Simplest method to check whether unordered_map of unordered_maps contains key

Question

I am using an unordered_map of unordered_maps, such that I can reference an element using the "multi key" syntax:

my_map[k1][k2].

Is there a convenient way to use the same "multi-key" syntax to check whether an element exists before trying to access it? If not, what is the simplest way?

Answer 1

If your intention is to test for the existence of the key, I would not use

my_map[k1][k2]

because operator[] will default construct a new value for that key if it does not already exist.

Rather I would prefer to use std::unordered_map::find. So if you are certain the first key exists, but not the second you could do

if (my_map[k1].find(k2) != my_map[k1].end())
{
    // k2 exists in unordered_map for key k1
}

If you would like to make a function that checks for the existence of both keys, then you could write something like

//------------------------------------------------------------------------------
/// \brief Determines a nested map contains two keys (the outer containing the inner)
/// \param[in] data Outer-most map
/// \param[in] a    Key used to find the inner map
/// \param[in] b    Key used to find the value within the inner map
/// \return True if both keys exist, false otherwise
//------------------------------------------------------------------------------
template <class key_t, class value_t>
bool nested_key_exists(std::unordered_map<key_t, std::unordered_map<key_t, value_t>> const& data, key_t const a, key_t const b)
{
    auto itInner = data.find(a);
    if (itInner != data.end())
    {
        return itInner->second.find(b) != itInner->second.end();
    }
    return false;
}