QString to const std::string

Question

I want to assign the value in QString to a const std::string

QString qfile("some value");

this is the const std::string variable

const std::string file

The code that I m using

file = qfile.toStdString();

above codes works fine for a normal std::string . But there is some problem because of the keyword 'const'. How do I solve it ?

Any work around? Yes: if you want to change something, don't declare it `const`. Or, if you want it to be immutable, then don't try to change it. — Mike Seymour, Jun 27 '12 at 12:28
Possibly a const_cast but what is the logic that requires you to assign to a const variable? Perhaps you should update that rather than applying a work around? — Stefan, Jun 27 '12 at 12:28
@Stefan: `const_cast` would make it compile, but trying to modify a `const` object gives undefined behaviour. — Mike Seymour, Jun 27 '12 at 12:29
note that `Qt::toStdString()` by default converts the string to Latin1 encoding, and because of that any characters which cant be represented in Latin1 will be lost. If you use that `std::string` for filenames, you wont be able to handle files with names containing special characters. You can use `const std::string file = qFile.toUtf8().constData();` intstead. But on Windows you wont be able to use the `std::string` for file operations anyway, because Windows does not support UTF8 properly. — smerlin, Jun 27 '12 at 12:36
@VihaanVerma: Why did you declare it `const` in the first place ? We really need to know that. — ereOn, Jun 27 '12 at 12:37

score 7 · Accepted Answer · answered Jun 27 '12 at 12:27

7

Don't use the assignment operator to initialize the std::string object but rather initialize the variable directly.

const std::string file = qfile.toStdString();

That aside, please make sure that you're aware of what encoding QString::toStdString() uses; unlike QString, std::string is encoding-agnostic (it's a string of bytes, not characters).

answered Jun 27 '12 at 12:27

Frerich Raabe

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or `const std::string file( qfile.toStdString() );` – stijn Jun 27 '12 at 12:28
1

(insert here the usual comment about most vexing parse) – Matteo Italia Jun 27 '12 at 12:29
the variable is already being used in code. I just want to update its content. – Vihaan Verma Jun 27 '12 at 12:30
6

@VihaanVerma: you can't, otherwise `const` would serve no purpose. – Matteo Italia Jun 27 '12 at 12:31

score 1 · Answer 2 · answered Jun 27 '12 at 12:27

1

Constants are expressions with a fixed value.

http://www.cplusplus.com/doc/tutorial/constants/

answered Jun 27 '12 at 12:27

Jake1164

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score 0 · Answer 3 · answered Jun 27 '12 at 12:57

0

You can also get rid of the const by making copy of the return value of toStdString():

QString str("text");
std::string std_str(str.toStdString());

answered Jun 27 '12 at 12:57

weggo

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QString to const std::string

3 Answers3