I got some problems in creating my program for mobile using QT C++
When i run it i get this: cannot convert 'std::string' to 'std::string*' in initialization
And theres code for that error:
void rozvrh_b17::pars(string par)
{
data = new std::string*(par);
printf(data->data());
}
//data and par are std::string
//without that new std::string*() it does similiar error
And i ask how to convert std::string to std::string* ??
EDIT:
i made this function to transfer data from one form to another and i need to remember that parameter...
I downvoted you because this question shows no research effort.
string * data = ∥
std::string* is a pointer type. You have a std::string, and it's address is the pointer type you want. This is one of the first principles of using pointers.
What do you really try to do?
If you just want to print the string then this should work:
void rozvrh_b17::pars(string par)
{
printf(par.c_str());
}
If you want to create a copy of string on the heap then you need this:
std::string* data = new std::string(par);
but that doesn't make much sense.
You are trying to assign a string from a pointer to string. In this particular case I see several things.
You don't need a copy at all - simply use par right away
You better make par a const ref: void rozvrh_b17::pars(const string& par)
You should use only string whaen calling new otherwise you create a pointer to string. So do: data = new std::string(par);
Related
Hello everyone I am trying to get to know pointers better and I stumbled into a Qt type change. I have made a QString array and gave the pointer to the array to a method. But when I try to use a QString functions it give a error and says that it is a QCharRef which does not have the member function isEmpty().
The code:
QString data_array[2][3] =
{
{"11:28:8","Room 1","Presence detected"},
{"11:38:8","Room 1","No presence"}
}
bool method(QString *_data_array)
{
QString *data_array = _data_array;
return data_array[0][1].isEmpty(); /* changed to QCharRef */
}
My question is why does this happen and how can I prevent it or change it?
The reason for which you are getting QCharRef is due to how QString is built. The [] operator returns one character from a QString (QString is built up from QChars, much like strings in C/C++ are character arrays). From the Qt documentation:
The return value is of type QCharRef, a helper class for QString. When you get an object of type QCharRef, you can use it as if it were a QChar &. If you assign to it, the assignment will apply to the character in the QString from which you got the reference.
So what that means for you is that when you use the lovely square bracket operators, you are no longer using a QString, you are using a QChar reference.
As for how to change it, QChar's isNull() seems like it would fit your uses. so instead try return data_array[0][1].isNull(); and that should work.
I would also look into QStringList if you're doing things with lists of strings
I have a data structure defined up here called this:
typedef list <classSpec*> ClassSpecList;
I'm trying to add stuff into the list here based on functions that return certain values of that match the same data type. In one function, I have a list pointer object defined here and I have another statement that calls a function.
ClassSpecList *answer = 0;
classSpec *thisanswer = parseClass(br);
Basically I'm trying to add the results of what thisanswer returns into my main ClassSpecList. Problem is, when I try
answer->push_back(new classSpec (*thisanswer));
It compiles but I get a seg fault
When I try somethign else like:
answer->insert(ClassSpecList.begin(), *thisanswer);
I keep getting primary expression errors and I do not know why. I even tried it with other list made without typedef and I still get those.
Thank you.
You should initialize the pointer answer first, like :
ClassSpecList *answer = new ClassSpecList;
then you can add thisAnswer into this list.
This should work:
ClassSpecList *answer = new ClassSpecList;
answer->push_back(thisAnswer);
as should this, which is usually recommended:
ClassSpecList answer;
answer.push_back(thisAnswer);
If possible, parseClass shouldn't return a pointer, and you should use typedef list <classSpec> ClassSpecList;.
I'm developing for an embedded hardware using C++ and I often use pointers to const (ROM) structs to minimize the object sizes.
When I get a pointer passed to my class constructor, I want the compiler to create another ROM object based on the one I passed but with one or two arguments changed and just then call the parent class constructor (Display_Element). Any ideas of how this could be done?
Since a const string can be declared within a parameter list I though possibly there could be a way of also declaring a const struct within a parameter list.
I wanted to do something like this, (which is illegal in C++)
Button::Button(const Colors_const* ecc, const Display_element_const* edc, const Element_const* eec, char* elabel,
Display_element* eparent, Screen* escreen) :
Display_element(ecc, cc,Display_element_const {
edc->xi+200,
edc->xf,
edc->yi,
edc->yf,
edc->font_size,
edc->image,
edc->image_x_offset,
edc->image_y_offset,
edc->label_x_offset,
edc->label_y_offset,
edc->mouse_down_ptr,
NULL,
edc->largura_borda_externa,
edc->borda_panel
},
eec,elabel,eparent,escreen) {
flags.largura_borda = 2;
//flags.largura_borda_externa = 3;
flags.borda_light_shadow = true;
flags.sliding_text = true;
flags.dont_paint_on_click = false;
}
Thanks
Well, it seems what I want to do is really illegal and can't be done in C. But philosophically I keep asking myself: if I can allocate a const char[n] written inside a parameter list such as fn(...,"The brow dog",...) why not a way to allocate a const struct the same way? If someone knows the answer, please post!
