Using set.insert( key ) as a conditional?
-
03-07-2019 - |
Question
I am trying to use set.insert (key)
as a conditional, where if the key is inserted correctly (meaning that the key does NOT already exist in the set ) then it should go on and perform some kind of code. For example, something like:
if (set.insert( key )) {
// some kind of code
}
Is this allowed? Because the compiler is throwing this error:
conditional expression of type 'std::_Tree<_Traits>::iterator' is illegal
Solution
The version of insert that takes a single key value should return a std::pair<iterator,bool>
, where the bool indicates whether an insertion was made. A value of true indicates that the value was inserted, and false indicates that the value was already present. So your conditional would look like this:
if( set.insert( key ).second ) {
// code
}
OTHER TIPS
set::insert returns a pair, try this:
if( set.insert( key ).second ) {
// some kind of code
}
Other answers have suggested just using '.second', and this will work - but if the processing that you need to perform uses the existing entry in the set, then you can store the full result of the insert:
std::pair<std::set<key>::iterator, bool> iResult = set.insert (key);
if (iResult.second) {
// some kind of code - insert took place
}
else {
// some kind of code using iResult.first, which
// is an iterator to the previous entry in the set.
}