Basic hash problem

[feraudyh]

Hi,
I declared a variable h of type QHash<const char *,const char *>.
I called h.insert("foo","bar")
then I test to see if h.contains("foo") returns true. It does. However I have another case where
I use a variable that points to a string s identical to foo in content and i call
h.insert(s,"baz")
this time h.contains("foo") returns false.
Any suggestions?

[yogeshgokul]

Hi,
I declared a variable h of type QHash<const char *,const char *>.
I called h.insert("foo","bar")
then I test to see if h.contains("foo") returns true. It does. However I have another case where
I use a variable that points to a string s identical to foo in content and i call
h.insert(s,"baz")
this time h.contains("foo") returns false.
Any suggestions?

You need to insert the string after converting it to char*.
Like this.

h.insert(s.toUtf8().data(), "baz");

h.insert(s.toUtf8().data(), "baz");

To copy to clipboard, switch view to plain text mode 

[feraudyh]

Thankyou Yogeshgokul,
I should have said that s is of type char *, and when I use the following program to print the keys they are as expected

QList<const char *> config_keys = config->keys();
int len = config_keys.count();
printf("len = %d\n",len);
for(int i=0; i < len; i++)
{
printf("key:%s\n",config_keys[i]);
}

In the above config is a pointer to my hash table.
However

if (!(config->contains("port")))
{
printf( "port is not a key\n");
}

prints "port is not a key", despite the fact it appears in my previous display of the keys.

[feraudyh]

OK I have somplifed my code to this which is surprising to me:

#include <QtCore/QCoreApplication>
#include <QHash>
#include <iostream>
 
 
QHash<const char*,const char*> *config;
int main(int argc, char *argv[])
{
    using std::cout;
 
    QCoreApplication a(argc, argv);
    char *key1, *val1;
    config = new QHash<const char *,const char *>;
    key1 = new char[5];
    key1[0] = 'p';
    key1[1] = 'o';
    key1[2] = 'r';
    key1[3] = 't';
    key1[4] = '\0';
    val1 = new char[2];
    val1[0] = '5';
    val1[1] = '\0';
    config->insert(key1,val1);
    if(!(config->contains("port")))
        cout<<"port is not a key";
    return a.exec();
}

#include <QtCore/QCoreApplication>
#include <QHash>
#include <iostream>


QHash<const char*,const char*> *config;
int main(int argc, char *argv[])
{
    using std::cout;

    QCoreApplication a(argc, argv);
    char *key1, *val1;
    config = new QHash<const char *,const char *>;
    key1 = new char[5];
    key1[0] = 'p';
    key1[1] = 'o';
    key1[2] = 'r';
    key1[3] = 't';
    key1[4] = '\0';
    val1 = new char[2];
    val1[0] = '5';
    val1[1] = '\0';
    config->insert(key1,val1);
    if(!(config->contains("port")))
        cout<<"port is not a key";
    return a.exec();
}

To copy to clipboard, switch view to plain text mode 

When run it prints
port is not a key.

Now of course there is something a bit fishy in my code: i am mixing char * and const char *. I would never have expected expected it could lead to this!!
I should point out I was a C programmer a long time ago and I am not used to the const
word.

[feraudyh]

I have just edited and simplified the previous post.

I'd like to point out that I just removed the word const from the above code and the problem is still there!!

Well, I'll just work around the problem by using QString for the first type instead of
char *.

[Tanuki-no Torigava]

Hi, it looks like the task you try to solve already done by QSettings. Please, have a look in assistant for it.

[wysota]

You have a hash that has a pointer to a const char as a key. If you have two character strings containing the same text (like "port" in your case) the pointers will be different, meaning that this returns false (check it out if you want):

const char *p1 = "port";
const char *p2 = "port";
if(p1==p2)
  printf("the same\n");
else
  printf("different\n");

const char *p1 = "port";
const char *p2 = "port";
if(p1==p2)
  printf("the same\n");
else
  printf("different\n");

To copy to clipboard, switch view to plain text mode 

If you use C++ then avoid using char* for strings as they have no semantics. Use std::string or QString (if you use Qt - and you do) instead.