STL maps and const keys

[Michael H Lees]

Hi there,

I'm trying to use an stl map where I have my own defined (templated)
class as a key. I have the class "variableId " below which I want to use
as the key for an stl map. It has two members agent and slot. I've
defined the < operator but I'm slightly confused, here's the complete
class anyhow,

template <typename agentId, typename slotId>
class variableId
{
public:
agentId agent;
slotId slot;
variableId(){}
variableId(agen tId aId, slotId sId);
bool operator<(varia bleId vId);
};

template <typename agentId, typename slotId>
variableId<agen tId,slotId>::va riableId(agentI d aId, slotId sId)
{
agent=aId;
slot=sId;
}

template <typename agentId, typename slotId>
bool variableId<agen tId,slotId>::
operator< (variableId vId)
{
return (slot<vId.slot && agent<vId.agent );
}


When I then try and do something like...

map<variableId< int,int>,int> m;
variableId<int, int> varname(1,1);
m[varname] = 1;


I get gcc reporting an error on the line m[varname]=1, something like

/usr/local/include/g++-v3/bits/stl_function.h: 141: passing `const
variableId<int, int>' as `this' argument of `bool variableId<agen tId,
slotId>::operat or<(variableId< agentId, slotId>) [with agentId = int,
slotId = int]' discards qualifiers

I guess I'm either calling incorrecty or I've specified the
class/operator incorrectly.


Any help much appreciated


Thanks

ps. remove ++ from address to reply

[tom_usenet]

Re: STL maps and const keys

On Wed, 25 Jun 2003 16:49:02 +0100, Michael H Lees
<mhl@cs.nott.ac .uk++> wrote:
[color=blue]
>Hi there,
>
>I'm trying to use an stl map where I have my own defined (templated)
>class as a key. I have the class "variableId " below which I want to use
>as the key for an stl map. It has two members agent and slot. I've
>defined the < operator but I'm slightly confused, here's the complete
>class anyhow,
>
>template <typename agentId, typename slotId>
>class variableId
>{
>public:
> agentId agent;
> slotId slot;
> variableId(){}
> variableId(agen tId aId, slotId sId);
> bool operator<(varia bleId vId);[/color]

Should be:
bool operator<(varia bleId vId) const;
or
bool operator<(varia bleId const& vId) const;
[color=blue]
>};
>
>template <typename agentId, typename slotId>
>variableId<age ntId,slotId>::v ariableId(agent Id aId, slotId sId)
>{
> agent=aId;
> slot=sId;
>}
>
>template <typename agentId, typename slotId>
>bool variableId<agen tId,slotId>::
>operator< (variableId vId)
> {
> return (slot<vId.slot && agent<vId.agent );
> }[/color]

template <typename agentId, typename slotId>
bool variableId<agen tId,slotId>::
operator< (variableId vId) const
{
return (slot<vId.slot && agent<vId.agent );
}

or

template <typename agentId, typename slotId>
bool variableId<agen tId,slotId>::
operator< (variableId const& vId) const
{
return (slot<vId.slot && agent<vId.agent );
}

[color=blue]
>
>
>When I then try and do something like...
>
> map<variableId< int,int>,int> m;
> variableId<int, int> varname(1,1);
> m[varname] = 1;[/color]

That's fine.
[color=blue]
>
>
>I get gcc reporting an error on the line m[varname]=1, something like
>
>/usr/local/include/g++-v3/bits/stl_function.h: 141: passing `const
>variableId<int , int>' as `this' argument of `bool variableId<agen tId,
>slotId>::opera tor<(variableId <agentId, slotId>) [with agentId = int,
>slotId = int]' discards qualifiers[/color]

Right, you need a const operator since the key in a map is const.
[color=blue]
>
>I guess I'm either calling incorrecty or I've specified the
>class/operator incorrectly.[/color]

The latter.

Tom

[Thomas Matthews]

Re: STL maps and const keys

Michael H Lees wrote:[color=blue]
> Hi there,
>
> I'm trying to use an stl map where I have my own defined (templated)
> class as a key. I have the class "variableId " below which I want to use
> as the key for an stl map. It has two members agent and slot. I've
> defined the < operator but I'm slightly confused, here's the complete
> class anyhow,
>
> template <typename agentId, typename slotId>
> class variableId
> {
> public:
> agentId agent;
> slotId slot;
> variableId(){}
> variableId(agen tId aId, slotId sId);
> bool operator<(varia bleId vId);[/color]

bool operator<(const variableId& vId) const;
[color=blue]
> };
>
> template <typename agentId, typename slotId>
> variableId<agen tId,slotId>::va riableId(agentI d aId, slotId sId)[/color]
: agent(aId), slot(sId) // prefer initialization lists.
[color=blue]
> {
> agent=aId;
> slot=sId;
> }[/color]

The above assignments should be replaced by the
initialization list above.


[color=blue]
>
> template <typename agentId, typename slotId>
> bool variableId<agen tId,slotId>::
> operator< (variableId vId)[/color]

operator< (const variableId& vId) const
[color=blue]
> {
> return (slot<vId.slot && agent<vId.agent );[/color]
// Perhaps some spaces would improve readability:
// (also, return is not a function and doesn't require
// an expression in parenthesis).
return (slot < vId.slot) && (agent < vId.agent);
[color=blue]
> }
>
>
> When I then try and do something like...
>
> map<variableId< int,int>,int> m;
> variableId<int, int> varname(1,1);
> m[varname] = 1;
>
>
> I get gcc reporting an error on the line m[varname]=1, something like
>
> /usr/local/include/g++-v3/bits/stl_function.h: 141: passing `const
> variableId<int, int>' as `this' argument of `bool variableId<agen tId,
> slotId>::operat or<(variableId< agentId, slotId>) [with agentId = int,
> slotId = int]' discards qualifiers
>
> I guess I'm either calling incorrecty or I've specified the
> class/operator incorrectly.
>
>
> Any help much appreciated
>
>
> Thanks
>
> ps. remove ++ from address to reply
>[/color]

Add the "const" qualifiers to the "operator <" method above.
The key of a map is constant, but the comparison function wasn't.


--
Thomas Matthews

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[Thomas Matthews]

Re: STL maps and const keys

Michael H Lees wrote:[color=blue]
> Replying to my own question.... I know.
>
> I managed to fix the problem by looking at stl::pair as an example.
>
> I redefined the < operator as follows.....
>
> template <class agentId, class slotId>
> inline bool operator<(const variableId<agen tId, slotId>& v1,
> const variableId<agen tId, slotId>& v2)
> {
> return (v1.slot<v2.slo t && v1.agent<v2.age nt);
> }
>
> I had tried all manner of things but missed the inline option. Should I
> define all my operators as inline if I'm using the class as a key for a
> map?
>
> Cheers
>
> _mike_
>[/color]
1. Don't top-post. Replies are either intermixed or appended to
the bottom of a post.
2. A rule of thumb of inlining: Inline simple functions. A get or
a set method is an example. If the overhead of the function
call is greater than the code in the function, then inline the
function.

--
Thomas Matthews

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http://www.slack.net/~shiva/welcome.txt

C++ Faq: http://www.parashift.com/c++-faq-lite
C Faq: http://www.eskimo.com/~scs/c-faq/top.html
alt.comp.lang.l earn.c-c++ faq:

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Other sites:
http://www.josuttis.com -- C++ STL Library book