C/C++ Programming Questions Not Worth Their Own Thread

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No Safe Word: Feb 26, 2005

Though 4(x) obviously needs to be changed to (4*x).

e:f;b ... and by like an hour, man I need to stop replying to tabs that have been open so long

# ? May 5, 2008 17:19

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# ? May 18, 2024 03:54

Llama Patrol: Mar 11, 2007

ColdPie posted:

Just kind of at a first glance (about to run out the door to school myself), this looks all right. I'd add in a check to make sure x isn't negative -- that'd create an infinite loop.

Or just

code:

int recursive(unsigned int x){ ... }

# ? May 5, 2008 17:45

Soldat: Jan 22, 2008; my name is soldat and I get respect, your cash and your jewelery is what I expect

Paniolo posted:

Here's a tutorial for writing a small Scheme interpreter in Scheme. I think it would be straightforward to take most of the concepts and re-implement them in C++.

For parsing, you could look into boost::spirit, which lets you specify grammars in EBNF-like syntax. Given the syntactical simplicity of Scheme you could probably get a parser up and running relatively easily.

This was very helpful, thank you. Our team has decided that we're going to use structs rather than classes, so we've entered the realm of C now.

# ? May 5, 2008 17:58

Smackbilly: Jan 3, 2001; What kind of a name is Pizza Organ! anyway?

Being aware that invoking reinterpret_cast is something that should be done with extreme care in good C++ code, I was wondering if there is anything wrong with this, or if there is a better way to do this:

code:


struct Packet {
  uint16_t foo;
  uint16_t bar;
  uint8_t isLong;
  char optData[];
};

struct LongPacket {
  Packet shortPacket;
  uint16_t extraField1;
  uint16_t extraField2;
};

void parsePacket(Packet* pkt) {
  std::cout << "Foo: " << pkt->foo << std::endl;
  std::cout << "Bar: " << pkt->bar << std::endl;
  std::cout << "Is Long: " << pkt->isLong << std::endl;

  if (pkt->isLong != 0) {
    LongPacket* l_pkt = reinterpret_cast<LongPacket*>(pkt);
    std::cout << "Extra Field 1: " << l_pkt->extraField1 << std::endl;
    std::cout << "Extra Field 2: " << l_pkt->extraField2 << std::endl;
  }
}

Where this packet is something that is going to be sent/received over a network socket, and assuming, of course, that the isLong field is accurate.

# ? May 5, 2008 22:03

JoeNotCharles: Mar 3, 2005; Yet beyond each tree there are only more trees.

Since this is C++ code, why doesn't LongPacket just inherit from Packet so you can use static_cast?

# ? May 5, 2008 22:35

Smackbilly: Jan 3, 2001; What kind of a name is Pizza Organ! anyway?

JoeNotCharles posted:

Since this is C++ code, why doesn't LongPacket just inherit from Packet so you can use static_cast?

I considered that, but - and I guess this should have been an alternate question - I couldn't find any definitive answer on whether or not that would preserve field ordering and packing.

Since it's a network packet, it needs to be bit-for-bit accurate. So, if I write:

code:

#pragma pack(1)

struct Packet { 
  uint16_t foo; 
  uint16_t bar; 
  uint8_t isLong; 
}; 

struct LongPacket : public ShortPacket { 
  uint16_t extraField1; 
  uint16_t extraField2; 
};

Am I absolutely guaranteed that if I copy the contents of a LongPacket struct into a char[] buffer that, for example, buf[56] is the first byte of extraField2?

# ? May 5, 2008 22:47

Lexical Unit: Sep 16, 2003

JoeNotCharles posted:

why doesn't LongPacket just inherit from Packet

When you get the data out of the socket on the receiving end, it'll just be a bunch of bytes. Understanding how these bytes are packed is important. So there's a desire for the datatypes to be POD. At least that's what I assume... assuming I'm not misunderstanding something.

