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

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Ms Jewish Anime: Feb 18, 2007; WATCH THE WAY "THE JEW" DODGES AND WEAVES LIKE A SNAKE IT IS SADLY ALL HE KNOWS

I have a question about fflush(stdin) in C programming.
It's supposed to clear some input buffer that holds input data. However I have yet to come across a situation that requires fflush for stdin. If possible, could someone explain a situation or give an example when fflush(stdin) or even stdout would be required?

# ? Apr 11, 2008 16:20

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# ? Apr 18, 2024 02:02

Plastic Jesus: Aug 26, 2006; I'm cranky most of the time.

Bush is a QT posted:

I have a question about fflush(stdin) in C programming.
It's supposed to clear some input buffer that holds input data. However I have yet to come across a situation that requires fflush for stdin. If possible, could someone explain a situation or give an example when fflush(stdin) or even stdout would be required?

Always flush shared file handles in multi-threaded applications. I can't for the life of my think of a time when multiple threads would be accessing stdin, but it happens all the time with stdout. If you have several threads that all want to print poo poo to the screen you have to make them stand in line to do so or they'll end up printing over the top of each other. So you make them acquire a mutex, then write to stdout. You want to flush stdout at that point to ensure that everything has been written before you release the mutex.

Fake edit: just thought of when you'd want to fflush(stdin)- when your application will be used in a pipe, e.g. 'cat somefile|yourapp'.

# ? Apr 11, 2008 16:48

Ms Jewish Anime: Feb 18, 2007; WATCH THE WAY "THE JEW" DODGES AND WEAVES LIKE A SNAKE IT IS SADLY ALL HE KNOWS

thank you

# ? Apr 11, 2008 16:54

Vanadium: Jan 8, 2005

I am pretty sure that fflushing input streams does not do anything.

# ? Apr 11, 2008 16:54

POKEMAN SAM: Jul 8, 2004

Vanadium posted:

I am pretty sure that fflushing input streams does not do anything.

From the man page:

code:

Flushing  an  input  stream  discards any buffered input and
adjusts the file pointer such that the next input  operation
accesses  the  byte  after  the  last one read.

# ? Apr 11, 2008 18:50

FigBug: Apr 27, 2002; Lemon Party - Lest we Forget

Ugg boots posted:

code:

Flushing  an  input  stream  discards any buffered input and
adjusts the file pointer such that the next input  operation
accesses  the  byte  after  the  last one read.

fflush(stdin) is undefined behavior in ANSI C. Some compilers implement it, some don't.

# ? Apr 11, 2008 19:09

Vanadium: Jan 8, 2005

Ugg boots posted:

From the man page:

code:

Flushing  an  input  stream  discards any buffered input and
adjusts the file pointer such that the next input  operation
accesses  the  byte  after  the  last one read.

It does not say that in my manpage. Are you sure you are not missing some "conforming to" part?

# ? Apr 11, 2008 19:26

haveblue: Aug 15, 2005; Toilet Rascal

The OS X 10.5 manpage for fflush makes no mention of input streams; it only says the function will fail if the stream is "not open for writing".

# ? Apr 11, 2008 19:36

6174: Dec 4, 2004

I've got a template function in which I'm doing a comparison like:

code:

if (number < 0) {
    number *= -1;
}

number is of type T. When T is an unsigned type the compiler gives me a warning:

../shared/number_stuff.h:102: warning: comparison of unsigned expression < 0 is always false

Is there anyway I can reasonably make these warnings go away? I tried enclosing it in an if (numeric_limits<T>::is_signed) block, but the compiler still complains.

# ? Apr 11, 2008 19:45

more falafel please: Feb 26, 2005; forums poster

6174 posted:

I've got a template function in which I'm doing a comparison like:
code:
if (number < 0) {
    number *= -1;
}
number is of type T. When T is an unsigned type the compiler gives me a warning:

../shared/number_stuff.h:102: warning: comparison of unsigned expression < 0 is always false

Is there anyway I can reasonably make these warnings go away? I tried enclosing it in an if (numeric_limits<T>::is_signed) block, but the compiler still complains.

Since numeric_limits<T>::is_signed is a compile-time constant, you should be able to make use of SFINAE to make one specialization for signed types and another for unsigned types. Otherwise, you can just turn off that warning with a #pragma around that block or for that file on the command line, but I'm pretty sure SFINAE is the proper way to do it. Concepts would make this easier.

# ? Apr 11, 2008 19:55

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

Try boost::enable_if

# ? Apr 11, 2008 19:57

chris_bacon: Feb 27, 2008

Is it possible to use a struct or class in a templated function? I've already made a working, templated, double-linked list and now I'm trying to use it in a call center simulation.

I made the struct:

code:

struct caller_t {
  public: 
    string          status; 
    string          name;
    int           timestamp;
    int		  duration;
};

and I'm trying to insert a struct into my double-linked list, so I made a dynamic array of the caller_t struct.

When I compile, I get this:

code:

cse1695p58% g++ -Wall -Werror -o call call.cpp
call.cpp:38:32: error: character constant too long for its type
call.cpp:42:32: error: multi-character character constant
call.cpp:46:32: error: character constant too long for its type
call.cpp:50:32: error: multi-character character constant
call.cpp: In function �int main()�:
call.cpp:38: error: no match for �operator==� in �(((caller_t*)(((long unsigned int)i) * 24ul)) + callers)->caller_t::status == 1768846701�
call.cpp:39: error: no matching function for call to �Dlist<caller_t>::insertFront(caller_t&)�
dlist.cpp:14: note: candidates are: void Dlist<T>::insertFront(T*) [with T = caller_t]
call.cpp:42: error: no match for �operator==� in �(((caller_t*)(((long unsigned int)i) * 24ul)) + callers)->caller_t::status == 1735355492�
call.cpp:43: error: no matching function for call to �Dlist<caller_t>::insertFront(caller_t&)�
dlist.cpp:14: note: candidates are: void Dlist<T>::insertFront(T*) [with T = caller_t]
call.cpp:46: error: no match for �operator==� in �(((caller_t*)(((long unsigned int)i) * 24ul)) + callers)->caller_t::status == 1819698546�
call.cpp:47: error: no matching function for call to �Dlist<caller_t>::insertFront(caller_t&)�
dlist.cpp:14: note: candidates are: void Dlist<T>::insertFront(T*) [with T = caller_t]
call.cpp:50: error: no match for �operator==� in �(((caller_t*)(((long unsigned int)i) * 24ul)) + callers)->caller_t::status == 1852796517�
call.cpp:51: error: no matching function for call to �Dlist<caller_t>::insertFront(caller_t&)�
dlist.cpp:14: note: candidates are: void Dlist<T>::insertFront(T*) [with T = caller_t]
call.cpp:60: error: request for member �name� in �curcaller�, which is of non-class type �caller_t*�
call.cpp:61: error: request for member �duration� in �curcaller�, which is of non-class type �caller_t*�

Here's my insertFront for reference:

code:

template <class T>
void Dlist<T>::insertFront(T *o)
{
  node *np = new node;
  np->next = first;
  np->o = o;
  if (isEmpty()) {
    last = np;
  } 
  else {
    first->prev = np;
  }
  first = np;
}

Am I using incorrect syntax or is it not possible to do?

# ? Apr 13, 2008 01:52

Avenging Dentist: Oct 1, 2005; oh my god is that a circular saw that does not go in my mouth aaaaagh

You can use objects in templates, that's the point. You're trying to call insertFront with a reference as the parameter and not a pointer. The types need to match up.

# ? Apr 13, 2008 02:10

lhunc: Jan 4, 2007

I'm trying to get myself back into C but things are going a bit slow...

fwrite requires a pointer to a string as the first argument so how would I properly cast a character constant to a pointer? ie:

// Wrong
fwrite('\n', sizeof(char), 1, fd);

// Wrong
fwrite((char *) '\n', sizeof(char), 1, fd);

# ? Apr 14, 2008 17:39

Avenging Dentist: Oct 1, 2005; oh my god is that a circular saw that does not go in my mouth aaaaagh

Don't bother with casts, just do fwrite("\n",1,1,fd);

# ? Apr 14, 2008 17:56

Standish: May 21, 2001

lhunc posted:

I'm trying to get myself back into C but things are going a bit slow...

fwrite requires a pointer to a string as the first argument so how would I properly cast a character constant to a pointer? ie:

// Wrong
fwrite('\n', sizeof(char), 1, fd);

// Wrong
fwrite((char *) '\n', sizeof(char), 1, fd);

You cannot take the address of a literal so you'll have to assign it to a variable:

code:

char c = '\n';
fwrite(&c, sizeof(char), 1, fd);

Although it's probably simpler just to write a string consisting of a single character, like the other guy said.

# ? Apr 14, 2008 18:04

lhunc: Jan 4, 2007

thanks guys, guess I'm just over thinking it.

# ? Apr 14, 2008 18:15

Avenging Dentist: Oct 1, 2005; oh my god is that a circular saw that does not go in my mouth aaaaagh

Oh, I forgot to mention, another option would be to use fputc('\n',fd); if you're only writing single characters.

# ? Apr 14, 2008 18:22

POKEMAN SAM: Jul 8, 2004

HB posted:

The OS X 10.5 manpage for fflush makes no mention of input streams; it only says the function will fail if the stream is "not open for writing".

I found it on this site http://www.hmug.org/man/3/fflush.php but I guess that's only for ISO C90?

# ? Apr 14, 2008 19:33

The Red Baron: Jan 10, 2005

I'm wondering if it's at all possible to further specialize a nested class of a base class in its subclass(es)?

What I want is something that works theoretically like this (ignore the actual types used, they're not important):

code:

struct base
{
  template <typename T>
  struct x;

  template <>
  struct x<float>
  {
    typedef float some_type;
  };

  // (More specializations...)
};

struct derived : base
{
  using base::x;

  template <>
  struct x<int>
  {
    typedef int some_type;
  };
};

which isn't allowed, at least not in MSVC++ 2008. My hope was that I can somehow keep most of the general code in a base class and that doing derived::x<int>::some_type and derived::x<float>::some_type will give the specialized results. I hope I managed to pose this question in an understandable way :shobon:

# ? Apr 14, 2008 20:23

That Turkey Story: Mar 30, 2003

The Red Baron posted:

I'm wondering if it's at all possible to further specialize a nested class of a base class in its subclass(es)?

What I want is something that works theoretically like this (ignore the actual types used, they're not important):

Not in standard C++, however the VC++ compilers have a little hacky way to make it work which is actually what is used to provide VC++ with native support for Boost.Typeof.

# ? Apr 14, 2008 20:32

Avenging Dentist: Oct 1, 2005; oh my god is that a circular saw that does not go in my mouth aaaaagh

Even if it did work, wouldn't you end up indirectly specializing base::x instead of making a specialized version just for derived::x? Also, I'm pretty sure explicit specializations aren't allowed inside a class.

The following is probably closer to what you want (compiles under GCC, Comeau, and VC++2005):

code:

struct base
{
	template <typename T>
	struct x;
};

template <>
struct base::x<float>
{
	typedef float some_type;
};

struct derived : base
{
	template<typename T>
	struct x : base::x<T>
	{};
};

template <>
struct derived::x<int>
{
	typedef int some_type;
};

int main()
{
	derived::x<int>::some_type i;
}

# ? Apr 14, 2008 20:43

The Red Baron: Jan 10, 2005

Avenging Dentist posted:

Even if it did work, wouldn't you end up indirectly specializing base::x instead of making a specialized version just for derived::x?

