Can't use STL algorithms and binders with function that has referenceparameter

[massysett]

Hello,

I've got a couple of simple functions to examine a vector of strings,
removing from the vector strings that do not begin with a particular
string. They compile just fine when they look like this:

#include <algorithm>
#include <functional>

bool startswith(std::string subject, std::string test)
{
return (subject.substr(0, test.length()) == test);
}

std::vector<std::string> matchAbbrev(std::vector<std::string> strings,
const std::string& abbrev)
{
// isolate all words with abbrev as a prefix
// Takes the list of all strings and removes strings that do NOT
// start with abbrev.
std::vector<std::string>::iterator it =
std::remove_if(strings.begin(), strings.end(),
std::not1(std::bind2nd(std:tr_fun(startswith), abbrev)));

strings.erase(it, strings.end());
return strings;
}

However, if I change the parameter list of the first function so it
looks like this:

bool startswith(std::string& subject, std::string& test)

(note the addition of the references) it won't compile and I get a
slew of errors. Why? Maybe I could work around this with pointers,
which would not be particularly pretty...

 

[werasm]

bool startswith(std::string& subject, std::string& test)

(note the addition of the references) it won't compile and I get a
slew of errors. Why? Maybe I could work around this with pointers,
which would not be particularly pretty...

You've omitted to included <vector> and <string> and for that
reason your code does not compile (at least the posted code).

Apart from that, use const references. Changing the binary
function declaration to:

bool startswith(
const std::string& subject, const std::string& test)

seems to do the trick, which is more correct anyway because
comparison does not imply modification, not?

Regards,

Werner

 

[massysett]

You've omitted to included <vector> and <string> and for that
reason your code does not compile (at least the posted code).

Yes, thanks for pointing that out (only noticed right after I
posted...)

Apart from that, use const references. Changing the binary
function declaration to:

Unfortunately that doesn't work for me--actually my original
implementation used constant references. I get a slew of errors,
starting with

/usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../include/c++/4.1.2/bits/
stl_function.h:435: error: forming reference to reference type 'const
std::string&'

I'm on g++ 4.1.2 as you can see...

 

[Abhishek Padmanabh]

Hello,

I've got a couple of simple functions to examine a vector of strings,
removing from the vector strings that do not begin with a particular
string. They compile just fine when they look like this:

#include <algorithm>
#include <functional>

bool startswith(std::string subject, std::string test)
{
return (subject.substr(0, test.length()) == test);

}

std::vector<std::string> matchAbbrev(std::vector<std::string> strings,
const std::string& abbrev)
{
// isolate all words with abbrev as a prefix
// Takes the list of all strings and removes strings that do NOT
// start with abbrev.
std::vector<std::string>::iterator it =
std::remove_if(strings.begin(), strings.end(),
std::not1(std::bind2nd(std:tr_fun(startswith), abbrev)));

strings.erase(it, strings.end());
return strings;

}

However, if I change the parameter list of the first function so it
looks like this:

bool startswith(std::string& subject, std::string& test)

(note the addition of the references) it won't compile and I get a
slew of errors. Why? Maybe I could work around this with pointers,
which would not be particularly pretty...

Use std::bind or tr1::bind instead of bind2nd. bind2nd and bind1st
work only for const arguments (or if they are passed by value). If you
make the arguments non-const, they fail. If you use a functor the
operator() need to be const as well. boost::bind/tr1::bind are the
solution to this.

 

[massysett]

/usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../include/c++/4.1.2/bits/

stl_function.h:435: error: forming reference to reference type 'const
std::string&'

Okay, found something--see

http://gcc.gnu.org/ml/libstdc++/2000-06/msg00210.html

 

[werasm]

Okay, found something--see

http://gcc.gnu.org/ml/libstdc++/2000-06/msg00210.html

O, my apology if that is the case. I should probably tell
you that I've used bind1st and bind2nd quite some time ago
but it worked with references then, but I must have used
another lib than you. Currently I'm using gcc 4.1.2 but I
use boost::bind for quite some time now and STL algorithms
has become a breeze. . You should try it too. Actually,
I think the libc++ is not distributed with tr1::bind which
works much better. You should have a look.

W

 

[werasm]

O, my apology if that is the case. I should probably tell
you that I've used bind1st and bind2nd quite some time ago
but it worked with references then, but I must have used
another lib than you. Currently I'm using gcc 4.1.2 but I
use boost::bind for quite some time now and STL algorithms
has become a breeze. . You should try it too. Actually,
I think the libc++ is not distributed with tr1::bind which

I meant [is] and not [is not]