using std::find_if() with a std::map

[Anonymous]

I have map variable that maps ids (unsigned int) to objects. Each object
has a Name() method.

I want to use std::find_if to fetch the object that name matches.

I defined my predicate like this:


class NameMatch
{
public:
inline NameMatch ( const std::string& name ) : m_name ( name )
{}

inline bool operator() ( const char* systemname ) const
{
return !stricmp(m_name .c_str(), systemname) ;
}

inline ~NameMatch(){}

private:
std::string m_name ;
};




I got the ff error:

error C2664: 'bool `anonymous-namespace'::Nam eMatch::operato r ()(const
char *) const' : cannot convert parameter 1 from 'std::pair<_Ty1 ,_Ty2>'
to 'const char *'

How may I implement this ?

[=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=]

Re: using std::find_if() with a std::map

On 2007-10-25 23:53, Anonymous wrote:

I have map variable that maps ids (unsigned int) to objects. Each object
has a Name() method.
>
I want to use std::find_if to fetch the object that name matches.
>
I defined my predicate like this:
>
>
class NameMatch
{
public:
inline NameMatch ( const std::string& name ) : m_name ( name )
{}
>
inline bool operator() ( const char* systemname ) const
{
return !stricmp(m_name .c_str(), systemname) ;
}
>
inline ~NameMatch(){}
>
private:
std::string m_name ;
};
>
>
>
>
I got the ff error:
>
error C2664: 'bool `anonymous-namespace'::Nam eMatch::operato r ()(const
char *) const' : cannot convert parameter 1 from 'std::pair<_Ty1 ,_Ty2>'
to 'const char *'

find_if() will call your comparison method with the objects stored in
the map, which (in your case) is an std::pair<unsig ned int, object>
where object is whatever kinds of objects you store.

How may I implement this ?

class NameMatch
{
std::string m_name;

public:
NameMatch(std:: string& name) : m_name(name) {}
bool operator()(cons t std::pair<unsig ned int, object>& o)
{
return (m_name == o.Name());
}
};

--
Erik Wikström

[James Kanze]

Re: using std::find_if() with a std::map

On Oct 26, 12:19 am, Erik Wikström <Erik-wikst...@telia. comwrote:

[On using find_if on a map...]

class NameMatch
{
std::string m_name;
>
public:
NameMatch(std:: string& name) : m_name(name) {}
bool operator()(cons t std::pair<unsig ned int, object>& o)

I seem to see this a lot, but... wouldn't it be preferable to
use Map::value_type rather than std::pair<...>? They mean the
same thing to the compiler, but not to whoever is reading the
program.

{
return (m_name == o.Name());
}
};

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[James Kanze]

Re: using std::find_if() with a std::map

On Oct 26, 12:33 am, "Alf P. Steinbach" <al...@start.no wrote:

* Anonymous:

I have map variable that maps ids (unsigned int) to objects. Each object
has a Name() method.

I want to use std::find_if to fetch the object that name matches.

I defined my predicate like this:

class NameMatch
{
public:
inline NameMatch ( const std::string& name ) : m_name ( name )
{}

I'm not sure whether it's valid to add 'inline' to a function defined
(not just declared) in the class, but anyway it's superfluous: the
function is already inline by virtue of being defined here.

It's valid, but I agree that it's just noise.

inline bool operator() ( const char* systemname ) const
{
return !stricmp(m_name .c_str(), systemname) ;
}

This operator returns true if the names are different.

How do you know? I'd have to see the specifications of stricmp
first. It's not a standard function, so I can't look it up in
the standard, but if it follows the conventions of strcmp, then
it returns 0 when the strings are equal. And !0 is true, !
anything else false, so his function works. It's not readable,
of course, and won't pass code review in any place I've ever
worked in, but if the function follows the strcmp convention, he
really should compare the results with 0. But technically, it
works (although I had to think it through carefully to realize
this---the code is obfuscated).

(Of course, according to the C standard, function names
beginning with str are reserved for future expansion, so he's
probably treading very close to undefined behavior using this
function name himself. But I'm not sure that C++ has the same
restriction.)

It should return true if the names are equal.

Also, unless your objects convert implicitly to 'const char*'
(which is generally a bad idea), the argument should be a
'std::pair<int, O const>' where O is your object type.

That's really his problem. std::pair (and thus,
Map::value_type ) doesn't convert implicitly to char const*.
Ever. And he can't change it. To use find_if on a map, the
operator() must take an argument which can be initialized with
an MapType::value_ type (which is guaranteed to be std::pair<
key_type const, mapped_type >, but using std::pair instead of
MapType::value_ type is another frequent obfuscation.)

