Delegating constructors in c++ () or {}

Question

I read this link of Stroustrup with the following code:

class X {
        int a;
    public:
        X(int x) { if (0<x && x<=max) a=x; else throw bad_X(x); }
        X() :X{42} { }
        X(string s) :X{lexical_cast<int>(s)} { }
        // ...
    };

My question is about the line:

X() X{42}{}

Is there any differences between parentheses and curly brackets?
If there is no differences can I use curly brackets in other function calls as well? Or is it just in constructor delegation? And at last Why we should have both syntaxes? It is a little ambigous.

Fir

"can I use curly brackets in othere functions calls as well or is it just in constructor delegation?" — Constructor delegation is not a function call. You can never use {} with function calls. – Adne

check this : #18223426 – Firstly

Answer 1

() uses value initialization if the parentheses are empty, or direct initialization if non-empty.

{} uses list initialization, which implies value initialization if the braces are empty, or aggregate initialization if the initialized object is an aggregate.

Since your X is a simple int, there's no difference between initializing it with () or {}.

Adne

If I understand right it means if I am wrinting X(): X(42){} then I am calling the inte constructor but if I am writing the X(): X{42}{} then I am just initializing memebr variables without calling another constructor? – Fir

@Fir - primitive types such as int and double aren't full-fledged objects. They don't have constructors. – Quadruplicate

@DavidHammen I meant X(int constructor) not primitive int constructior – Fir

@Fir Since int has no constructor there is no constructor to be called while initializing X. Both X(42) and X{42} will perform the direct initialization, and since X is of type int, that means an implicit conversion from 42 to int will be performed to initialize X. So in the case of a simple int, both () and {} do the same thing. – Adne

"Since X is of type int" - no it isn't! It's a fully fledged class. Admittedly it only has one data member of type int, but it has three user-defined constructors. – Skyscape

Answer 2

Initialization values can be specified with parentheses or braces.

Braces initialization was introduced with C++11 and it is meant to be "uniform initialization" that can be used for all non-static variables.

Braces can be used in the place of parentheses or the equal sign and were introduced to increase uniformity and reduce confusion.

It is only a syntactical construct and does not result in performance benefits or penalties.

Annabelle