std::reverse_copy() woes

[Eric Lilja]

Hello, consider the following program:

#include <algorithm>
#include <cstring>
#include <iostream>
#include <iterator>
#include <string>

int main()
{
const char *foo = "hello";
std::string s;

/* Prints olleh */
std::reverse_copy(foo, foo + std::strlen(foo),
std:stream_iterator<char>(std::cout));

std::reverse_copy(foo, foo + std::strlen(foo), s.begin());

/* Prints nothing, why? */
std::cout << std::endl << s << std::endl;
}

The second call to reverse_copy doesn't work as expected because when I
print s I get nothing. Using a debugger, I inspect s just before and
right after the call to reverse_copy:
(gdb) p s
$1 = {static npos = 4294967295, _M_dataplus = {<allocator<char>> =
{<new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p =
0x44212c ""}}
(gdb) n
(gdb) p s
$2 = {static npos = 4294967295, _M_dataplus = {<allocator<char>> =
{<new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p =
0x71254c "olleh"}}
(gdb)

So it seems to contain the reversed string, but it won't output?!
What's going on?

- Eric

 

[Ondra Holub]

Eric Lilja napsal:

Hello, consider the following program:

#include <algorithm>
#include <cstring>
#include <iostream>
#include <iterator>
#include <string>

int main()
{
const char *foo = "hello";
std::string s;

/* Prints olleh */
std::reverse_copy(foo, foo + std::strlen(foo),
std:stream_iterator<char>(std::cout));

std::reverse_copy(foo, foo + std::strlen(foo), s.begin());

/* Prints nothing, why? */
std::cout << std::endl << s << std::endl;
}

The second call to reverse_copy doesn't work as expected because when I
print s I get nothing. Using a debugger, I inspect s just before and
right after the call to reverse_copy:
(gdb) p s
$1 = {static npos = 4294967295, _M_dataplus = {<allocator<char>> =
{<new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p =
0x44212c ""}}
(gdb) n
(gdb) p s
$2 = {static npos = 4294967295, _M_dataplus = {<allocator<char>> =
{<new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p =
0x71254c "olleh"}}
(gdb)

So it seems to contain the reversed string, but it won't output?!
What's going on?

- Eric

Copy it to back_inserter iterator:

std::reverse_copy(foo, foo + std::strlen(foo),
std::back_inserter(s));

 

[Ondra Holub]

Eric Lilja napsal:

Hello, consider the following program:

#include <algorithm>
#include <cstring>
#include <iostream>
#include <iterator>
#include <string>

int main()
{
const char *foo = "hello";
std::string s;

/* Prints olleh */
std::reverse_copy(foo, foo + std::strlen(foo),
std:stream_iterator<char>(std::cout));

std::reverse_copy(foo, foo + std::strlen(foo), s.begin());

/* Prints nothing, why? */
std::cout << std::endl << s << std::endl;
}

The second call to reverse_copy doesn't work as expected because when I
print s I get nothing. Using a debugger, I inspect s just before and
right after the call to reverse_copy:
(gdb) p s
$1 = {static npos = 4294967295, _M_dataplus = {<allocator<char>> =
{<new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p =
0x44212c ""}}
(gdb) n
(gdb) p s
$2 = {static npos = 4294967295, _M_dataplus = {<allocator<char>> =
{<new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p =
0x71254c "olleh"}}
(gdb)

So it seems to contain the reversed string, but it won't output?!
What's going on?

- Eric

Writing to s.begin() is wrong - it overwrites items in s starting with
first item. But s is currently empty, so it may write boyond the
allocated area.