printf with reference arguments - c

Question

Imagine having a label that after created updates x amount of times / sec. The text of the label is given as a format-specifier text (ala printf), and any arguments for the format-specifier is updated on redraw, because the arguments for the format specifier is pointers to their respective values.
Does any variant of sprintf work like this?
The code would work something like this:
/* client */
createLabel("Value is %f", &myFloatValue);
I haven't quite figured out a way to do this yet, does anyone have any ideas? I guess one could parse the format text, retrieve the pointers (and types), and store them as some object in a list, where you later could reprint the text and maybe delegate the formatting to the objects themselves, passing them only a textbuffer.. hmmm
Btw, the interface is C, but the host is C++.
Okay i got a "working" prototype, but it's written mainly in assembler. Anyway it demonstrates the supposed use of the api. Can anyone see a portable way to do this / have a better idea for the implementation?
It's pretty large so i'm posting it on pastebin:
http://pastebin.com/H8ZpWb4u

Answer 1

So your createLabel interface would store the format string, along with the addresses of the variables you're wanting to display within the string. Then just use standard old sprintf to reformat the text. Just be careful with those pointers to the data, and make sure you invalidate them when necessary.
I'm not really sure what the problem is. What else are you looking for? sprintf is capable of doing what you want, but you're going to have to track the format string and variable addresses yourself.

Answer 2

Okay i suddenly got an idea .. stringstream + templated polymorphism. I ended up writing the thing in C++ in 5 mins, and at the very least it's a huge improvement.
#include <string>
#include <iostream>
#include <vector>
#include <sstream>
class CBaseValue
{
public:
virtual void toString(std::stringstream & buf) = 0;
};
template< typename T >
class CValue : public CBaseValue
{
typedef T type;
typedef T * ptr_type;
type * val;
public:
CValue(void * val)
{
this->val = reinterpret_cast<ptr_type>(val);
}
CValue(type * val) : val(val) {}
virtual void toString(std::stringstream & buf) {
buf << *val;
}
};
class CLabel
{
std::stringstream ss;
std::vector<CBaseValue *> valueList;
std::string format;
public:
CLabel() {};
void reset() {
format.clear();
ss.str("");
for(unsigned i = 0; i < valueList.size(); i++) {
delete valueList[i];
}
valueList.clear();
}
void setFormat(const char * fmt, ...) {
reset();
format = fmt;
va_list args;
va_start(args, fmt);
for(unsigned i = 0; i < format.size(); ++i) {
if(format[i] == '%') {
++i;
switch(fmt[i])
{
case 'd':
valueList.push_back(new CValue<unsigned int>( va_arg(args, void *) ));
break;
case 'f':
valueList.push_back(new CValue<float>( va_arg(args, void *) ));
break;
}
}
}
va_end(args);
}
std::string get() {
ss.str("");
unsigned count(0);
for(unsigned i = 0; i < format.size(); i++) {
if(format[i] == '%') {
i++; // ignore type specifiers, already polymorphically solved
valueList[count++]->toString(ss);
} else {
ss << format[i];
}
}
return ss.str();
}
~CLabel() {
reset();
}
};
int main() {
int test = 2;
float val = 3.14f;
CLabel myLabel;
myLabel.setFormat("Stringstream test, float: %f, and an int: %d \n", &val, &test);
std::cout << myLabel.get();
test = 3;
std::cout << myLabel.get();
system("pause");
}

Answer 3

You could do something relatively simple with std::bind or boost::bind. I'll leave it as an exercise on how to massage a C interface on top of this.
#include <functional>
int main() {
int test = 2;
float val = 3.14f;
std::function<int()> label = std::bind(
printf,
"Stringstream test, float: %f, and an int: %d \n",
std::ref(val),
std::ref(test));
label();
test = 3;
label();
}