Smackbilly posted:

I was wondering if there is anything wrong with this

Isn't there a problem if optData isn't always the same length? If not, then it seems like it'll work.

Smackbilly posted:

or if there is a better way to do this

What about this?

code:

struct Packet {
  uint16_t foo;
  uint16_t bar;
  uint8_t isLong;
  uint16_t extraField1;
  uint16_t extraField2;
  char optData[];
};

Can't you just ignore extraField1 and extraField2 if isLong isn't set?

Lexical Unit fucked around with this message at 22:51 on May 5, 2008

# ? May 5, 2008 22:48

Vanadium: Jan 8, 2005

I do not think the language guarantees absence of padding in any case, with or without inheritance.

# ? May 5, 2008 23:09

Smackbilly: Jan 3, 2001; What kind of a name is Pizza Organ! anyway?

Vanadium posted:

I do not think the language guarantees absence of padding in any case, with or without inheritance.

In gcc/g++, the #pragma pack(1) directive guarantees the absence of padding in plain structs. However I'm looking but I can't find out if this also applies if you use a struct more like a class (for example, adding constructors, using inheritence, etc).

I'm not terribly concerned about portability to other compilers at the moment, but this will have to build on gcc for several platforms, so I don't want to rely on something that might work on x86 but not on ARM.

# ? May 5, 2008 23:14

Mr VacBob: Aug 27, 2003; Was yea ra chs hymmnos mea

Classes will usually have a vtable pointer at the beginning, so you can't rely on offsets from the beginning of the class. You can rely on offsets from other members, I guess.

Also, use __attribute__((packed)) if you don't care about compiler portability, otherwise you'll forget to undo the #pragma and it'll be slow.

# ? May 5, 2008 23:29

Paniolo: Oct 9, 2007; Heads will roll.

There's only a vtable if there is a virtual function anywhere in the inheritance tree. Using #pragma pack(1) you can be fairly sure about the run-time size of a struct. However, the correct way to do what you're trying to do is to implement a serialize/unserialize method:

code:

class ShortPacket : public Packet {
   static ShortPacket* unserialize(const char* buf) {
      // pull data out of the short packet
   }
};

class LongPacket : public Packet {
   static LongPacket* unserialize(const char* buf) {
      // pull data out of the long packet
   }
};

Packet* accept_packet(const char* buf) {
   if(buf[PACKET_TYPE_OFFSET] == PACKET_TYPE_SHORT)
      return ShortPacket::unserialize(buf);
   else if(buf[PACKET_TYPE_OFFSET] == PACKET_TYPE_LONG)
      return LongPacket::unserialize(buf);
   throw std::runtime_error("Unknown packet type");
}

As a general rule you really don't want to initialize a class by memcpying into it or just casting a void pointer to a class pointer, there's just so many things that can go wrong (for example, padding, network byte order vs host byte order, corrupted or malicious data, etc.)

# ? May 5, 2008 23:45

Mustach: Mar 2, 2003; In this long line, there's been some real strange genes. You've got 'em all, with some extras thrown in.

Paniolo posted:

As a general rule you really don't want to initialize a class by memcpying into it or just casting a void pointer to a class pointer, there's just so many things that can go wrong (for example, padding, network byte order vs host byte order, corrupted or malicious data, etc.)

Specifically, if you have the choice, serialize/deserialize through text and not directly as binary.

# ? May 6, 2008 00:14

Viper2026: Dec 15, 2004; I got iRaped by Steve Jobs and all I got was this custom title.

If anyone could take a look at this bit of code for me I'd appreciate it:

http://rafb.net/p/Xr46gD55.html

Line 61 is giving me the following error:
Blob.cpp:61: error: cannot convert 'color' to 'color (*)[5]' for argument '1' to 'int count_cells(color (*)[5], int, int)'

And I can't seem to figure out what's wrong.
The count_cells function returns the number of cells marked "abnormal" or '1' which are connected to the cell being tested.
The array it's trying to read in from the file is a 5x5 grid, and I'm attempting to fill it up with the color enumeration types depending if a 1 or 0 is read (the other 2 infile variables at the beginning are the test row/column, the size of the grid and the test cell are the first 4 characters in the file). As I said before, the grid is 5x5 so once I figure out how to fix this error I'll figure out some way to make the function called on a grid of arbitrary size.

