returning a pointer to stack variable

Hi,
while I know that returning pointers to stack/local/automatic variables is wrong:

Code:

---

int *f()
{
  int i = 7;
  return &i;
}

---

I'm not sure how correct is this:

Code:

---

const char *f()
{
  string s("whatever");
  return s.c_str();
}

---

the "s" var should be destroyed after the function returns, right? But what happens to the memory pointed to by the pointer returned by c_str()? Do I have to free/delete the memory pointed to by the pointer returned by c_str()?

10x

It is the same problem as the first example: s has been destroyed, so what you get from s.c_str() no longer exists, at least conceptually. You have no control over the string that you access via c_str(), so trying to use free() or delete[] on that invalid pointer would be wrong too.

10x for your answer

then why the return type of c_str() is const char * ?
and what is the right way to take in such situations?
for example if I don't have control over the caller function

so if I dynamically create a string object (string *s = new string("hello")) and if I free the memory pointed to by the pointer returned by s->c_str() do I also have to free/delete "s"?

10x

Quote:

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Originally Posted by justAlamer

then why the return type of c_str() is const char * ?

---

Because you are not supposed to modify that string.

Quote:

---

Originally Posted by justAlamer

and what is the right way to take in such situations?
for example if I don't have control over the caller function

---

If you want to keep the return type as a pointer, you may have little choice but to write:

Code:

---

const char* f()
{
    string s("whatever");

    // ...

    char* ret = new char[s.size() + 1];
    std::strcpy(ret, s.c_str());
    return ret;
}

---

On the other hand, there is also the option of:

Code:

---

std::string f()
{
    std::string s("whatever");

    // ...

    return s;
}

---

or the use of an in/out parameter:

Code:

---

void f(std::string& s)
{
    s = "whatever";

    // ...
}

---

Quote:

---

Originally Posted by justAlamer

so if I dynamically create a string object (string *s = new string("hello")) and if I free the memory pointed to by the pointer returned by s->c_str() do I also have to free/delete "s"?

---

You are not allowed to directly free the memory pointed to by the pointer returned by s->c_str(). What you can do in such a case is to delete s, or a copy of s... but why bother with manual memory management when you don't have to?

Thank you very much! :)
I get it now.
I don't wanna bother with manual memory management I just want to understand how it works and what is the right way to do it.

thanks again

You're welcome :)

By the way, to emphasize how little control you have over the null terminated C-style string you can access by calling c_str(), consider:

Code:

---

std::string s = "hello world";
const char* p = s.c_str();
s.push_back('!');
// p is no longer valid

---

This does not just apply to push_back(), but to any non-const member function called on the corresponding std::string object.

That's wicked :)
btw c_str() does not allocate additional memory, right?

Quote:

---

Originally Posted by justAlamer

btw c_str() does not allocate additional memory, right?

---

It might; it depends on the implementation.

So do I have to care about freeing it?

Quote:

---

Originally Posted by justAlamer

So do I have to care about freeing it?

---

No, you don't, no more than you have to care about freeing the other memory in use by the std::string object that will be destroyed when it goes out of scope.

Thanks a lot!
You've been extremely helpful!

cheers! :)