C / C++ / MFC

Re: deleting from a vector

24-Dec-23 20:37

I wrote some C++ code to create a vector and then try to delete all occurrences of 3 in it:

C++

vector<int> myVector = { 1, 2, 3, 4, 5, 3, 2 };

	for (auto i = myVector.begin(); i != myVector.end(); i++)
	{
		if (*i == 3) myVector.erase(i);
	}

However, it doesn't work right. It gives this error:

Expression: can't increment invalidated vector iterator

I guess you can't increment the iterator after you erase the element.

Anyone know how to fix this?

Thanks.

Richard MacCutchan24-Dec-23 21:39

Re: deleting from a vector

24-Dec-23 21:39

The problem comes because as soon as you remove an element then all the rest effectively move nearer the beginning, and thus their positions are out of sync with the iterator. You could try a reverse iterator as detailed at std::vector - cppreference.com[^].

Mircea Neacsu25-Dec-23 2:19

25-Dec-23 2:19

As you guessed, erase invalidates all iterators on the container. Your problem is known as the Erase–remove idiom[^] There are different ways of solving it:
1. The "manual" way:

C++

auto pos = myVector.begin();
size_t i=0;
while (pos != myVector.end())
{
  if (*pos ==3)
  {
    myVector.erase(pos);
    pos = myVector.begin() + i;
  }
  else
  {
    ++i;
    ++pos;
  }
}

2. The "semi-automatic" way:

C++

auto pos = myVector.begin();
while (pos != myVector.end())
{
  if (*pos ==3)
    pos = myVector.erase(pos); //erase returns an iterator pointing to next element
  else
    ++pos;
}

3. The "old fully automatic way":

C++

myVector.erase(std::remove(myVector.begin(), 
                              myVector.end(), 3),
               myVector.end());

4. The "new fully automatic way" (since C++20):

C++

std::erase (myVector, 3);

Disclaimer: I didn't compile any of the code above; some errors may/will exist Smile | :)

Mircea

std::async

utcode23-Dec-23 16:31

utcode

23-Dec-23 16:31

Hi,
Is it OK to call std::async(BYTE *p) from inside a worker thread?.
How would it be possible to make sure async calls gets executed in order of first came , first executed just like calling a function synchronously?

Re: std::async

Richard MacCutchan23-Dec-23 21:32

design question : variadic functions use ? .

23-Dec-23 21:32

If you want things run in order then do not use async. See the example at std::async - cppreference.com[^].

Maximilien19-Dec-23 2:34

19-Dec-23 2:34

(probably a TOmato vs toMAto kind of questions... )

At what point should I use variadic functions vs. keeping it local ?

I have a library that handles SQL queries.

is there a difference between something like :

(overly simplified)

C++

// do the formatting before calling the function.
std::string fullyFormedQuery = sprintf (queryFormat, param1, param2, ... ); 
void DoRequest( std::string fullyFormedQuery){ 
  // call SQL with the fullyFormedQuery
} 

// do the formatting in the function.
void DoRequest( std::string queryFormat, param1, param2, ... ){ 
  std::string fullyFormedQuery= queryFormat(format, param1, param2, ... ); 
  // call SQL ... 
}

CI/CD = Continuous Impediment/Continuous Despair

Mircea Neacsu19-Dec-23 2:42

19-Dec-23 2:42

As you say it's more of a style question. My preference would be for the first option (pass fully formed string) because:
- there is less documenting to do
- some compilers (MSVC for one), check matching between format strings and parameters in (s)printf functions but don't do that in a user defined variadic function.

Mircea

Maximilien19-Dec-23 2:54

19-Dec-23 2:54

thanks.

CI/CD = Continuous Impediment/Continuous Despair

jschell19-Dec-23 5:08

jschell

19-Dec-23 5:08

Just noting that your examples leave out using bind variables.

My understanding, and I just googled to see if that seems to be correct, is that bind variables make optimizations in the database easier.

Not to mention of course that they are a better fit for avoiding SQL injection problems.

Setting up bind variables is rather complex so putting it in the method is going to be better.

Maximilien19-Dec-23 7:36

19-Dec-23 7:36

Thanks.

We already have a complete (decades old) framework for SQL. (seems to be working fine)

maybe I should have left out the SQL reference and make it a more generic question.

CI/CD = Continuous Impediment/Continuous Despair

jschell20-Dec-23 5:31

jschell

20-Dec-23 5:31

Maximilien wrote:
complete (decades old) framework for SQL.

Then you should use the same idiom as the existing code. Changing it for some and not others just adds to the confusion. Although if a preference for one is given and there is a large amount of new work (new files not modifying existing code) then maybe go with there.

modified 20-Dec-23 12:00pm.

Maximilien20-Dec-23 6:55

20-Dec-23 6:55

I will not change existing code; it's just too dangerous.

I'm just curious; if there's a best practice.

