convertion from QString to char pointer generates empty string

Question

Here's some test code:

QString qstr_test("TEST");
const char *p = qstr_test.toStdString().c_str();
cout << p << endl;

Nothing is output as p is an empty string.

Here's what I got at debugging:

std::basic_string<char,std::char_traits<char>,std::allocator<char> >::c_str 
returns:    0x003bf9d4 "TEST"   

p      :    0x003bf9d4 ""

It seems p is pointing to the right location but doesn't display the right content.

Why is p empty ?

are you sure that std::string returned by toStdString() is still exists in memory when you get to cout << p << endl; line? — westwood
– westwood, Commented Jun 27, 2013 at 12:55

Pavel Strakhov · Accepted Answer · 2013-06-27 13:11:44Z

5

std::string object is temporary and is destroyed right after c_str() is completed. But std::string owns char* buffer returned by c_str() and this buffer is also destroyed. So your code is incorrect and dangerous. You need to store std::string as long as you use char* buffer:

std::string s = qstr_test.toStdString();
const char* p = s.c_str();

Also it seems pointless to create std::string just to convert it to char*. QString has better methods: toLatin1, toLocal8bit, and toUtf8. Note that returned QByteArray has the same issue that is a common source of mistakes. QByteArray also must be stored if you want to use its buffer.

QByteArray array = qstr_test.toUtf8();
const char* p = array.constData();

I think this method is better because here you specify explicitly the encoding you need to use. And toStdString result depends on QTextCodec::codecForCStrings() current value.

edited Jun 27, 2013 at 13:11

answered Jun 27, 2013 at 13:05

Pavel Strakhov

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3 Comments

WoJo Over a year ago

toAscii should not longer be used, it's removed in Qt 5. you should use toLatin1

user1095108 Over a year ago

@WoJo Nay, toUtf8() is superior.

Frank Osterfeld Over a year ago

toUtf8 is only superior if you want UTF-8.

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