遍历给定枚举的所有枚举变体

Question

Is it possible to iterate through all enum variants of a given enum. I have an std::unordered_map<Enum, Value_Type> with enum variant as key. I need to check whether all the enum variants of a given enum exist as key in std::unordered_map<Enum, Value_Type> .

Answer 1

Consider making a helper method like AllKeyPresent illustrated in a sample program below

#include <iostream>
#include <unordered_map>
//declare an enum
enum DIR : int {NORTH, SOUTH, WEST, EAST}; 

/**
 * AllKeysPresent: return true if and only if map contains all enum keys
 */
template<typename T = int> 
bool AllKeysPresent(const std::unordered_map<DIR, T>& myMap)
{
    bool allKeysPresent = true;
    //loop over enum, starting with the first entry up and including the last entry
    for (int key = DIR::NORTH; key <= DIR::EAST; key++)
    {
        if (myMap.find(static_cast<DIR>(key)) == myMap.end())
        {
            allKeysPresent = false;
            break;
        }
    }
    //TODO: check if at least one valid key is present. return false if the map is empty
    return allKeysPresent;
}

int main()
{
    //make a map with all keys
    std::unordered_map<DIR, int> mapWithAllKeys = { {DIR::NORTH, -1}, {DIR::SOUTH, -2}, {DIR::WEST, -3}, {DIR::EAST, -4} };
    std::unordered_map<DIR, int> mapWithSomeKeys = { {DIR::NORTH, -1}, {DIR::SOUTH, -2} };
    std::cout << AllKeysPresent(mapWithAllKeys) << std::endl;
    std::cout << AllKeysPresent(mapWithSomeKeys) << std::endl;
    return 0;
}

I would also recommend declaring your enum like this enum DIR : int {DIR_FIRST, NORTH, SOUTH, WEST, EAST, DIR_LAST}; where DIR_FIRST and DIR_LAST are just dummy entries, and change for loop iteration to for (int key = DIR::DIR_FIRST + 1; key < DIR:DIR_LAST; key++) . This way, if someone decides to add a new entry to your enum like SOUTHWEST , then AllKeysPresent will still work