std::list::reverse_iterator not working right.

[Jim Langston]

Expected output is:
Three
One
One

Actual output is:
Three
One
Two

If I comment out the block that displays the first One, the last line is
blank (in real program it causes memory fault)

#include <string>
#include <iostream>
#include <list>

std::list< std::string > SendHistory;
std::list< std::string >::reverse_iterator SHi;

int main ()
{

SendHistory.push_back( "One" );
SendHistory.push_back( "Two" );
SendHistory.push_back( "Three" );

SHi = SendHistory.rbegin();
std::cout << (*SHi) << std::endl;

// Comment out following three lines changes output
SHi = SendHistory.rend();
--SHi;
std::cout << (*SHi) << std::endl;

// This is the line that's causing problems. Why isn't it working?
SHi-- = SendHistory.rend();
std::cout << (*SHi) << std::endl;

std::string wait;
std::cin >> wait;
}

The line that is causing the problems is:
SHi-- = SendHistory.rend();

Why is that failing? It should be the same as
SHi = SendHistory.rend();
--SHi;

but it's not working the same way. Is this a fault in my compiler maybe?
Microsoft Visual C++ .net 2003

 

[Kai-Uwe Bux]

Jim Langston wrote:
[snip]

The line that is causing the problems is:
SHi-- = SendHistory.rend();

Why is that failing? It should be the same as
SHi = SendHistory.rend();
--SHi;

Where did you read that?


Best

Kai-Uwe Bux

 

[Jonathan Mcdougall]

The line that is causing the problems is:
SHi-- = SendHistory.rend();

You may think this means "assign the result of rend() so SHi and then
decrement". People seem to confuse the use of postfix operator--, for
example with for loops, although this is the first time I see something
like that.

SHi-- decrements the iterator, but you then lose that value because you
are assigning the result of SendHistory.rend() to it.

Why is that failing? It should be the same as
SHi = SendHistory.rend();
--SHi;

but it's not working the same way. Is this a fault in my compiler maybe?
Microsoft Visual C++ .net 2003

Do not be to quick to blame your compiler. It very improbable for a
well-known commercial compiler to barf on such trivial statements.


Jonathan

 

[Jim Langston]

You may think this means "assign the result of rend() so SHi and then
decrement". People seem to confuse the use of postfix operator--, for
example with for loops, although this is the first time I see something
like that.

SHi-- decrements the iterator, but you then lose that value because you
are assigning the result of SendHistory.rend() to it.

Oh man, you're totally right. This totally excaped me. I guess because I
often do things such as:

bool result = MyFunction( parm1, SH1++, parm2 );

that I just always thought of SH1 not getting incremented until after the
statement was complete, but this is not really true. Hmmm... I better make
sure I'm not doing this anywhere else in my program!

Maybe a search for "++ =" and "-- =" will let me know.

Thanks!

 

[Jonathan Mcdougall]

This is wrong, see below.

Oh man, you're totally right. This totally excaped me. I guess because I
often do things such as:

bool result = MyFunction( parm1, SH1++, parm2 );

that I just always thought of SH1 not getting incremented until after the
statement was complete, but this is not really true.

Not really, no

SH1 (or was that SHi?) is incremented before the function is called.
However, what MyFunction() receives as is second parameters is the
original value, not the incremented one (assuming operator++ follows
the usual semantics).

Note that there is no magic here, it is only because postfix operator++
returns the original value. A valid (but silly) postfix operator++
could return the incremented value, or anything else for that matter.

Hmmm... I better make
sure I'm not doing this anywhere else in my program!

Maybe a search for "++ =" and "-- =" will let me know.

A statement such as

SHi-- = SendHistory.rend();

is misleading. SHi-- decrements SHi but returns a temporary on which
operator= is called. What you are doing here is assigining the value of
rend() to a temporary which gets destroyed at the end of the statement.
SHi retains its decremented value.

However,

--SHi = SendHistory.rend();

is undefined behavior (and will probably crash) because --SHi returns a
reference to SHi. That means rend() is assigned SHi which you try to
output later on.


Jonathan