Just take std::vector as an example.
I wonder which constructor function is called when executing std::vector<int> a{1,2,3}.
Thanks for your help.

That particular form of vector initialization is using list initialization, introduced in C++11.

https://en.cppreference.com/w/cpp/language/list_initialization

The std::vector had/has quite many constructors, as shown in https://en.cppreference.com/w/cpp/container/vector/vector

The {1,2,3} initializer syntax was introduced in C++11 and along with that:

The constructor which initialises the container with the contents of the initializer list
The one marked (10) in this list of overloads: https://en.cppreference.com/w/cpp/container/vector/vector

Note that the presence of list-initializing constructor (10) means list initialization and direct initialization do different things: https://en.cppreference.com/w/cpp/container/vector/vector#Notes

Thanks for all answers. I have got it.