🔒 Locked

Decimal string to Pointer via StringStream

Started by Cygon Aug 8, 2012 at 12:36 PM 3 replies 3.2k views

Original Post

Aug 8, 2012 at 12:36 PM

C++ iostreams provide a method to output (void *) pointers. The pointer is written to and read from a stream in hexadecimal notation.

How can I use decimal notation without resorting to a reinterpret_cast and ugly #ifdefs?

I tried this:


 std::stringstream ss;
 
 void *test = (void *)0x12345678;
 
 // Output pointer as decimal number
 ss << std::ios::dec << test; // Works!
 
 // Read pointer from decimal number
 void *x;
 ss << dec; // Nope
 ss.setf(std::ios::dec); // No change
 ss >> dec >> x; // Neither

All the input variants do nothing. The pointer will not be assigned and keeps its "help I'm uninitialized" value of 0xcccccccc. The fail bit of the stream is set - probably because it tried to _still_ parse the number in hex and it was too large.

Bregma

Aug 8, 2012 at 12:53 PM

It might not answer your question, but why do you want to (a) write the pointer out in decimal, and (2) why on Mars would you want to read in a pointer?

rip-off

Aug 8, 2012 at 1:01 PM

Why are you trying to read a pointer from a stream? That seems incredibly dangerous.

Interestingly, the following code appears to compile on my system:


 #include <iostream>
 
 
 int main() {
 void *foo;
 if(std::cin >> foo) {
 std::cout << "Read a pointer?\n";
 } else {
 std::cout << "Failed\n";
 }
 }

However explicitly trying to call the operator fails:


 #include <iostream>
 
 
 int main() {
 void *foo;
 if(operator>>(std::cin, foo)) {
 std::cout << "Read a pointer?\n";
 } else {
 std::cout << "Failed\n";
 }
 }

[s]I suspect the former program is actually calling a built in operator >> after some type converting, for example converting the two sides to booleans or something.[/s] (See Alvaro's response, I am wrong).

alvaro

Aug 8, 2012 at 3:00 PM

Calling

operator>>(std::cin, foo)' explicitly doesn't work if foo is an int either.
 
 Reading a pointer from a stream is a valid operation, but there is no guarantee that the accepted format is decimal. In g++ the format seems to be hexadecimal. I can't bother to look it up in the standard, but my guess is that the only guarantee is that it will understand whatever format

<< foo' spits out.

rip-off

Aug 8, 2012 at 5:12 PM

Calling `operator>>(std::cin, foo)' explicitly doesn't work if foo is an int either.
[/quote]
Interesting, Needless to say, I don't explicitly call operators like that often. I was hoping an explicit call would allow me to "jump to declaration" in my IDE.

Reading a pointer from a stream is a valid operation...
[/quote]
Also interesting. Yet another entry in the C++ minefield to watch out for.

Topic Locked

This topic has been locked by a moderator. New replies are not allowed.