Quite possible. Hadn't thought about that, to be honest.

quote:

The following is probably closer to what you want (compiles under GCC, Comeau, and VC++2005):

That seems like a far better approach, I'll give it a try. Many thanks to you both!

# ? Apr 14, 2008 20:56

6174: Dec 4, 2004

I've got a C program that I've got to insert some sorting operations into. I've got several arrays, say a, b, and c. I need to sort a into non-decreasing order, however the order of b and c must change in the same manner as a does. This is easy enough to do by swapping the elements of b and c in the same manner as a when sorting a. The problem comes into when later I've got a similar situation with arrays x and y. I don't want to have to be coding up 4-5 slight variants of the same sorting algorithm (and the number of "dependent" arrays varies from 1 to 4). How can I reasonably make only one sorting algorithm deal with this situation? Would a function pointer to a swap routine be reasonable? If needed I can restructure things to make this easier.

# ? Apr 14, 2008 22:11

lhunc: Jan 4, 2007

Another C newb question, this time regarding the passing of structures to functions.

I've got a structure like this:

code:

struct note {
    time_t creation_date;
    int priority;
    int *note_text;
} notes[100];

and then a function which takes an array of note structures, which I'm sure is wrong...

code:

void get_notes(note *some_notes);

The compiler (gcc) is giving me an error (at the function declaration line):

code:

error: expected ')' before '*' token

Can anyone point me in the right direction?

# ? Apr 14, 2008 22:20

haveblue: Aug 15, 2005; Toilet Rascal

6174 posted:

Would a function pointer to a swap routine be reasonable? If needed I can restructure things to make this easier.

This would be quite reasonable. You'd have the indices as its parameters and just have it perform the same swap on any of an arbitrary set of arrays.

quote:

Can anyone point me in the right direction?

You need the struct keyword before the type of the parameter (or a typedef statement for the structure definition).

haveblue fucked around with this message at 22:24 on Apr 14, 2008

# ? Apr 14, 2008 22:22

6174: Dec 4, 2004

HB posted:

This would be quite reasonable. You'd have the indices as its parameters and just have it perform the same swap on any of an arbitrary set of arrays.

That is what I figured, I just wanted to make sure I wasn't missing something stupid.

# ? Apr 14, 2008 22:24

Avenging Dentist: Oct 1, 2005; oh my god is that a circular saw that does not go in my mouth aaaaagh

lhunc posted:

Another C newb question, this time regarding the passing of structures to functions.

..

Can anyone point me in the right direction?

If you're doing pure C, you need either struct note in place of note, or the following:

code:

typedef struct { /* ... */ } note;
note notes[1000];

# ? Apr 14, 2008 22:25

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

lhunc posted:

Another C newb question, this time regarding the passing of structures to functions.

I've got a structure like this:
code:
struct note {
    time_t creation_date;
    int priority;
    int *note_text;
} notes[100];

As a general principle, you're probably better off splitting this into a declaration of a struct type, and a declaration of an array of that type.

code:

struct note { ... };
struct note notes[100];

It's just less confusing that way.

quote:

and then a function which takes an array of note structures, which I'm sure is wrong...
code:
void get_notes(note *some_notes);

This is one of the areas where C and C++ differ. In C, it's

code:

void get_notes(struct note *some_notes);

You use struct note { ... }; to declare a structure, and just struct note to use it.