CI/CD = Continuous Impediment/Continuous Despair

jschell21-Dec-23 3:31

jschell

21-Dec-23 3:31

My best practice doesn't fit either of your suggestions...

If I am creating a new database layer (which I have numerous times) then the caller would never provide the SQL.

If the business layer(s) need new database logic then the database layer should be modified to provide it.

The caller provides the parameters that would be used in the call.

Then within that database layer method it would decide how to process the parameters and SQL (provided by the method.)

So it looks like the following.

void DatabaseDoX(param1, param2,...)
{
// multiple variations possible
std::string fullyFormedQuery = sprintf (SQL_X, param1, param2, ... );

There are quite a few variations on the above. For example consider the complication of a user initiated call where they might specify different 'query' terms (like data range, customer name, etc)

But all still hide the SQL.

Certainly there are design considerations for example your code might be coming from within the database layer (not a business call at all.) If so then I suspect that usage would actually vary depending on what the database layer was specifically doing in the code path.

strcpy_s weirdness

mike741119-Dec-23 1:31

19-Dec-23 1:31

I was playing around in Microsoft Visual C++, and I noticed something weird about this code:

C++

void setname(char name[80])
{
    strcpy_s(name, "joebob");

    char name2[80];
    strcpy_s(name2, "joebob");
}

It looks like the compiler complains about the first strcpy_s but not the second one. If I comment out the first one, the program compiles fine.

Can anyone explain this?

BTW, I know strcpy_s should probably take 3 arguments, but I'm trying to understand this weirdness.

Thanks.

Re: strcpy_s weirdness

Mircea Neacsu19-Dec-23 1:43

19-Dec-23 1:43

Remember that in C arrays and pointers are the same thing so the argument name is a pointer to char. For the compiler, your code fragment is the same as:

C++

void setname(char *name)
{
    strcpy_s(name, "joebob");

    char name2[80];
    strcpy_s(name2, "joebob");
}

In the case of the second call to strcpy_s, the compiler can figure out the size parameter through some template magic. For the first one however you have to use the explicit size form.

Mircea

Re: strcpy_s weirdness

Richard MacCutchan19-Dec-23 2:04

19-Dec-23 2:04

It would help when posting questions that you include the exact error message(s), so it makes it clear what your problem is.

Re: strcpy_s weirdness

CPallini19-Dec-23 2:09

CPallini

19-Dec-23 2:09

See, for instance
Array Decay In C - GeeksforGeeks[^].

"In testa che avete, Signor di Ceprano?"
-- Rigoletto

Should this work in C++?

mike741118-Dec-23 7:07

Re: Should this work in C++?

18-Dec-23 7:07

Should this work in C++?

C++

long add1(a)
 long a;
 {
 return a+1;
 }

It doesn't seem to compile in Visual Studio. I got it from MIT's OpenCourseware.

Thanks.

Mircea Neacsu18-Dec-23 7:17

Re: Should this work in C++?

18-Dec-23 7:17

That is the so called K&R syntax[^]. You can compile as C code but not as C++. The C++ version would be:

C++

long add1 (long a) {
  return a+1;
}

Mircea

Maximilien19-Dec-23 5:11

19-Dec-23 5:11

mike7411 wrote:
I got it from MIT's OpenCourseware.

I see the MIT is still at the cutting edge of technology !! Roll eyes | :rolleyes:

CI/CD = Continuous Impediment/Continuous Despair

C++ memory freeing

mike741116-Dec-23 3:04

16-Dec-23 3:04

I am a little confused about when to use delete and delete[].

Is this correct?

C++

int* numArray = new int[100];
delete numArray;

Or should it be this?

C++

int* numArray = new int[100];
delete[] numArray;

I saw the former in an MIT lecture, and it looked suspicious. Both of them compile.

Thanks.

Richard MacCutchan16-Dec-23 3:21

16-Dec-23 3:21

If you create an array with operator new as above, then you must tell delete that it is an array. So example 2 is the correct way to do it.

Mircea Neacsu16-Dec-23 3:21

16-Dec-23 3:21

Array delete operator, delete[] invokes the destructor of each array element before freeing the array. For an array of integers, it doesn't make any difference. Meanwhile, if the objects in the array have a non-trivial destructor you should call the delete[] operator.

Mircea

Daniel Pfeffer21-Dec-23 9:41

Daniel Pfeffer

21-Dec-23 9:41

Mircea Neacsu wrote:
For an array of integers, it doesn't make any difference.

It is true that no destructor is called for each element of an int[] array, but that does not mean that the memory layout is compatible with delete. For example, the compiler might allocate additional space for a variable containing the size of the array and return a pointer to memory after this variable. When using delete, the heap will be corrupted.

The heap layout is implementation-dependent, and therefore you should always match new/delete and new[]/delete[]!

Freedom is the freedom to say that two plus two make four. If that is granted, all else follows.
-- 6079 Smith W.

Mircea Neacsu21-Dec-23 15:26