'STL map - insert or update

I have a map of objects and I want to update the object mapped to a key, or create a new object and insert into the map. The update is done by a different function that takes a pointer to the object (void update(MyClass *obj))

What is the best way to "insert or update" an element in a map?

c++stl


Solution 1:[1]

The operator[]    

Solution 2:[2]

With something like the following snippet:

std::map<Key, Value>::iterator i = amap.find(key);

if (i == amap.end())
    amap.insert(std::make_pair(key, CreateFunction()));
else
    UpdateFunction(&(i->second));

If you want to measure something that might improve performance you might want to use .lower_bound() to find where an entry and use that as a hint to insert in the case where you need to insert a new object.

std::map<Key, Value>::iterator i = amap.lower_bound(key);

if (i == amap.end() || i->first != key)
    amap.insert(i, std::make_pair(key, CreateFunction()));
                                       // Might need to check and decrement i.
                                       // Only guaranteed to be amortized constant
                                       // time if insertion is immediately after
                                       // the hint position.
else
    UpdateFunction(&(i->second));

Solution 3:[3]

something like:

map<int,MyClass*> mymap;
map<int,MyClass*>::iterator it;

MyClass* dummy = new MyClass();
mymap.insert(pair<int,MyClass*>(2,dummy));

it = mymap.find(2);
update(it.second);

here a nice reference link

Solution 4:[4]

The operator[] already does, what you want. See the reference for details.

Solution 5:[5]

The return value of insert is "a pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool denoting whether the insertion took place."

Therefore you can simply do 

auto result = values.insert({ key, CreateFunction()});
if (!result.second)
    UpdateFunction(&(result.first->second));

NOTE: Since your question involved raw pointers, and you said you wanted your Update function to take a pointer, I have made that assumption in my snippet. Assume that CreateFunction() returns a pointer and UpdateFunction() expects a pointer.

I'd strongly advise against using raw pointers though.

Solution 6:[6]

In C++17, function insert_or_assign insert if not existing and update if there.

Sources

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Source: Stack Overflow

Solution Source
Solution 1
Solution 2
Solution 3
Solution 4 ablaeul
Solution 5
Solution 6 Abdelmonem Mahmoud Amer

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