In case anyone wants to see the contents of the file in question:

code:

So the expected printout of my function call would be 10. (grid and row indexes start at 0)

Viper2026 fucked around with this message at 05:08 on May 6, 2008

# ? May 6, 2008 05:04

Paniolo: Oct 9, 2007; Heads will roll.

code:

// This doesn't pass an array, this passes the cell at [r_size][c_size]
// Even if it did compile, this would cause a segfault
count_cells(grid[r_size][c_size], r_test, c_test)

// Try this
count_cells(grid, r_test, c_test)

Also you can't declare an array like this:

code:

int x = 5; // or whatever
int array[x];

// You have to dynamically allocate the array
int* array = new int[x];

Finally, if you do that, you'll have bugs if the array you're dynamically allocating isn't 5x5 because what you're passing to the count_cells function is. Frankly, multidimensional arrays are more trouble than they are worth, and are typically a sign of a newbie programmer. A better solution is this:

code:

int* grid = new int[width*height];

int get_cell_state(int* grid, unsigned int width, unsigned int height, int x, int y) {
   assert(width > 0);
   assert(height > 0);
   assert(y < height);
   assert(x < width);
   
   return grid[(width*y)+x];
}

# ? May 6, 2008 05:14

Viper2026: Dec 15, 2004; I got iRaped by Steve Jobs and all I got was this custom title.

Thanks for the help, but this is some crap my teacher got out of a textbook and gave us that count_cells() function along with enumeration type at the top, then was like "make it do this this and this". If I had a choice of doing it some other way I would :mad:

I changed the row/col size variables to type of size_t and now it lets me create the array given the values read in from my file. However, when I try to call my function:

cout << count_cells(the_grid[r_size][c_size], r_test, c_test) << "\n";

again, I'm getting this:

error: cannot convert 'color' to 'color (*)[5]' for argument '1' to 'int count_cells(color (*)[5], int, int)'

The code in question:

code:

	ifstream infile("tests/myfile.txt", ios::in);
	size_t r_size = 0;
	size_t c_size = 0;
	int r_test, c_test, read_data;
	
	infile >> r_size >> c_size >> r_test >> c_test;
	color the_grid[r_size][c_size];

Viper2026 fucked around with this message at 05:56 on May 6, 2008

# ? May 6, 2008 05:21

elevatordeadline: Jan 29, 2008

TSDK posted:

I think your dev environment is misconfigured. By the looks of it, either there's a mismatch in the calling convention or the long double support between your executable and the library you're linking against.

I guess so. As it turns out, changing long doubles and %Lfs, respectively, to doubles and %fs made everything work as it should've in the first place.

# ? May 6, 2008 07:58

Incoherence: May 22, 2004; POYO AND TEAR

Viper2026 posted:

I changed the row/col size variables to type of size_t and now it lets me create the array given the values read in from my file. However, when I try to call my function:

cout << count_cells(the_grid[r_size][c_size], r_test, c_test) << "\n";

again, I'm getting this:

error: cannot convert 'color' to 'color (*)[5]' for argument '1' to 'int count_cells(color (*)[5], int, int)'

As mentioned before, the_grid[r_size][c_size] is a cell (which is why the compiler says it's of type color). Moreover, it's a cell that's not in the grid, so even if it compiled it would immediately segfault and you'd have even less feedback.

Go look at Paniolo's post again, particular the first code block.

# ? May 6, 2008 09:08

JoeNotCharles: Mar 3, 2005; Yet beyond each tree there are only more trees.

