Determine if map contains a value for a key?

As long as the map is not a multimap, one of the most elegant ways would be to use the count method

if (m.count(key))
    // key exists

The count would be 1 if the element is indeed present in the map.

I just noticed that with C++20, we will have

bool std::map::contains( const Key& key ) const;

That will return true if map holds an element with key key.

Does something along these lines exist?

No. With the stl map class, you use ::find() to search the map, and compare the returned iterator to std::map::end()

so

map<int,Bar>::iterator it = m.find('2');
Bar b3;
if(it != m.end())
{
   //element found;
   b3 = it->second;
}

Obviously you can write your own getValue() routine if you want (also in C++, there is no reason to use out), but I would suspect that once you get the hang of using std::map::find() you won't want to waste your time.

Also your code is slightly wrong:

m.find('2'); will search the map for a keyvalue that is '2'. IIRC the C++ compiler will implicitly convert '2' to an int, which results in the numeric value for the ASCII code for '2' which is not what you want.

Since your keytype in this example is int you want to search like this: m.find(2);