printf question with a const char* variable
I am stuck in a printf problem. I would appreciate if I can get some help here: In the below code, I can see the font family get displaced correctly in first printf(), but if I set it to variable, i only get an empty string. How can I put it in a variable and have the right values? I just don't want to type 'font.family().family().string().utf8().data()' everywhere?
I did this in the same method:
void myMethod() {
const char* fontFamily = font.family().family().string().utf8().data();
// get displayed correctly
printf ("drawText1 %s \n", font.family().family().string().utf8().data());
// get an empty string
printf ("drawText2 %s \n", fontFamily);
}
A开发者_高级运维nd the signature of 'data()' is
class CString {
public:
CString() { }
CString(const char*);
CString(const char*, unsigned length);
CString(CStringBuffer* buffer) : m_buffer(buffer) { }
static CString newUninitialized(size_t length, char*& characterBuffer);
const char* data() const;
//...
}
The signature of utf8() is
class String {
CString utf8() const;
}
Thank you.
Something in the chain of font.family().family().string().utf8().data()
is returning a temporary object. In your first printf
, the temporary object doesn't go out of scope until the printf
returns. In the second printf
, the temporary has been destroyed after the pointer assignment was made, and the pointer is now invalid. You're seeing a classic example of "undefined behavior".
There are two ways to fix this. Either make a copy of the data before the temporary gets destroyed, or make a reference to the temporary. The copy is probably easiest and clearest, as long as the class has a copy operator. Assuming that utf8()
generates a temporary CString
, this would be
CString fontFamily = font.family().family().string().utf8();
printf ("drawText2 %s \n", fontFamily.data());
You are caching a pointer that resides in the temporary returned by utf8()
(as Mark and Neil have argued about). You'll have to change fontFamily
to either a CString
or const CString &
to keep the result from utf8()
in scope.
The call to data()
(assuming it is called on a std::string) does not necessarily return a null-terminated string. You almost certainly want c_str()
, which is defined to do so.
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