Paniolo posted:

Also you can't declare an array like this:
code:
int x = 5; // or whatever
int array[x];

Actually you can in gcc - it's a compiler extension. (You shouldn't, though, because it makes your code non-portable.)

# ? May 6, 2008 15:57

Lexical Unit: Sep 16, 2003

Why does this assert?

code:

#include <vector>
#include <fstream>
#include <iterator>
#include <iostream>

using namespace std;

int main()
{
	fstream f ("blob", ios::in | ios::out | ios::trunc);
	streampos start = f.rdbuf ()->pubseekoff (0, ios::cur);
	char data[] = "this is just a test";
	f.write (data, sizeof (data));
	f.rdbuf ()->pubseekpos (start);
	vector<char> v;
	v.insert (v.end (), istream_iterator<char> (f), istream_iterator<char> ());
	assert (!f.fail ());
	return 0;
}

Edit: Never mind, I should have tried harder with google.

Answer: "After reading the last available character, the extraction not only reads past the end of the input sequence; it also fails to extract the requested character. Hence, failbit is set in addition to eofbit."

Lexical Unit fucked around with this message at 17:26 on May 6, 2008

# ? May 6, 2008 16:48

Smackbilly: Jan 3, 2001; What kind of a name is Pizza Organ! anyway?

This may be silly/stupid, but is it in any way possible to do this?

code:

class Foo : public Bar<Foo::Things::Thing> {

public:

  struct Things {
    enum Thing {
      // ...
    };
   };

   // ...

};

Basically I have an enum declared (publicly) within a class. The enum is contained by an otherwise empty struct for the purpose of giving the enumerated values their own namespace. I want to pass the enum type as a template argument to a class that I want to inherit from. I can't seem to find any way to forward-declare things that occur inside a top-level class, and I'm thinking this might be impossible simply because I'm asking a base class to do something with a type declared in a derived class, but I'm wondering if there is any way to get this effect.

The motivation behind this is that I'm writing a number of classes that all have several identical member functions that accept enumerated values, but the enumerated values are different for each class. I was thinking it would be handy to have a common templated base class that handled the implementation of these member functions so that I wouldn't have to duplicate the code. The other way to do this (my present solution) is to have each class call templated utility functions that are in an unrelated clsas, but in that case there is still an identical interface to the utility function in each class.

Obviously one solution is to move the Things struct outside of the class, but then the namespace for the Thing values would have to be FooThings:: instead of Foo::Things:: which seems inelegant to me.

# ? May 6, 2008 17:23

Mr VacBob: Aug 27, 2003; Was yea ra chs hymmnos mea

JoeNotCharles posted:

Actually you can in gcc - it's a compiler extension. (You shouldn't, though, because it makes your code non-portable.)

It's legal C99, but that assumes a C99 compiler.

vv That means they're not perfect (see the note), but I've never had any trouble with them in practice.

Mr VacBob fucked around with this message at 20:22 on May 6, 2008

# ? May 6, 2008 18:50

KaeseEs: Feb 23, 2007; by Fragmaster

Mr VacBob posted:

It's legal C99, but that assumes a C99 compiler.

IIRC variable-length arrays are one of the most problematic C99 features with respect to the implementations of various compilers

edit: this page says they're still broken in gcc 4.3

# ? May 6, 2008 19:31

JoeNotCharles: Mar 3, 2005; Yet beyond each tree there are only more trees.

Smackbilly posted:

This may be silly/stupid, but is it in any way possible to do this?
code:
class Foo : public Bar<Foo::Things::Thing> {

public:

  struct Things {
    enum Thing {
      // ...
    };
   };

   // ...

};

Off the top of my head, try

code:

class FooBase {
public:
  struct Things { ... };
};

class Foo : public Bar<FooBase::Things::Thing>, public FooBase {
  ...
};

Since Foo inherits FooBase, I believe you can now refer to FooBase::Things as Foo::Things. (Not in the actual definition of Foo, but at least you're containing the inelegance.)

