How to use range-based for() loop with std::map?
Solution 1
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.
Solution 2
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";
}
Solution 3
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
Solution 4
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
Solution 5
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;
}
Stéphane
Linux, Ubuntu, C++ developer. https://www.linkedin.com/in/scharette http://www.ccoderun.ca/
Updated on September 17, 2020Comments
-
Stéphane over 3 years
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 anint
. 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.)
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isoiphone over 10 yearsFirst snippet of code is not using a "C++11 range-based for()". It is not an answer to "C++11: how to use range-based for() loop with std::map?"
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balki almost 10 years@ytj It is already mentioned in the answer that it doesn't work. I don't want to remove that so that new users don't have to try it and find out the fact again.
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peterchen over 6 yearsIst it possible to get a
const &
to the key, but a non-const reference to the value? (because that's what map::value_type does...) -
dalle over 6 years@peterchen:
k
isconst
if you usefor(auto&[k,v]:testing)
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TankorSmash over 6 yearscpppreference on structured bindings en.cppreference.com/w/cpp/language/structured_binding
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csknk almost 6 yearsIf you're compiling with GCC you need version 7 or better for structured bindings: gcc.gnu.org/projects/cxx-status.html
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Gabriel Staples over 3 yearsRelated links I've found really helpful as I've studied this, in this order from most-to-least helpful: 1) [MOST EXCELLENT article] thispointer.com/how-to-iterate-over-an-unordered_map-in-c11, 2) [EXCELLENT GENERAL INFO on iterators] cplusplus.com/reference/iterator, 3) [cplusplus.com
std::unordered_map
reference pg] cplusplus.com/reference/unordered_map/unordered_map, 4) [cppreference.comstd::unordered_map
reference pg] en.cppreference.com/w/cpp/container/unordered_map, 5) see also my answer here.