How to find if a given key exists in a C++ std::map

Use map::find

if ( m.find("f") == m.end() ) {
  // not found
} else {
  // found
}

To check if a particular key in the map exists, use the count member function in one of the following ways:

m.count(key) > 0
m.count(key) == 1
m.count(key) != 0

The documentation for map::find says: "Another member function, map::count, can be used to just check whether a particular key exists."

The documentation for map::count says: "Because all elements in a map container are unique, the function can only return 1 (if the element is found) or zero (otherwise)."

To retrieve a value from the map via a key that you know to exist, use map::at:

value = m.at(key)

Unlike map::operator[], map::at will not create a new key in the map if the specified key does not exist.

C++20 gives us std::map::contains to do that.

#include <iostream>
#include <string>
#include <map>

int main()
{
    std::map<int, std::string> example = {{1, "One"}, {2, "Two"}, 
                                     {3, "Three"}, {42, "Don\'t Panic!!!"}};

    if(example.contains(42)) {
        std::cout << "Found\n";
    } else {
        std::cout << "Not found\n";
    }
}