Stéphane Stéphane - 2 months ago 5
C++ Question

How to use range-based for() loop with std::map?

The common example for C++11 range-based for() loops is always something simple like this:

std::vector<int> numbers = { 1, 2, 3, 4, 5, 6, 7 };
for ( auto xyz : numbers )
{
std::cout << xyz << std::endl;
}


In which case
xyz
is an
int
. But, what happens when we have something like a map? What is the type of the variable in this example:

std::map< foo, bar > testing = { /*...blah...*/ };
for ( auto abc : testing )
{
std::cout << abc << std::endl; // ? should this give a foo? a bar?
std::cout << abc->first << std::endl; // ? or is abc an iterator?
}


When the container being traversed is something simple, it looks like range-based for() loops will give us each item, not an iterator. Which is nice...if it was iterator, first thing we'd always have to do is to dereference it anyway.

But I'm confused as to what to expect when it comes to things like maps and multimaps.

(I'm still on g++ 4.4, while range-based loops are in g++ 4.6+, so I haven't had the chance to try it yet.)

Answer

Each element of the container is a map<K, V>::value_type, which is a typedef for std::pair<const K, V>. Consequently, you'd write this as

for (auto& kv : myMap) {
    std::cout << kv.first << " has value " << kv.second << std::endl;
}

For efficiency, it is a good idea to make the parameter in the loop a reference. You could also consider making it const if you want a read-only view of the values.

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