Whats the difference between using () or {} for an initializer list

Question

I was looking at the cppreference for constructors and I came across this:

class X {
    int a, b, i, j;
public:
    const int& r;
    X(int i)
      : r(a) // initializes X::r to refer to X::a
      , b{i} // initializes X::b to the value of the parameter i
      , i(i) // initializes X::i to the value of the parameter i
      , j(this->i) // initializes X::j to the value of X::i
    { }
};

When constructing for b and i, why do they use different brackets? The first one uses {} and the second one uses ()

Can someone explain this to me? I tried using both and I can't find a difference.

Answer 1

In your example there is no difference, you can use either of them, but, there exist some differences in some other context, for example, you can use curly braces to initialize a vector see below program.

#include<iostream>
#include<vector>

int main(){

    std::vector<int> first(12,3,5,6);   // Compile time error
    std::vector<int> second{12,3,5,6};  // Ok 

    return 0;
}

Hope, this helps you understand the difference, for more information, you should check the link mentioned by @Thomas Salik. Link.