Questions on dynamic allocation

[gallows]

In this code:

void f()
{
char* s1 = new char;
s1 = "fred";

char* s2 = new char[4];
s2 = "fred";
}

What is the difference between s1 and s2?
When I should use new[]?
Why if I append "delete[] s1" I get: "*** glibc detected *** ./foo:
free(): invalid pointer: 0x0804874c ***" ?

I know that seems stupid, but it's not clear for me.
Thanks.

s.

 

[red floyd]

In this code:

void f()
{
char* s1 = new char; allocates a single character.
s1 = "fred";

memory leak. You've lost the pointer you allocated with new, s2 now
points to the first character of the string literal "fed".

char* s2 = new char[4];

you've allocated four characters.

s2 = "fred";

memory leak. You've lost the pointer you allocated with new, s2 now
points to the first character of the string literal "fred".

}

What is the difference between s1 and s2?
When I should use new[]?

You want to use new[] when you allocate multiple items.

Why if I append "delete[] s1" I get: "*** glibc detected *** ./foo:
free(): invalid pointer: 0x0804874c ***" ?

Because s1 is not allocated with new[], it isn't even allocated anymore,
it's pointing to a string literal.

I know that seems stupid, but it's not clear for me.
Thanks.

You're better off not using new and delete for this. Use std::string,
or std::vector.

 

[gallows]

-- snip --
Because s1 is not allocated with new[], it isn't even allocated anymore,
it's pointing to a string literal.

Oh, _now_ it's clear. Thanks.

You're better off not using new and delete for this. Use std::string,
or std::vector.

I will do, thanks.

 

[red floyd]

-- snip --
Because s1 is not allocated with new[], it isn't even allocated anymore,
it's pointing to a string literal.

Oh, _now_ it's clear. Thanks.

You're better off not using new and delete for this. Use std::string,
or std::vector.

I will do, thanks.

You're welcome. I hope my explanation was moderately clear -- I usually
start out with an idea of what I want to explain, but fail miserably.