segmentation fault...

[Fei Liu]

What's wrong with the following code? It seems I can't assign to a
vector<stringelement...

#include <vector>
#include <iostream>
#include <iterator>
#include <functional>
#include <string>

using namespace std;

struct c{
vector<stringa;
void set_size(int n) { a.reserve(n); }
void set_val(int i, string v) { a[i] = v; }
};

void foo(c & cc){
cc.set_size(10);
cc.set_val(3, "value 3");
}

int main(){

vector<inta;
a.reserve(10);
a[3] = 10;
copy(a.begin(), a.end(), ostream_iterator<int>(cout, " "));
cout << "a[3]: " << a[3] << endl;
{
vector<stringa;
a.reserve(10);
a[3] = "value 3";
copy(a.begin(), a.end(), ostream_iterator<string>(cout, " "));
cout << "a[3]: " << a[3] << endl;
}

c cc;
foo(cc);
}

[Jerry Coffin]

In article <f8**********@aioe.org>, fe****@aepnetworks.com says...

[ ... ]

int main(){

vector<inta;
a.reserve(10);
a[3] = 10;

This gives undefined behavior. a.reserve(10) only _reserves_ space for
10 elements, but does NOT actually add those 10 elements to the vector.
This ensures that you can add 10 elements to 'a' without a reallocation
taking place, so (for example) all iterators into 'a' remain valid
until/unless you add an eleventh element. It does NOT, however, make it
valid to index into those 10 elements until you add them (e.g. with
push_back).

If you change the 'reserve' to 'resize', this part of the code will
become valid -- resize actually adds elements to the vector, so you can
then index into the vector and use those elements.

[ ... ]

vector<stringa;
a.reserve(10);
a[3] = "value 3";

This has the same problem, which can be fixed in the same way (i.e.
change 'reserve' to 'resize').

--
Later,
Jerry.

The universe is a figment of its own imagination.

[Victor Bazarov]

Fei Liu wrote:

What's wrong with the following code? It seems I can't assign to a
vector<stringelement...
[..]

RTFM about the differences between 'reserve' and 'resize'.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Robert Bauck Hamar]

Fei Liu wrote:

What's wrong with the following code? It seems I can't assign to a
vector<stringelement...

You have no elements to assign to. I believe you don't understand what
reserve() actually does, it does not create any elements, it only reserves
space for them. Try using a.at(i) instead of a[i].

What you really want is the resize() member function of vector.

#include <vector>
#include <iostream>
#include <iterator>
#include <functional>
#include <string>

#include <algorithm>

>
using namespace std;

struct c{
vector<stringa;
void set_size(int n) { a.reserve(n); }
void set_val(int i, string v) { a[i] = v; }
};

void foo(c & cc){
cc.set_size(10);
cc.set_val(3, "value 3");
}

int main(){

vector<inta;
a.reserve(10);

a.size() == 0

a[3] = 10;

Undefined behaviour. Use
a.at(3) = 10;

copy(a.begin(), a.end(), ostream_iterator<int>(cout, " "));

OK. a.begin() == a.end()

cout << "a[3]: " << a[3] << endl;

UB.

{
vector<stringa;
a.reserve(10);

a.size() == 0

a[3] = "value 3";

Undefined behaviour. While the previous errors are also undefined behaviour,
i guess this is what actually _caused_ the crash? A likely reason is that
a[3] returns some raw memory, on which no string has been constructed.

copy(a.begin(), a.end(), ostream_iterator<string>(cout, " "));

again, a.begin() == a.end(), so this is a no-op.

cout << "a[3]: " << a[3] << endl;

But this is UB again.

}

c cc;
foo(cc);

And UB.

}

--
rbh

[Fei Liu]

Jerry Coffin wrote:

In article <f8**********@aioe.org>, fe****@aepnetworks.com says...

[ ... ]

>int main(){

vector<inta;
a.reserve(10);
a[3] = 10;

This gives undefined behavior. a.reserve(10) only _reserves_ space for
10 elements, but does NOT actually add those 10 elements to the vector.
This ensures that you can add 10 elements to 'a' without a reallocation
taking place, so (for example) all iterators into 'a' remain valid
until/unless you add an eleventh element. It does NOT, however, make it
valid to index into those 10 elements until you add them (e.g. with
push_back).

If you change the 'reserve' to 'resize', this part of the code will
become valid -- resize actually adds elements to the vector, so you can
then index into the vector and use those elements.

[ ... ]

> vector<stringa;
a.reserve(10);
a[3] = "value 3";

This has the same problem, which can be fixed in the same way (i.e.
change 'reserve' to 'resize').

Thank you!