The workaround I found is to do it the canonical way: declare a const struct and then later assign the appropriate pointer to the struct (something I wanted to be done inside Display_element function on the first place). It solves my problem, but not the conceptual question I've been trying to formulate...
const Display_element_const new_ec = {
edc->xi+200,
edc->xf,
edc->yi,
edc->yf,
edc->font_size,
edc->image,
edc->image_x_offset,
edc->image_y_offset,
edc->label_x_offset,
edc->label_y_offset,
edc->mouse_down_ptr,
NULL,
edc->largura_borda_externa,
edc->borda_panel
};
Button::Button(const Colors_const* ecc, const Display_element_const* new_edc, const Element_const* eec, char* elabel,
Display_element* eparent, Screen* escreen) :
Display_element(ecc, edc,eec,elabel,eparent,escreen) {
//previously dc = edc, assigned inside Display_element fn
dc = &new_ec;
I am coming from the C++ world and i want to do some simple stuff with Actionscript 3.0.
Have search around this site and google and haven't found a universally accepted way to do so. I will give you the C++ code of the analogous of what I am trying to do in Actionscript 3.0.
Pass by reference:
void somefunction (string &passvariable);
Create instance of, deep copy:
string something;
string somethingelse;
something = "randomtext";
somethingelse = something;
Pass by reference
Every object is passed by reference. As far as I know, there are no explicit & address of or * dereference operators. Actionscript is a higher level language than that.
Primitive types (and Strings are primitive - see link) are Immutable in Actionscript, so pass by value / pass by reference are effectively the same.
Deep Copy / Instance of
ObjectUtil.clone / ObjectUtil.copy will create sometimes-deep copies of Objects, if you're working in Flex. I usually don't rely on it for anything deep, however. In most cases you will want to create your own clone style method to create a deep copy.
A generic, flexible clone method can be found here
The rules for pass as reference are different for simple data types like string and number than they are for objects and complex data types.
If you are passing a string to a function, it creates a copy, leaving the original untouched.
So to pass by reference, try creating an object:
var str:Object = {string:"foo"};
passByref(str);
trace(str.string);
private function passByref(str:Object):void
{
str.string = str.string + "bar";
trace("inside", str);
}
As for deep object cloning, this works great:
package
{
import flash.utils.ByteArray;
public class DeepCopyUtil
{
public static function clone (source : Object) : *
{
var array : ByteArray = new ByteArray ();
array.writeObject (source);
array.position = 0;
return array.readObject ();
}
}
}
Credit where credit is due:
http://cookbooks.adobe.com/post_How_to_create_deep_copies_of_objects_and_arrays-19261.html
In Actionscript you have to define all things with function, var or const.
You should define the (return type) after the variable name, like var:String
Creating a function
function someFunction (var:String):void
{
}
Copy a string
var something:String;
var somethingElse:String;
something = "randomtext";
somethingelse = something;
This c++ code is working fine , however memory validator says that I am using a deleted pointer in:
grf->filePath = fname; Do you have any idea why ? Thank you.
Dirloader.h
// Other code
class CDirLoader
{
public:
struct TKnownGRF
{
std::string filePath;
DWORD encodingType;
DWORD userDataLen;
char *userData;
};
// Other Code
CDirLoader();
virtual ~CDirLoader();
Dirloader.cpp
// Other code
void CDirLoader::AddGroupFile(const std::string& _fname)
{
// Other code including std::string fname = _fname;
TKnownGRF *grf = new TKnownGRF;
grf->filePath = fname;
delete grf; // Just for testing purposes
P.S.: This is only an code extract. Of course if I define a struct TKnownGRF inside .cpp and use it as an actual object, gfr.filepath = something, instead of pointer grf->filepath=something, than it is ok, but I do need to have it inside *.h in CDirLoader class, due to many other vector allocations.
Since the function returns void
void CDirLoader::AddGroupFile(const std::string& _fname)
the question is what are you going to do with grf?
Are you going to delete it? If so, then, why do a new? you can just declare a TKnownGRF variable on the stack! In that case, _fname is not contributing to the logic of this method.
I guess that the class CDirLoader has a member variable of type TKnownGRF, say grf_, and that need to be used in the AddsGroupFile() method, e.g.:
grf_.filepath = _fname;
Does this happen to be using an older version of STL, say, VC6, and running multithreaded? Older versions of STL's string class used a reference counted copy on write implementation, which didn't really work in a multithreaded environment. See this KB article on VC 6.
Or, it's also possible that you are looking at the wrong problem. If you call std::string::c_str() and cache the result at all, the cached result would probably be invalidated when you modified the original string. There are a few cases where you can get away with that, but it's very much implementation specific.