How to resolve this "cast to pointer from type of different size" warning?

Question

I have two programs that communicate over the D-Bus. In one of the there is a function that takes two parameters namely const char* data and int size and in another one I have a function that returns a value of the type unsigned char. I place a function call in the first program like this function((char*)program2->function().value(), 1) but i get the warning cast to pointer from integer of different size [-Wint-to-pointer-cast]. how should i resolve this warning? also I'm using Qt libraries.

EDIT:

actually in the function that takes the const char* and int I append the data to a QByteArray which accepts const char* and a size of type int in it's constructor as provided by Qt and the other program is supposed to return a number in the range 0-255 hence the unsinged char. If what I'm doing is wrong what's the best way to obtain the desired result?

Also call to program2->function invokes the following:

inline QDBusPendingReply<uchar> type()
    {
        QList<QVariant> argumentList;
        return asyncCallWithArgumentList(QLatin1String("type"), argumentList);
    }

this class has a function called value() which invokes:

inline typename Select<0>::Type value() const
    {
        return argumentAt<0>();
    }

NOTE: my main purpose of this question was to find out a way to create a reference to the returned result of calling a function defined within the proxy class not how to convert it to something acceptable for creation of a QByteArray so please stop adding unrelated tags like QByteArray or QtCore etc.

Answer 1

What you need is the address of the data... which would look like

function2(& program2->function().value(), 1);

and would be perfectly safe, since the lifetime of the temporary variable is until the end of the complete expression, which is long enough¹ for function2 to use it.

Unfortunately, you can't use the & address-of operator on an rvalue. But there is a workaround, since you can bind a const lvalue reference to an rvalue, and you want a pointer to const data anyway:

template<T>
const T* get_addr(const T& v) { return &v; }

function2( get_addr(program2->function().value()), 1 );

If you get a signed-vs-unsigned, mismatch, try

function2( get_addr<char>(program2->function().value()), 1 );

Of course this is a lot of work just to avoid giving the variable a name, as Captain Obvlious has done.

---

¹ Unless function2 saves that pointer for later, but if it does then you shouldn't be using a local variable to hold the value either, but pay very close attention to object lifetime.

Answer 2

I do not think what you are doing is safe, hence your code might not work at all. If your function() is returning an "unsigned char", its going to make a copy of it and return it. At the same time your other function() is taking a "const char*". I suggest to modify your program2->function() to return a "char *".

Answer 3

Converting an integer to a pointer is not a really good idea especially if you don't know why the conversion is necessary and what exactly it does. It looks like the conversion isn't even necessary and you really need a solution to properly call the function instead of fixing the cast warning. From the code and description you provided it appears you want to send a single byte returned by the call to program2->function(). Since the value returned by this function is an rvalue you can't directly take it's address. Instead you can to store it in a variable and then pass a pointer to that instead. Something like the code below should let you do what you want.

char value = program2->function();
function(&value, 1);