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=]

Re: using std::find_if() with a std::map

On 2007-10-26 10:32, James Kanze wrote:

On Oct 26, 12:19 am, Erik Wikström <Erik-wikst...@telia. comwrote:
>
[On using find_if on a map...]
>

>class NameMatch
>{
> std::string m_name;
>>
>public:
> NameMatch(std:: string& name) : m_name(name) {}
> bool operator()(cons t std::pair<unsig ned int, object>& o)

>
I seem to see this a lot, but... wouldn't it be preferable to
use Map::value_type rather than std::pair<...>? They mean the
same thing to the compiler, but not to whoever is reading the
program.

Yes, except that it is more to type if you do not have a typedef for the
map. :-)

--
Erik Wikström

[James Kanze]

Re: using std::find_if() with a std::map

On Oct 26, 10:52 am, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2007-10-26 10:32, James Kanze wrote:

[...]

bool operator()(cons t std::pair<unsig ned int, object>& o)

I seem to see this a lot, but... wouldn't it be preferable to
use Map::value_type rather than std::pair<...>? They mean the
same thing to the compiler, but not to whoever is reading the
program.

Yes, except that it is more to type if you do not have a
typedef for the map. :-)

But if you're worried about typing (or even only line length),
you do have the typedef, because otherwise, declaring an
iterator (e.g. to capture the results of find()) is also too
much to type:-).

By using value_type, you have a lot less to change when the key
is changed to unsigned long. And you're sure that you've got
the type right. (Note that Alf put the const on the wrong
element. And Alf's hardly a beginner. It's easy to make such
small errors.)

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[Rajesh S R]

Re: using std::find_if() with a std::map

On Oct 26, 3:19 am, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2007-10-25 23:53, Anonymous wrote:
>
>
>

I have map variable that maps ids (unsigned int) to objects. Each object
has a Name() method.

>

I want to use std::find_if to fetch the object that name matches.

>

I defined my predicate like this:

>

class NameMatch
{
public:
inline NameMatch ( const std::string& name ) : m_name ( name )
{}

>

inline bool operator() ( const char* systemname ) const
{
return !stricmp(m_name .c_str(), systemname) ;
}

>

inline ~NameMatch(){}

>

private:
std::string m_name ;
};

>

I got the ff error:

>

error C2664: 'bool `anonymous-namespace'::Nam eMatch::operato r ()(const
char *) const' : cannot convert parameter 1 from 'std::pair<_Ty1 ,_Ty2>'
to 'const char *'

>
find_if() will call your comparison method with the objects stored in
the map, which (in your case) is an std::pair<unsig ned int, object>
where object is whatever kinds of objects you store.
>

How may I implement this ?

>
class NameMatch
{
std::string m_name;
>
public:
NameMatch(std:: string& name) : m_name(name) {}
bool operator()(cons t std::pair<unsig ned int, object>& o)
{
return (m_name == o.Name());

Is it not return (m_name == o.second.Name() ); ?

}
>
};

[=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=]

Re: using std::find_if() with a std::map

On 2007-10-26 20:54, Rajesh S R wrote:

On Oct 26, 3:19 am, Erik Wikström <Erik-wikst...@telia. comwrote:

>On 2007-10-25 23:53, Anonymous wrote:
>>
>>
>>

I have map variable that maps ids (unsigned int) to objects. Each object
has a Name() method.

>>

I want to use std::find_if to fetch the object that name matches.

>>

I defined my predicate like this:

>>

class NameMatch
{
public:
inline NameMatch ( const std::string& name ) : m_name ( name )
{}

>>

inline bool operator() ( const char* systemname ) const
{
return !stricmp(m_name .c_str(), systemname) ;
}

>>

inline ~NameMatch(){}

>>

private:
std::string m_name ;
};

>>

I got the ff error:

>>

error C2664: 'bool `anonymous-namespace'::Nam eMatch::operato r ()(const
char *) const' : cannot convert parameter 1 from 'std::pair<_Ty1 ,_Ty2>'
to 'const char *'

>>
>find_if() will call your comparison method with the objects stored in
>the map, which (in your case) is an std::pair<unsig ned int, object>
>where object is whatever kinds of objects you store.
>>

How may I implement this ?

>>
>class NameMatch
>{
> std::string m_name;
>>
>public:
> NameMatch(std:: string& name) : m_name(name) {}
> bool operator()(cons t std::pair<unsig ned int, object>& o)
> {
> return (m_name == o.Name());

>
Is it not return (m_name == o.second.Name() ); ?

Yes, sorry about that.

--
Erik Wikström