Initializer

(), {} and std::initializer_list

1. Commons and distinctions between () and {}

		()	{}
Commons		Direct initialization int x(3); int x{3}; std::string name("Some Name"); std::string name{"Some Name"}; int* y = new int(1); int* y = new int{1}; Call constructors MyClass mc(1); MyClass mc{1}; // If no initializer list constructor exists.
Distincetions	Narrowing Conversions	int pi(3.14); // OK -- pi == 3.	int pi{3.14}; // Compile error: narrowing conversion.
	Uniform Initialization	std::vector v(100, 1); // A vector containing 100 items: All 1s.	std::vector v{100, 1}; // A vector containing 2 items: 100 and 1.

2. std:: initializer_list

Using std::initializer_list allows a class to be constructed with a list of values, providing flexibility and ease of initialization.

template<typename T>
struct S {
    S(std::initializer_list<T> data) : _data(data) {
        std::cout << "initializer_list contructor called\n";
    }
    S(T a, T b, T c) {
        std::cout << "3 params contructor called\n";
    }
private:
    std::initializer_list<T> _data;
};

int main() {
    S<int> s1{1, 2, 3}; // Ok, initializer_list contructor called.
    S<int> s2(1, 2, 3); // Ok, 3 params contructor called.
    S<int> s3{1.1, 2.2, 3.3}; // Error, narrowing conversion.
    return 0;
}

{} provides a more safer way(prevent type conversion) for initialization. In situations where {} and () are equivalent, we should choose {} over ();

References

• https://google.github.io/styleguide/cppguide.html#Variable_and_Array_Initialization
• https://en.cppreference.com/w/cpp/utility/initializer_list