C++Talk.NET

Erik · Guest

Hello,

I was a little bit surprised when I tested the string members resize
and reserve.

I thought that:
- reserve only change the capacity, if the paramater is higher or equal
than the actual capacity. It never should reduce the capacity.
- reserve increase the capacity exactly to the given size.
- resize changes the capacity and the size (I read that in some C++
Books, but I think I should throw them away)

but the results delivers:
- resize do not change the capacity of a string. Thus you cannot reduce
the memory
already allocated by a string.
- reserve is able to decrease the capacity of a string (borland, VC++)
- reserve allocates the required memory or even more
- resize do not change the capacity of a string (only the size)
- the initial capacity of a string depends on the used compiler

Is each compiler comform to the C++ Standard?
Is it possible to reduce the capacity of a string?

//-------------------------------------------------------------------------------
#include <iostream>
#include <string>
#include <iomanip>
using namespace std;

void print_header()
{
cout << "+------+----------+" << endl
<< "| " << "size" << " | " << "capacity" << " |"
<< endl;
}

void print_data(string & s)
{
cout << "|------+----------|" << endl
<< '|'
<< setw(5) << s.length() << " |"
<< setw(9) << s.capacity() << " |"
<< endl;
}

int main()
{
print_header();
string s1;
print_data(s1);
s1.reserve(20);
print_data(s1);
s1.resize(5);
print_data(s1);
s1.reserve(10);
print_data(s1);
s1.reserve(1000);
print_data(s1);
s1.resize(1);
print_data(s1);
}

Let's see the output different compilers produced.

//--output gcc-----------------
+------+----------+

Bo Persson · Guest

"Erik" <erik.griessmann (AT) fmc-ag (DOT) com> skrev i meddelandet
news:1123050764.832100.325480 (AT) f14g2000cwb (DOT) googlegroups.com...

James Daughtry · Guest

Alf P. Steinbach · Guest

* Bo Persson:

Erik · Guest

Hi James,
Thanks for your competent explanations. I think I have now a better
understanding for this subject.

kanze@gabi-soft.fr · Guest

Bo Persson wrote:

Erik · Guest

I think there is a little bug in your code.
Please take a look at my toy code I compiled using gcc.
It seemed that the operator= and the copy constructor is allowed to
overtake the capacity of its string parameter. When I used the
constructor using iterators the swap trick is working.
//----------------------------------------------------
#include <iostream>
#include <string>
using namespace std;

string & reduce_capacity(string & str)
{
string reducer(str.begin(),str.end());
str.swap(reducer);
return str;
}

string & no_reduce_of_capacity(string & str)
{
//copy constructor do not match requirement
//to reduce capacity
string reducer(str);
str.swap(reducer);
return str;
}

void out(string & s)
{
cout << "value : " << s << endl
<< "lenght: " << s.length() << endl
<< "capacity: " << s.capacity() << endl;
}

int main()
{
string s("I wanna grow");
out(s);
s.reserve(1000);
out(s);
out(no_reduce_of_capacity(s));
out(reduce_capacity(s));
}
//----------------------------------------------------

// output -----------------------------------------
value : I wanna grow
lenght: 12
capacity: 12
value : I wanna grow
lenght: 12
capacity: 1123
value : I wanna grow
lenght: 12
capacity: 1123
value : I wanna grow
lenght: 12
capacity: 12

[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

kanze@gabi-soft.fr · Guest

Alf P. Steinbach wrote:

Pete Becker · Guest

[email]kanze (AT) gabi-soft (DOT) fr[/email] wrote:

Alf P. Steinbach · Guest

* [email]kanze (AT) gabi-soft (DOT) fr[/email]: