C++ - value of uninitialized vector<int>

I understand from the answer to this question that values of global/static uninitialized int will be 0. The answer to this one says that for vectors, the default constructor for the object type will be called.

I am unable to figure out - what happens when I have vector<int> v(10) in a local function. What is the default constructor for int? What if I have vector<int> v(10) declared globally?

What I am seeing is that vector<int> v(10) in a local function is resulting in variables being 0 - but I am not sure if that is just because of my compiler or is the fixed expected behaviour.

Solution 1:

The zero initialization is specified in the standard as default zero initialization/value initialization for builtin types, primarily to support just this type of case in template use.

Note that this behavior is different from a local variable such as int x; which leaves the value uninitialized (as in the C language that behavior is inherited from).

Solution 2:

It is not undefined behaviour, a vector automatically initialises all its elements. You can select a different default if you want.

The constructor is:

vector( size_type, T t = T() )

and for int, the default type (returned by int()) is 0.

In a local function this:

int x;

is not guaranteed to initialise the variable to 0.

int x = int();

would do so.

int x();

sadly does neither but declares a function.