Use of parenthesis or curly braces in C++ constructor initializer list

Question

I have a question on constructor initializer list as follows : while specifying the initial values of members, initial values are written in ()- parenthesis according to C++ Primer book (author - Stanley Lippman). However, I have also seen {} being used to specify initial values (please refer to link - https://en.cppreference.com/w/cpp/language/constructor) can someone explain when to use () - parenthesis and when to use {} - curly braces thanks and regards, -sunil puranik

Answer 1

According to Scott meyors Effective Modern C++, Item 7, you should basically be using {} wherever you can in the initializer list. If you are initialising a type that takes a std::initializer_list then you will need to think about it a bit more. But outside of std::vector and templates, you should basically always be using {} to construct. Why? From Scott Meyors:

Braced initialization is the most widely usable initialization syntax, it prevents narrowing conversions, and it’s immune to C++’s most vexing parse.

Answer 2

Using T x{}; where T is some type, is called zero initialization.

Parenthesis () is Pre-C++11 while braces {} is from C++11 and onwards(like c++11, c++14, etc). This is just one of the many differences between the two. For example,

Pre C++11

class MyVector
{
    int x;
    MyVector(): x()
    {
    }
};

C++11

From C++11 and onwards, you can use {} instead as shown below:

class MyVector
{
    int x;
    MyVector(): x{}
    {
    }
};

In the context of constructor initializer list(which is what your question is about) they are used to ensure proper initialization of non-static data members of a class template as explained here.