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]

kap wrote:

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;
}
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

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]

Mark P wrote:

kap wrote:

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;
}

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

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

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

[Kai-Uwe Bux]

kap wrote:

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.

It's a FAQ:

http://www.parashift.com/c++-faq-lit...html#faq-10.19
Best

Kai-Uwe Bux

[kap]

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