Does STL Map auto-initialize values? [duplicate]

Question

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Why does a class used as a value in a STL map need a default constructor in …?

When I'm using a map, do values definitely get initialized to defaults or should I not rely on this?

For example, say I have the following code:

map<string, int> myMap;
cout << myMap["Hey"];

This will output "0" with my compiler. Is this guaranteed behavior? Is it possible that this wouldn't always initialize to 0?

Answer 1

Quoth the standard:

ISO/IEC 14882 §23.4.4.3

T& operator[](const key_type& x);

1. Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.
2. Requires: key_type shall be CopyConstructible and mapped_type shall be DefaultConstructible.
3. Returns: A reference to the mapped_type corresponding to x in *this.
4. Complexity: logarithmic.

So, not only is it guaranteed, but evaluating myMap["Hey"] also causes the value 0 to be inserted into the map, if there was no entry for it before.

Answer 2

It value constructs the value for a new key. Take a look at some documentation:

 A call to this function is equivalent to:
 insert(
    make_pair(x,T())
 );

This will translate into

 insert(make_pair("Key", int()));

So yes: your values will be zero at first.