Free up malloced char * in a vector?

Question

We are given:

typedef const char* V;
typedef std::vector<V> Record;

We instantiate the record somewhere in code like:

Record rec;

We populate it with data via malloc:

void fixed_len_read(void *buf, int size, Record *record){
    char *c_buf = (char *)malloc(size);
    memcpy(c_buf, buf, size);
    record->push_back(c_buf);
}

Now after usage, I wish to free the memory allocated. Would it go away on it's own? Or would I have to free() every element? I tried to call free like this:

for (int i =0; i < 100; i++) {
   free(record.at(i));
}

But it complains about const char.

    " error: no matching function for call to 'free'
    free(record.at(i));"

I then used a cast:

  for (int i =0; i < 100; i++) {
      free((char *) record.at(i));
  }

Now it seems to run, but I'm not sure if it works? Does that look right?

[EDIT]
As per comments and answer below. Casting it is ok. An alternative is to use delete() as per: Unable to free const pointers in C (C's free has an issue, which was fixed in delete() in c++).

C and C++ are different languages. This is C++. Whatever you `malloc`, has to be `free`'d. It won't go away on its own. Is there a reason you cannot just use `std::vector`? Then you don't need to worry about any of this. — Neil Kirk, Nov 10 '15 at 03:09
Code was given by our professor. It is odd that he mixes C and C++ like this. — Leo Ufimtsev, Nov 10 '15 at 03:09
Very odd. Is there a special reason he wants you to learn how to write code that is error prone and difficult to maintain? — Neil Kirk, Nov 10 '15 at 03:10
I'm asking myself the same question :-|, it's a horrible assignment. I've asked the prof if we can use strings instead. — Leo Ufimtsev, Nov 10 '15 at 03:12
It's not just error prone and difficult to maintain, it's complete nonsense. I wouldn't take too much from this professor. — Collin, Nov 10 '15 at 03:12
In any case *"it complains about const char"* - you need to share the error message. — Neil Kirk, Nov 10 '15 at 03:13
Mmm... maybe we need AssignmentReview.SE on [Area 51](http://area51.stackexchange.com)? — 3442, Nov 10 '15 at 03:19
Hm it does look like `free` cannot delete `const char *` without casting, which I didn't know about nor do I understand why. You'd be better off asking the question with just the sample code such as: http://ideone.com/wk6xfs — Neil Kirk, Nov 10 '15 at 03:21
http://stackoverflow.com/questions/33622382/why-cant-free-work-with-const-pointers — Neil Kirk, Nov 10 '15 at 03:29

score 1 · Accepted Answer · answered Nov 10 '15 at 03:20

What you're doing (I'm pretty sure) is technically allowed. However, it really makes no sense to malloc a const char*, as you wouldn't be able to initialize the memory after allocation without a cast.

free takes a void*, which is why the compiler complains when you pass it a const char*. const pointers are not implicitly convertible to void*. When you cast away the constness using (char*), you make it convertible, and free can take the variable.

To answer your question, yes, you do need to free each element before the vector is destroyed. It won't happen on it's own.

Truth be told, the code your professor is giving you is complete nonsense. If you're talking about binary data, perhaps you should be using std::vector<std::vector<std::uint8_t>> rather than vector<const char*>. If you're really holding strings, std::vector<std::string> makes more sense. This way, the memory is automatically free'd for you, because the container's destructors take care of it.

Thank you for your answer. The only thing, we don't malloc a const char. We malloc a char *? — Leo Ufimtsev, Nov 10 '15 at 03:37
Right, you malloc a `char*`, but then almost immediately assign it `const char*` by pushing it into the vector. That's fine, but you need to cast it back before you can free it. — Collin, Nov 10 '15 at 03:40

Free up malloced char * in a vector?

1 Answers1