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c++dictionaryinitializationstdmap

How does std::map::operator[] initialize values (especially fundamental types)?

I was doing a simple map program but ended up with this question. cplusplus.com says this:

Access element

If k matches the key of an element in the container, the function returns a reference to its mapped value.

If k does not match the key of any element in the container, the function inserts a new element with that key and returns a reference to its mapped value. Notice that this always increases the container size by one, even if no mapped value is assigned to the element (the element is constructed using its default constructor).

The part I don't really get is where it says "the element is constructed using its default constructor".

I gave it a try and made this:

std::map<string, int> m;
m["toast"];

I just wanted to see what value the mapped element of "toast" would be, and it ended up being zero, but why? Do fundamental types have a default constructor, or what is happening?

Solution

  • The statement of "using its default constructor" is confusing. More precisely, for std::map::operator[], if the key does not exist, the inserted value will be value-initialized.

    When the default allocator is used, this results in the key being copy constructed from key and the mapped value being value-initialized.

    For int, it means zero-initialization.

    4) otherwise, the object is zero-initialized.