I have a vector of a map of int and float std::vector<std::map<int, float>> datad;

And I want to iterate over the maps in vector and inside over pairs in map

for (std::vector<std::map<int, float>>::iterator currentMap = datad.begin(); currentMap < datad.end();   currentMap)
{ 
 for (std::map<int, float>::iterator it = currentMap->begin(); it < currentMap->end();   it)
     {/*some code here*/}
}

But in the second loop g compiler gives an error:

no match for ‘operator<’ (operand types are ‘std::_Rb_tree_iterator<std::pair<const int, float> >’ and ‘std::map<int, float>::iterator’ {aka ‘std::_Rb_tree_iterator<std::pair<const int, float> >’})

But isn't it looks like the types

std::_Rb_tree_iterator<std::pair<const int, float>>
std::_Rb_tree_iterator<std::pair<const int, float>>

are the same?

Yes, I see that compiler says the currentMap->end() have the type std::map<int, float>::iterator but...it is aka _Rb_tree_iterator...

maybe

static_cast<std::_Rb_tree_iterator<std::pair<const int, float>>>(currentMap->end())

What is the easiest way to continue using iterators and fix the problem?

PS. static_cast don't work, it gives

no match for ‘operator<’ (operand types are ‘std::map<int, float>::iterator’ {aka ‘std::_Rb_tree_iterator<std::pair<const int, float> >’} and ‘std::map<int, float>::iterator’ {aka ‘std::_Rb_tree_iterator<std::pair<const int, float> >’})

PSS. I've also tried auto but it also doesn't work (the same error as the first one)

CodePudding user response：

std::map's iterators are bidirectional iterators, not random access iterators. Therefore they do not provide relational comparison operators. You can only equality-compare them.

So replace < with !=, which is the standard way of writing iterator loops. Or even better, replace the loop with a range-for loop:

for (auto& currentMap : datad)
{ 
    for (auto& el : currentMap)
    {
        /* Use el here as reference to the (pair) element of the map. */
    }
}