C++ pointer to objects which go out of scope when function returns - why does this work?

Question

Both these calls to get_string and get_string2 return objects which go out of scope when the function returns. Shouldn't the returned object be an address in memory which goes out of scope after the function returns? This is using Visual Studio 2008. Should this always work? Why?

#include <iostream>

enum myID { SMALL, MEDIUM, LARGE };

const char* get_string(myID id) {
   switch(id){
      case SMALL: return "small";
      case MEDIUM: return "medium";
      case LARGE: return "large";
      default: return "unknown";
   }
}

const char* get_string2(myID id) {
   char* s = 0;
   switch(id){
      case SMALL:  s = "small"; return s;
      case MEDIUM: s = "medium"; return s;
      case LARGE:  s = "large"; return s;
      default: return "unknown";
   }
}

int main() {
   std::cout << get_string(SMALL) << std::endl;
   std::cout << get_string2(SMALL) << std::endl;
   return 0;
}

possible duplicate of returning a pointer to a literal (or constant) character array (string)? — Mark B
– Mark B, Commented May 16, 2013 at 15:44

Armen Tsirunyan · Accepted Answer · 2013-05-16 15:48:55Z

12

String literals are the only literals that are lvalues and have static storage duration. So returning a pointer to the first element of a string literal is very safe. Compare

const char* f1()
{
      return "Hello"; // OK
} 

const char* f2()
{
      const char s[6] = "Hello";
      return s; // Undefined Behavior
} 


const char* f3()
{
      const char* s = "Hello";
      return s; //OK
}

Judging from comments, this answer needs some elaboration. OK, see, a string literal is stored in the memory until the program has finished. In case 1 you return a pointer to it. The object pointer to is alive, so it's fine. In case 3, you take the address of the literal into a local variable s and return the latter by copy. The copy of the address still points to a valid memory, so it's fine. In the second case, however, we copy the string literal into a local array s which is destroyed upon function exit. Returning the address of a local variable is undefined behavior.

Further, if what you thought was undefined behavior were actually undefined then you shouldn't be surprised either because undefined behavior can mean

doesn't work
works sometimes
always works
does funny inexplicable stuff
anything

So if something works it doesn't mean it's OK. In this case it is, but not in the general case.

edited May 16, 2013 at 15:48

answered May 16, 2013 at 15:33

Armen Tsirunyan

134k61 gold badges336 silver badges442 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Angus Comber Over a year ago

So because it is a string literal it is not stored as a stack variable? It is stored in a global variable? Do you have a web link with details of this?

mr5 Over a year ago

would you mind to explain a bit further on the undefined behavior case? I just like to know

James Kanze Over a year ago

Just a nit (because the effect is the same), but in f2, you don't copy anything (officially, at least). It's exactly as if you'd written char const s[6] = { 'H', 'e', 'l', 'l', '0', '\0' };.

Armen Tsirunyan Over a year ago

@JamesKanze: You are of course right, since it is initialization, but I think it is easier to understand it this way. At least in this question.

mr5 Over a year ago

now I think I need to go back again study the difference of an array and a pointer

|

nullptr · Accepted Answer · 2013-05-16 15:33:38Z

4

This memory does not go out of the scope. These are string literals, they exist all time while the program runs.

answered May 16, 2013 at 15:33

nullptr

11.1k1 gold badge27 silver badges19 bronze badges

Collectives™ on Stack Overflow

C++ pointer to objects which go out of scope when function returns - why does this work?

2 Answers 2

7 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

7 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related