const

[Massimiliano Alberti]

Sometimes I've seen something like:

const char* ReturnNull()
{
return "";
}

Is this legal? Why? Is this because the string is a constant?

And... Is this legal:

const int* ReturnZero()
{
const int i = 0;
return &i;
}

 

[Massimiliano Alberti]

const char* ReturnNull()

{
return "";
}

Wrong example:

const char* ReturnNull()
{
const char *p = "";
return p;
}

 

[Ron Natalie]

Sometimes I've seen something like:

const char* ReturnNull()
{
return "";
}

This is legal because a string literal is persistant. It's a bunch of const chars
at an undetermined location. They are not local to the block.

const int* ReturnZero()
{
const int i = 0;
return &i;
}

This will compile, but any attempt to use the returned pointer will be problematic
as in this case the value is a pointer to an object that doesn't exist anymore.

 

[Rolf Magnus]

Sometimes I've seen something like:

const char* ReturnNull()
{
return "";
}

Is this legal?
Yes.

Why?

What makes you think it's not?

Is this because the string is a constant?

No, it's because it's a literal, and string literals exist through the
whole life time of the program.

And... Is this legal:

const int* ReturnZero()
{
const int i = 0;
return &i;
}

No. You're returning a pointer to a local variable that gets destroyed
when the function returns.

 

[Leor Zolman]

This will compile, but any attempt to use the returned pointer will be problematic
as in this case the value is a pointer to an object that doesn't exist anymore.

It will compile, but every self-respecting compiler will at least issue a
warning about it. If you don't see a warning, consider getting a newer
compiler (or tightening up the warning level options).
-leor


Leor Zolman
BD Software
(e-mail address removed)
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

 

[Nick Hounsome]

Wrong example:

const char* ReturnNull()
{
const char *p = "";
return p;
}

No - his example is fine - yours just wastes a bit of stack space to acheive
exactly
the same thing.

Actually according to the principle of calling it what it is it should be

const char* ReturnNull() { return NULL; }
or
const char* ReturnEmptyString() { return ""; }

 

[Rolf Magnus]

No - his example is fine - yours just wastes a bit of stack space to
acheive exactly the same thing.

The "his" and the "yours" you are talking about are the same person.

 

[Roger Leigh]

Sometimes I've seen something like:

const char* ReturnNull()
{
return "";
}

Is this legal? Why? Is this because the string is a constant?

Yes.

[OT: On a UNIX-like OS (e.g. Linux/ELF), this will probably be put in
the .rodata (read-only data) section of the executable. This part is
mapped into memory with read-only permissions, so if you actually
attempt to modify it, you'll get a SEGV.]

And... Is this legal:

const int* ReturnZero()
{
const int i = 0;
return &i;
}

It's "legal" in that it will compile. However, it's wrong because i
will be destroyed when it goes out of scope when the function returns.
This will leave you with a pointer to a nonexistent object, and
probably a crash.