std::map::operator[]

Question

I was doing a simple map program but ended up with this question. The c++ doc says this:

Access element If k matches the key of an element in the container, the func

Answer 1

Map values are value-initialized by the operator[], which, for int means zero-initialization.

As defined by the standard (§23.4.4.3):

Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.

T() is explained as (§8.5/10):

An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized ​

which means (§8.5/8): ​

To value-initialize an object of type T means:

[...]

— otherwise, the object is zero-initialized.

and zero-initialization is defined as (§8.5/6):

To zero-initialize an object or reference of type T means:

— if T is a scalar type, the object is set to the value 0 (zero), taken as an integral constant expression, converted to T

[...]

_{all quotes taken from n4140}

Answer 2

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.