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std::string problems / C++

Started by PhiberOptic Jan 1, 2004 at 1:24 PM 20 replies 3k views

Original Post

Jan 1, 2004 at 1:24 PM

Okay, I got a function that uses std::string a lot for some operations. This is the function: bool MyClass::func() { std::string buffer; while( something ) { buffer = bla bla // lots of "buffer =" here.. //then in the end of the function I have these lines: buffer = readLine(fin); sscanf(buffer.c_str(),"%f %f %f %f", &b->rot.w, &b->rot.x, &b->rot.y, &b->rot.z); //Break! Here buffer starts to act wierd! } } When it passes that sscanf function, the buffer variable starts to act wierd. No matter what you do with it, all it does is make itself equals "D". I''ve tried a lot of stuff like: buffer = "asdf"; buffer = szSomeString; buffer = otherStdString; buffer = readLine(); buffer.clear(); buffer = std::string("hello world"); Nothing helped.. buffer still equals "D".. :-( Does anyone know anything about this behaviour? ---------------------------------------------- Petter Nordlander "There are only 10 kinds of people in the world. They who understand binary and those who do not"

Fruny

Jan 1, 2004 at 1:35 PM


#include <iostream>
#include <string>
#include <sstream>

void func()
{
  std::string buffer;
  std::stringstream ss;

  while( condition() )
  {
    std::getline(fin, buffer);
    ss.str(buffer)
    ss >> b->rot.w >> b->rot.x >> b->rot.y >> b->rot.z;
    ss.clear(); // clears the EOF, fail, bad... flags

  }
}

You can use stream member functions like fail() to check whether any given IO operation failed.

[edited by - Fruny on January 1, 2004 2:35:29 PM]

PhiberOptic

Jan 1, 2004 at 1:39 PM

I think you got me wrong. The input didn''t fail. It was good. The problem is that the "buffer" variable can''t be used after that line. When I try to assign a string to it, it only assigns a "D".

----------------------------------------------
Petter Nordlander

"There are only 10 kinds of people in the world. They who understand binary and those who do not"

Fruny

Jan 1, 2004 at 1:44 PM

What do you use to show that "D" ?

PhiberOptic

Jan 1, 2004 at 1:46 PM

Visual Studio.NET debuger.. using breakpoints and stuff..

----------------------------------------------
Petter Nordlander

"There are only 10 kinds of people in the world. They who understand binary and those who do not"

Premandrake

Jan 1, 2004 at 1:56 PM

Are you possibly debugging in release mode? I''ve done that more than a few times and it gives very strange results.

From the code I''ve posted I don''t see how it would be incorrect, everything seems fine.

Fruny

Jan 1, 2004 at 1:58 PM


#include <cstdio>
#include <iostream>

int main()
{
    float a,b,c,d;
    std::string buffer = "1000 1000 1000 1000";
    std::sscanf( buffer.c_str(), "%f %f %f %f", &a, &b, &c, &d);
    std::cout << a << ' ' << ' ' << b << ' ' << c << ' ' << d << std::endl;
}

Works without a hitch for me (VS 7.1)

[edited by - Fruny on January 1, 2004 2:59:02 PM]

PhiberOptic

Jan 1, 2004 at 2:12 PM

of course it does. I have plenty of similar code too.

Now converted to stringstreams instead of sscanf..


// Loc

		buffer = readLine(fin);
		ss.str(buffer);
		ss >> b->loc.x >> b->loc.y >> b->loc.z;
		ss.clear();
		// Buffer works	

		// Head

		buffer = readLine(fin);
		ss.str(buffer);
		ss >> b->head.x >> b->head.y >> b->head.z;
		ss.clear();
		// Buffer still works..

		// Tail

		buffer = readLine(fin);
		ss.str(buffer);
		ss >> b->tail.x >> b->tail.y >> b->tail.z;
		ss.clear();
		// buffer still works

		// Quat

		buffer = readLine(fin);
		ss.str(buffer);
		ss >> b->rot.w >> b->rot.x >> b->rot.y >> b->rot.z;
		ss.clear();
		// Buffer don''t work!

----------------------------------------------
Petter Nordlander

"There are only 10 kinds of people in the world. They who understand binary and those who do not"

Fruny

Jan 1, 2004 at 2:28 PM

No clue...

Zipster

Jan 1, 2004 at 11:25 PM

Fruny not having a clue is a sign that the Apocalypse is near...

It could be something specifically with the b->rot object causing sscanf to fail. Or even the stringstreams. I used to notice strange behavior whenever the types were mismatched.

PhiberOptic

Jan 2, 2004 at 4:36 AM

quote:
Original post by Zipster
It could be something specifically with the b->rot object causing sscanf to fail. Or even the stringstreams. I used to notice strange behavior whenever the types were mismatched.

Shouldn''t be.. The b->rot seem to work perfectly and with all variables set correctly. It has to be something with the input I do to buffer. but why can that mess it up?

----------------------------------------------
Petter Nordlander

"There are only 10 kinds of people in the world. They who understand binary and those who do not"

emileej

Jan 2, 2004 at 9:21 AM

I am quite clueless here, but try creating a char buffer for use in the scanf and then assign the value to your string. If no other reason - do it for kicks.

cgoat

Jan 2, 2004 at 10:50 AM

Was "b" properly allocated?

Have you tried not looking at it in the debugger? Just have your code print out the value to the screen. (buffer = "Hello"; cout << buffer; ) Those STL classes do some strange stuff, and I've found it difficult to look at them in the debugger with Visual C++, though I haven't tried .NET ever.

[edited by - cgoat on January 2, 2004 11:51:18 AM]

Pipo DeClown

Jan 2, 2004 at 11:11 AM

Structure of b? Can you show us that?

--
You''re Welcome,
Rick Wong
- sitting in his chair doing the most time-consuming thing..

Daniel Benmergui

Jan 2, 2004 at 11:24 AM

Well, the STL documentation regarding c_str states that:
"The returned C-style string should not be modified, as this could invalidate the pointer to the string, or deleted, as the string has a limited lifetime and is owned by the class string."

So you shouldn''t modify it

Assign the new value to a TCHAR/char/whatever buffer, sscanf it, and assign it to the STL string

Good Luck,
Daniel.

cgoat

Jan 2, 2004 at 2:39 PM

He''s not changing the string returned by c_str.

Kylotan

Jan 2, 2004 at 5:48 PM

Ok... so you have two almost identical blocks of code, one for Tail, one for Quat. And after the 2nd, you say that buffer stops working. However I notice you''ve not checked the status of buffer on each of those 4 lines. eg.

- what is the return value of readLine
- what is buffer after it''s been assigned to ss
- what is buffer after you''ve read the 4 values out of ss

Maybe examining those will shed some light on things? After all, I don''t know what your readLine function does, or why its needed when there are existing line-reading functions in the library. Perhaps it returns an invalid string somehow.

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Hull

Jan 3, 2004 at 12:30 AM

Are b->rot.w, x, y, z double data types?

If so, change them to floats.

MauMan

Jan 3, 2004 at 12:41 AM

If you''re using VS 2002 there''s a bug in std::string''s evaluation in the debugger. If the string is less that 16 characters (I may be wrong with this number) it stores the string in a static buffer. This is what the debugger shows. The first time you exceed that buffer length it puts the string in a dynamically-allocated buffer. If you want to look at what''s in it you have to expand the string to look at its members.

temp_ie_cant_thinkof_name

Jan 3, 2004 at 1:59 AM

Why do you use std::string. What''s wrong with char * const?

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