# ? May 6, 2008 21:18

Paniolo: Oct 9, 2007; Heads will roll.

JoeNotCharles posted:

Since Foo inherits FooBase, I believe you can now refer to FooBase::Things as Foo::Things. (Not in the actual definition of Foo, but at least you're containing the inelegance.)

http://codepad.org/exT9EKui

You actually can refer directly to Foo::Thing from within the definition of Foo. What you have to be careful with is in the Base class.. if you do something like this:

code:

template <typename T>
struct Base {
   typedef typename T::Thing Thing;
};

Then referring to Foo::Thing is ambiguous (is it FooThing::Thing or Base<FooThing>::Thing? Even though they are identical types C++ isn't clever enough to know this.)

# ? May 6, 2008 22:54

Professor Science: Mar 8, 2006; diplodocus + mortarboard = party

What the gently caress, Qt 4.4 supports futures? Holy poo poo.

# ? May 7, 2008 04:48

vanjalolz: Oct 31, 2006; Ha Ha Ha HaHa Ha

code:

void runlinked(const char* cmd1, const char* argv1[], const char* cmd2, const char* argv2[], int *retval)
{
	...
	/* warning: passing argument 2 of �execvp� from incompatible pointer type */
	execvp(cmd1, argv1);

Why does this code produce a warning on argument 2 of execvp?
I checked the man page and its meant to be a const char* :|
(actually it says char * const, but i tried that too and it didn't help)
What am i missing?

gcc version 4.0.1 (Apple Inc. build 5465)

# ? May 8, 2008 03:15

Paniolo: Oct 9, 2007; Heads will roll.

You're passing a pointer to a array of const char* to a function which takes a pointer to a const char*. Try this:

code:

execvp(cmd1, argv1[0]);

# ? May 8, 2008 03:44

vanjalolz: Oct 31, 2006; Ha Ha Ha HaHa Ha

Oh I see, thanks.

# ? May 8, 2008 04:06

JoeNotCharles: Mar 3, 2005; Yet beyond each tree there are only more trees.

Professor Science posted:

What the gently caress, Qt 4.4 supports futures? Holy poo poo.

I took a look for that and couldn't find any docs. Can you point me to a reference?

# ? May 8, 2008 04:17

Id4ever: Dec 2, 2004

JoeNotCharles posted:

I took a look for that and couldn't find any docs. Can you point me to a reference?

http://doc.trolltech.com/4.4/qfuture.html
http://labs.trolltech.com/blogs/2007/03/08/making-asynchronous-function-calls-with-qfuture/

# ? May 8, 2008 15:25

JoeNotCharles: Mar 3, 2005; Yet beyond each tree there are only more trees.

Oh, I completely missed that 4.4 already had an entry on the main doc page - I was looking around for previews and getting pissed off that all the Google links were outdated.

Wow, the whole QConcurrent framework looks fantastic.

# ? May 8, 2008 15:29

Professor Science: Mar 8, 2006; diplodocus + mortarboard = party

JoeNotCharles posted:

Wow, the whole QConcurrent framework looks fantastic.

Any idea how it's implemented on the backend?

# ? May 8, 2008 21:00

Soldat: Jan 22, 2008; my name is soldat and I get respect, your cash and your jewelery is what I expect

code:

bool primcheck_add (char exp[])
{
  char begin[10];
 
  if (length(exp) > 1)
    {
      for (int x=0; x < 4; x++)
    begin[x]=exp[x];
      begin[4] = '\0';
      if (begin[0] == 'a')
    if (begin[1] == 'd')
      if (begin[2] == 'd')
        if (begin[3] == '1')
          return true;
        else
          return false;
    }
  else
    return false; 
}

bool primcheck_sub (char exp[])
{
  char begin [10];

  if (length(exp) > 1)
    {  
      for (int x=0; x < 4; x++)
    begin[x]=exp[x];
      begin[4] = '\0';
      if (begin[0] == 's')
    if (begin[1] == 'u')
      if (begin[2] == 'b')
        if (begin[3] == '1')
          return true;
        else
          return false;
    }
  else
    return false; 
}

bool longprim_check (char exp[])
{
  if (primcheck_add(exp))
    return true;
  else if (primcheck_sub(exp))
    return true;
  else
    return false;
}

When I call longprim_check, I just went it to return true if the input is simply 'add1' or 'sub1'. Anything else I want it to return false. Right now it's returning true for everything. What glaringly obvious thing am I missing?

