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

368
votes

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.)

5
c++c++11for-loopdictionary
The range for statement does an unholy dance with the standard library std::begin and std::end functions or member functions under the same name. – Gene Bushuyev
@will On a 3-line example, you're getting caught up on the fake variable name? – Stéphane

5 Answers

551
votes

Each element of the container is a map<K, V>::value_type, which is a typedef for std::pair<const K, V>. Consequently, in C++17 or higher, you can write

for (auto& [key, value]: myMap) {
    std::cout << key << " has value " << value << std::endl;
}

or as

for (const auto& [key, value]: myMap) {
    std::cout << key << " has value " << value << std::endl;
}

if you don't plan on modifying the values.

In C++11 and C++14, you can use enhanced for loops to extract out each pair on its own, then manually extract the keys and values:

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

You could also consider marking the kv variable const if you want a read-only view of the values.

102
votes

In C++17 this is called structured bindings, which allows for the following:

std::map< foo, bar > testing = { /*...blah...*/ };
for ( const auto& [ k, v ] : testing )
{
  std::cout << k << "=" << v << "\n";
}
25
votes

From this paper: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2049.pdf

for( type-specifier-seq simple-declarator : expression ) statement

is syntactically equivalent to

{
    typedef decltype(expression) C;
    auto&& rng(expression);
    for (auto begin(std::For<C>::begin(rng)), end(std::For<C>::end(rng)); begin != end; ++ begin) {
        type-specifier-seq simple-declarator(*begin);
        statement
    }
}

So you can clearly see that what is abc in your case will be std::pair<key_type, value_type >. So for printing you can do access each element by abc.first and abc.second

16
votes

If you only want to see the keys/values from your map and like using boost, you can use the boost adaptors with the range based loops:

for (const auto& value : myMap | boost::adaptors::map_values)
{
    std::cout << value << std::endl;
}

there is an equivalent boost::adaptors::key_values

http://www.boost.org/doc/libs/1_51_0/libs/range/doc/html/range/reference/adaptors/reference/map_values.html

3
votes

If copy assignment operator of foo and bar is cheap (eg. int, char, pointer etc), you can do the following:

foo f; bar b;
BOOST_FOREACH(boost::tie(f,b),testing)
{
  cout << "Foo is " << f << " Bar is " << b;
}