Operator[] in std::map implementation

Question

I am trying to implement a container like std::map in C++. I have a small query while overloading operator []. I see that this operator works in two ways:

mymap[2] - For this case it looks for key 2 in map and return the value against this key.
mymap[2]=3 - For this case it looks for key 2 in map and if key is not found than 3 is inserted in map.

I see that declaration for this operator looks like: Mapped_T &operator[](const Key_T &); but what I am not getting is that in case the key is not found I will have to insert a new element in Map but in declaration of operator[] function I don't see value being passed anywhere. So how would overloaded operator know what is value against key to be inserted?

They are both the same case. writing X = Y first evaluates X and Y before performing the assignment, and X is evaluated the same way regardless of context. — M.M
– M.M, Commented Oct 12, 2016 at 23:52

Sam Varshavchik · Accepted Answer · 2016-10-12 23:25:37Z

4

When the key does not exist, std::map constructs a new value using the value class's default constructor, and returns a reference to the newly-inserted value.

answered Oct 12, 2016 at 23:25

Sam Varshavchik

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5 Comments

Mayank Jain Over a year ago

But in this case how would actual value be inserted against that key?

Sam Varshavchik Over a year ago

As I explained to you: the actual value is constructed using the object's default constructor, and inserted. For simple types, the default constructor is zero-initialization.

Mayank Jain Over a year ago

Sorry, but I am still not getting it. If I call mymap[2]=3 (assume key doesn't exists for now, then 1. overloaded operator for [] is called 2. Key 2 is inserted with some default value assume it is NULL for now. But in this whole process how is 5 inserted?

Sam Varshavchik Over a year ago

The default value is 0. And where did 5 come from? There's no 5 here. Key 2 is inserted with the value of 0, operator[] then returns a reference to this value, and the = operator assigns 3 to it.

krzaq Over a year ago

Not sure why you're getting downvoted. It's a valid answer.

krzaq · Accepted Answer · 2016-10-12 23:37:23Z

3

It would be value-initialized - as if with {}, here's some pseudocode:

Mapped_T & operator[](const Key_T &k) {
    if(!has_key(k))
        insert(k, Mapped_T{});
    return at(k);
}

The important part for your question is Mapped_T{}. For aggregates it means zero-initialization, for classes with default constructor it means using that constructor.

edited Oct 12, 2016 at 23:37

answered Oct 12, 2016 at 23:26

krzaq

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2 Comments

Mayank Jain Over a year ago

So you mean intializer_list would be called to copy the actual value against the key in map?

krzaq Over a year ago

No, there's no initializer_list anywhere in this code.

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