problem with map construction

[kap]

Hi all,
I am facing a weired problem when I try using maps. Here is the
sample code.
Can anyone please tell me where am I going wrong.

int main() {
less<int> ltvar;
std::map<int, int> m2(less<int>()) ;
m2[1]; // compilation fails at this point
return 0;
}

whereas this particular code works fine
int main() {
less<int> ltvar;
std::map<int, int> m2( ltvar ); // less<int>() changed to ltvar
m2[1]; // compilation succeeds
return 0;
}

I would appreciate if someone can comment on what the issue is.

[Mark P]

Re: problem with map construction

kap wrote:[color=blue]
> Hi all,
> I am facing a weired problem when I try using maps. Here is the
> sample code.
> Can anyone please tell me where am I going wrong.
>
> int main() {
> less<int> ltvar;
> std::map<int, int> m2(less<int>()) ;
> m2[1]; // compilation fails at this point
> return 0;
> }[/color]

I'm not an expert but it seems, from looking at VC++EE compiler messages
(btw, always post compiler messages if you can't compile), that this is
interpreted as a _declaration of a function_ which takes a ptr to
function as an argument and returns a map.

You can get around this like so:

std::map<int, int> m2 = std::map<int, int> (less<int>());

However, there's no reason why you need to do any of this since
less<Key> is the default comparison function used by map anyway.

-Mark
[color=blue]
>
> whereas this particular code works fine
> int main() {
> less<int> ltvar;
> std::map<int, int> m2( ltvar ); // less<int>() changed to ltvar
> m2[1]; // compilation succeeds
> return 0;
> }
>
> I would appreciate if someone can comment on what the issue is.
>[/color]

[Mark P]

Re: problem with map construction

Mark P wrote:[color=blue]
> kap wrote:[color=green]
>> Hi all,
>> I am facing a weired problem when I try using maps. Here is the
>> sample code.
>> Can anyone please tell me where am I going wrong.
>>
>> int main() {
>> less<int> ltvar;
>> std::map<int, int> m2(less<int>()) ;
>> m2[1]; // compilation fails at this point
>> return 0;
>> }[/color]
>
> I'm not an expert but it seems, from looking at VC++EE compiler messages
> (btw, always post compiler messages if you can't compile), that this is
> interpreted as a _declaration of a function_ which takes a ptr to
> function as an argument and returns a map.
>
> You can get around this like so:
>
> std::map<int, int> m2 = std::map<int, int> (less<int>());
>
> However, there's no reason why you need to do any of this since
> less<Key> is the default comparison function used by map anyway.
>
> -Mark
>[/color]

I knew I'd seen this in the FAQ before...

http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.19

[Kai-Uwe Bux]

Re: problem with map construction

kap wrote:
[color=blue]
> Hi all,
> I am facing a weired problem when I try using maps. Here is the
> sample code.
> Can anyone please tell me where am I going wrong.
>
> int main() {
> less<int> ltvar;
> std::map<int, int> m2(less<int>()) ;
> m2[1]; // compilation fails at this point
> return 0;
> }
>
> whereas this particular code works fine
> int main() {
> less<int> ltvar;
> std::map<int, int> m2( ltvar ); // less<int>() changed to ltvar
> m2[1]; // compilation succeeds
> return 0;
> }
>
> I would appreciate if someone can comment on what the issue is.[/color]

It's a FAQ:

http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.19

Best

Kai-Uwe Bux

[kap]

Re: problem with map construction

hey Mark thanks a lot for the prompt reply and for pointing me to the
faq.
I highly appreciate that.