returning a pointer to stack variable
Hi,
while I know that returning pointers to stack/local/automatic variables is wrong:
I'm not sure how correct is this:Code:int *f() { int i = 7; return &i; }
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()?Code:const char *f() { string s("whatever"); return s.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.
Look up a C++ Reference and learn How To Ask Questions The Smart WayOriginally Posted by Bjarne Stroustrup (2000-10-14)
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
Because you are not supposed to modify that string.
If you want to keep the return type as a pointer, you may have little choice but to write:
On the other hand, there is also the option of:Code:const char* f() { string s("whatever"); // ... char* ret = new char[s.size() + 1]; std::strcpy(ret, s.c_str()); return ret; }
or the use of an in/out parameter:Code:std::string f() { std::string s("whatever"); // ... return s; }
Code:void f(std::string& s) { s = "whatever"; // ... }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?
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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:
This does not just apply to push_back(), but to any non-const member function called on the corresponding std::string object.Code:std::string s = "hello world"; const char* p = s.c_str(); s.push_back('!'); // p is no longer valid
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That's wicked
btw c_str() does not allocate additional memory, right?
It might; it depends on the implementation.
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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.
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Thanks a lot!
You've been extremely helpful!
cheers!