Since this is really annoying, generally you take that structure and typedef it to an easier name.

code:

struct note_struct { ... };
typedef note_struct note;
note notes[100];

void get_notes(note *some_notes);

Since the "struct and then typedef" idiom is so common, people usually combine it into one block, like you did with the array declaration - which is why you shouldn't do that with array declarations, because at a quick glance people will assume it's a typedef. (I was mighty confused for about 10 seconds why you were naming your note type "notes[100]".)

code:

typedef struct note_struct { ... } note;
note notes[100];

void get_notes(note *some_notes);

EDIT: oh yeah, you can just cut the word note_struct completely, create an anonymous struct, and then typedef it to the name "note". Been so long since I did C that I forgot that.

code:

typedef struct { ... } note;
note notes[100];

void get_notes(note *some_notes);

# ? Apr 14, 2008 22:31

lhunc: Jan 4, 2007

Avenging Dentist posted:

If you're doing pure C, you need either struct note in place of note, or the following:
code:
typedef struct { /* ... */ } note;
note notes[1000];

Thanks, I've modified it to read:

code:

typedef struct {
   /*...*/
} note;

note notes[100];

However, I'm still getting an error at the function declaration line [void get_notes(note *notes)];

code:

error: expected �)� before �*� token

Any ideas?

# ? Apr 14, 2008 22:34

Avenging Dentist: Oct 1, 2005; oh my god is that a circular saw that does not go in my mouth aaaaagh

Could you just post your code on https://www.pastebin.com or something?

# ? Apr 14, 2008 22:38

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

Did you declare that struct in a header file? Are you sure you're not including an older version of the header or something?

# ? Apr 14, 2008 22:40

lhunc: Jan 4, 2007

edit: thanks guys, I got it - I had the structures defined after the function prototypes, switching them did the trick.

lhunc fucked around with this message at 02:22 on Apr 15, 2008

# ? Apr 15, 2008 02:19

almostkorean: Jul 9, 2001; eeeeeeeee

Can someone tell me why this doesn't work?

code:

 
  char operand;
  double *number;
  *number = atof( operand );

When I compile I get:

code:

calc.cpp:31: error: invalid conversion from �char� to �const char*�
calc.cpp:31: error:   initializing argument 1 of �double atof(const char*)�
calc.cpp:31: error: cannot convert �double� to �double*� in assignment

I feel like this should work, but I can't figure it out.

# ? Apr 15, 2008 21:31

haveblue: Aug 15, 2005; Toilet Rascal

almostkorean posted:

Can someone tell me why this doesn't work?

code:

 
  char operand;
  double *number;
  *number = atof( operand );

When I compile I get:

code:

calc.cpp:31: error: invalid conversion from �char� to �const char*�
calc.cpp:31: error:   initializing argument 1 of �double atof(const char*)�
calc.cpp:31: error: cannot convert �double� to �double*� in assignment

I feel like this should work, but I can't figure it out.

Well, first, atof expects a string, not a single char (and it expects a null terminator, so don't just pass it &operand).

Second, once you get it to run, it's going to crash because you didn't allocate any memory for number to point to.

# ? Apr 15, 2008 21:35

Lexical Unit: Sep 16, 2003

atof takes a const char*, you're giving it a char.

Beaten of course, I meant to hit preview and not post.

# ? Apr 15, 2008 21:35

Avenging Dentist: Oct 1, 2005; oh my god is that a circular saw that does not go in my mouth aaaaagh

operand is a single character and should be a char*. I'm not sure why you're using atof to parse a single character though...

Also, you aren't allocating any memory for *number.

EDIT: haha

# ? Apr 15, 2008 21:35

Zombywuf: Mar 29, 2008

almostkorean posted:

Can someone tell me why this doesn't work?
code:
 
  char operand;
  double *number;
  *number = atof( operand );

operand is a char, atof takes a char *. You want something more like char operand[] = "123";.

# ? Apr 15, 2008 21:37

Steampunk Mario: Aug 12, 2004; DIAGNOSIS ACQUIRED

Also, operand is uninitialized and therefore this snippet won't do anything useful (and may possibly have undefined behavior).

# ? Apr 15, 2008 21:38

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# ? Apr 18, 2024 02:02

Lexical Unit: Sep 16, 2003

Hopefully that's just a dumbed down snippet of code and the real code allocates memory to number and populates operand... at least, that's what I assumed.

# ? Apr 15, 2008 21:40

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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