遍历 C++ 17 中的映射键

Question

I am looking for a way to iterate through the keys of a map in C++ 17. The way that I have in mind right now is based on the answer of thisquestion , the way is presented below.

for (auto const& [i, val] : myMap)
...

However, I do not need to use the value val , I just need the value i . Therefore, the piece of code ... does not contain any call to the value val . Consequently, whenever I compile the code, the following warning message appears:

warning: unused variable ‘val’ [-Wunused-variable]
         for (auto const& [i, val] : myMap){
                                  ^

What I want is to find a way to iterate through (only) the keys of the map, ignoring the values. Does anyone have any idea how to do it?

Answer 1

Two options:

1. A ranged-for version of @πάνταῥεῖ 's answer :

    for (auto const& pair : myMap) { auto key = pair.first; // etc. etc. }

2. Use the ranges-v3 library (or std::ranges in C++20) to adapt the range myMap.begin() and myMap.end() by projecting it onto its first coordinate. Then you'd write something like:

    for (auto key : keys_of(myMap) ) { // etc. etc. }

   you can do this without ranges if keys_of() materializes all of the keys, but that could be expensive for a larger map.

   _{(If you have weird, heavy keys, then const auto& key instead of auto key .)}

Answer 2

Does anyone have any idea how to do it?

Sure! You can just use a traditional for loop with an iterator:

for(auto it = myMap.begin(); it != myMap.end(); ++it) {
    auto i = it->first;
    // ...
}

Answer 3

How about using Go -like unused variable declaration with _ :

for(auto const& [i, _] : myMap)
...

Answer 4

You can avoid the unused warnings by casting to void :

for (auto const& [key, val] : m) {
  (void)val;  // To avoid unused warnings

  // use key
}

Answer 5

for(auto const & pair : myMap) since map internally stored element in std::pair format, hence you can access the element by writing, pair.first; for key or pair.second; for value.

Answer 6

for (auto& [key, std::ignore] : m) {
  // use key
}