# ? May 8, 2008 22:08

ShoulderDaemon: Oct 9, 2003; support goon fund; Taco Defender

Soldat posted:

When I call longprim_check, I just went it to return true if the input is simply 'add1' or 'sub1'. Anything else I want it to return false. Right now it's returning true for everything. What glaringly obvious thing am I missing?

The strcmp function?

# ? May 8, 2008 22:10

haveblue: Aug 15, 2005; Toilet Rascal

Soldat posted:

When I call longprim_check, I just went it to return true if the input is simply 'add1' or 'sub1'. Anything else I want it to return false. Right now it's returning true for everything. What glaringly obvious thing am I missing?

The strcmp function and the fact that most of your nested if statements do not have else cases. If you pass "adff" to primcheck_add it's just going to fall out in the middle, returning garbage (which would be interpreted as true unless it was 0). Your compiler should be warning you about this.

# ? May 8, 2008 22:20

Soldat: Jan 22, 2008; my name is soldat and I get respect, your cash and your jewelery is what I expect

code:

bool primcheck_mult (char exp[])
{
  char begin[4];
  char add[] = "add1";
  char sub[] = "sub1";
 
 
  if (length(exp) > 1)
    {
      for (int x=0; x < 4; x++)
    begin[x]=exp[x];
      begin[4] = '\0';
      if (strcmp (begin,add))
    return true;
      else if (strcmp (begin,sub))
    return true;
      else
    return false;
    }
  else
    return false;
}

Sorry to make you bear with me here, I'm not asking to get my work done, but I'm really beginning to get frustrated. I'm getting no warnings from the compiler, but no matter what the input is, it returns true. This doesn't need to be a very robust program, it just needs these two specific cases. I can change the first if to: if ((length(exp) > 1) && (length(exp) < 5)) if I need to, but it still is coming back true even if the input isn't specifically add1 or sub1...any advice appreciated again.

Soldat fucked around with this message at 23:22 on May 8, 2008

# ? May 8, 2008 23:02

TheSleeper: Feb 20, 2003

In addition to what they say before, if you really refuse to use strcmp, have you never heard of the && operater?

edit: and why are you using an array of 10 characters when you only ever use 4 in both those functions.

TheSleeper fucked around with this message at 23:13 on May 8, 2008

# ? May 8, 2008 23:11

Soldat: Jan 22, 2008; my name is soldat and I get respect, your cash and your jewelery is what I expect

oops, accidental post

# ? May 8, 2008 23:23

crazypenguin: Mar 9, 2005; nothing witty here, move along

strcmp returns 0 when the strings are the same.

so basically, stick a ! before those strcmps

# ? May 8, 2008 23:32

Adbot: ADBOT LOVES YOU

# ? May 18, 2024 03:54

StickGuy: Dec 9, 2000; We are on an expedicion. Find the moon is our mission.

Soldat posted:

code:
  char begin[4];
...
      begin[4] = '\0';
Sorry to make you bear with me here, I'm not asking to get my work done, but I'm really beginning to get frustrated.

It's time to for you to familiarize yourself with two of a programmer's best friends: the debugger and the documentation. If you step through the code with a debugger, you'll see precisely where it's going wrong. Then you can look up the relevant things in the documentation to understand what's causing it. As a freebie, though, begin now only has 4 characters and you're setting the 5th to null.

# ? May 8, 2008 23:34

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > C/C++ Programming Questions Not Worth Their Own Thread

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