In this syntax, what is the actual type of auto?

Question

In the C++17 for loop syntax for(auto [key, val]: students), what is auto replacing?

If students was, for example std::map<int,char*>, what would be written if not auto? I don't understand what it even is taking the place of. [int,char*]?

Answer 1

The std::map is filled with std::pair<const int, char*>, so one way to write this loop would be

for (std::pair<const int, char*> pair : students)

We could also separate the pair with std::tie, but this syntax cannot be used in the loop:

int key;
char* value;
std::pair<int, char*> pair;
std::tie(key, value) = pair;

Statement auto [key, val] replaces std::tie. This syntax is called structured binding.