated at compile time. https://woboq.com/blog/qstringliteral.html – Robin Jul 06 '17 at 13:34

score 12 · Answer 2 · edited May 23 '17 at 12:33

12

(EDIT: this is an answer to the question before the edit that mentioned QString. For QString, see the newer answer)

This can be done as a very similar one-liner using C++11:

int i = 5;
std::string directory = ":/karim/pic" + std::to_string(i) + ".jpg";

With older compilers, it can be substituted with Boost:

int i = 5;
std::string directory = ":/karim/pic" + boost::lexical_cast<std::string>(i) + ".jpg";

But the classic way to do it is with a string stream object.

int i = 5;
std::ostringstream oss;
oss << ":/karim/pic" << i << ".jpg";
std::string directory = oss.str();

edited May 23 '17 at 12:33

Community

answered Aug 10 '11 at 13:25

Cubbi

Interesting. I never knew, all this time that there was an overload for `operator+(const char *, const std::string &)`, and so I would always have to wrap my literals in a string constructor. – Benjamin Lindley Aug 10 '11 at 13:51
@Karim M. El Tel: Ohh, since it's a QString (after edit), go with hexa's comment. Hopefully it will get reopened so it can be posted as an answer. – Cubbi Aug 10 '11 at 14:05

score 2 · Answer 3 · answered Aug 10 '11 at 13:25

2

#include <sstream>
#include <string>

int i = 5;

std::stringstream s;
s << ":/karim/pic" << i << ".jpg";

std::string directory = s.str();

answered Aug 10 '11 at 13:25

Tobias Schlegel

1

I think it's `s.str()`, not `s.string()` – Hunter McMillen Aug 10 '11 at 13:26

score 2 · Answer 4 · answered Aug 10 '11 at 13:25

2

Have a look at stringstream:

ostringstream oss(ostringstream::out);

oss << ":/karim/pic";
oss << i
oss << ".jpg";

cout << oss.str();

answered Aug 10 '11 at 13:25

Jens Luedicke

4 Answers4