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How to format a QString?

Posted by: admin November 30, 2017 Leave a comment

Questions:

I’d like to format a string for Qt label, I’m programming in C++ on Qt.

In ObjC I would write something like:

NSString *format=[NSString stringWithFormat: ... ];

How to do something like that in Qt?

Answers:

You can use QString.arg like this

QString my_formatted_string = QString("%1/%2-%3.txt").arg("~", "Tom", "Jane");
// You get "~/Tom-Jane.txt"

This method is preferred over sprintf because:

Changing the position of the string without having to change the ordering of substitution, e.g.

// To get "~/Jane-Tom.txt"
QString my_formatted_string = QString("%1/%3-%2.txt").arg("~", "Tom", "Jane");

Or, changing the type of the arguments doesn’t require changing the format string, e.g.

// To get "~/Tom-1.txt"
QString my_formatted_string = QString("%1/%2-%3.txt").arg("~", "Tom", QString::number(1));

As you can see, the change is minimal. Of course, you generally do not need to care about the type that is passed into QString::arg() since most types are correctly overloaded.

One drawback though: QString::arg() doesn’t handle std::string. You will need to call: QString::fromStdString() on your std::string to make it into a QString before passing it to QString::arg(). Try to separate the classes that use QString from the classes that use std::string. Or if you can, switch to QString altogether.

UPDATE: Examples are updated thanks to Frank Osterfeld.

UPDATE: Examples are updated thanks to alexisdm.

Questions:
Answers:

You can use the sprintf method, however the arg method is preferred as it supports unicode.

QString str;
str.sprintf("%s %d", "string", 213);

Questions:
Answers:

Use QString::arg() for the same effect.