Why returning a const char * from function is not a good idea?

Question

Here is another question about returning const char * from function.

But I have tried it out with the following codes, it looks like everything works fine.

#include <iostream>
#include <string>

using namespace std;

const char * tocstring(){
    string str = "abcd";
    return str.c_str();
}

int main(){
    const char * p = tocstring();
    cout << p << endl;
    return 0;
}

it will output abcd as what I want. Any idea about this?

I can't explain it as well as [this post](http://stackoverflow.com/questions/6441218/can-a-local-variables-memory-be-accessed-outside-its-scope/6445794#6445794) — Tas, Jul 15 '15 at 04:36

score 2 · Answer 1 · answered Jul 15 '15 at 04:34

2

it will output abcd as what I want. Any idea about this?

The function tocstring returns a pointer being held by the automatic variable str. The pointer is invalid after the function returns. Your program is subject to undefined behavior. Unfortunately, seemingly sane behavior is a form of undefined behavior.

answered Jul 15 '15 at 04:34

R Sahu

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I have put undefined behavior through stress tests and it is still sane, yet still undefined. – Russell Greene Jul 15 '15 at 04:44

score 0 · Answer 2 · answered Jul 15 '15 at 04:34

0

You were lucky. tocstring is returning a pointer to memory that has already been freed so this is undefined behavior.

answered Jul 15 '15 at 04:34

sfjac

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2

lucky or unlucky? I wish it failed first time than surprising me later. – Mohit Jain Jul 15 '15 at 04:58

score 0 · Answer 3 · answered Jul 15 '15 at 04:37

const char * tocstring(){                                                                         
    string str = "abcd";                                                                        
    return str.c_str();                                                                          
}

Here str is a local variable. Once it goes out of scope, you should not point it to anymore. It is an UB.

Why returning a const char * from function is not a good idea?

3 Answers3