Re: unordered_map non-existent key

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 21 Jul 2010 19:00:31 CST

Message-ID:

<ec6d1830-b6a1-4236-835a-bd35d6bdfb82@g19g2000yqc.googlegroups.com>

On 22 Jul., 01:30, Helmut Jarausch <jarau...@igpm.rwth-aachen.de>
wrote:

Hi,

what happens if I try to access a non-existent key by using
operator[] ?

e.g.
#include <unordered_map>
typedef std::unordered_map<int,int> IntHash;
typedef IntHash::value_type ValuePair;

int main() {
  IntHash H;
  H.insert(ValuePair(3,9));
  int v = H[7];
  cout << v << endl;

}

gcc-4.4.4 does not throw an exception and returns 0.
What should it do? Throwing an exception or returning a
default value (if this, how to set the default value)?

This member function is similarly specified as
the corresponding operator[] of std::[multi]map:
If not found, a value-initialized (aka default-
constructed) key will be inserted.

There is no way to specify a different default
key. If you prefer an exception, you should
use the new member function at(), which throws
std::out_of_range if no such element is present.

HTH & Greetings from Bremen,

Daniel Kr?gler

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