What is the meaning of this constructor definition in C++?

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I have this definition:

struct A
{
    int x;
    A(int x = 1): x(x) {} // user-defined default constructor
};

but not sure what is the benefit of doing x(x)?
furthermore, I want to understand what is the real meaning of this x(x) expression?

try to validate my understanding with gurus here.

What I have tried:

x(x) is to initialize a type int with value x.

Posted 29-May-21 9:35am

Southmountain

Updated 29-May-21 9:58am

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Richard MacCutchan 29-May-21 16:19pm

It is just a shorthand way of saying this.x = x. As explained in the C++ documentation.

Southmountain 29-May-21 18:21pm

thank you for this explanation!

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Greg Utas · Accepted Answer · 2021-05-29T09:58:00

Solution 1

It says that if the constructor is called without an explicit argument, x defaults to 1. The constructor can then be called two ways:

C++

A one();
A two(2);

Posted 29-May-21 9:58am

Greg Utas

Updated 29-May-21 9:59am

v2

Comments

Southmountain 29-May-21 16:39pm

thank you! any link could you share for your statement?

Greg Utas 29-May-21 17:05pm

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

A great site that's my go-to for info on C++.

Southmountain 29-May-21 18:13pm

thank you very much for this link! I like that site too.

I think I got this trick: one stone hits two birds:)

Greg Utas 29-May-21 17:14pm

A couple of more points.

First, I hate the fact that "x" is used as a parameter name here, because it's also the name of A's field. There is undoubtedly a rule which says when "x" means the parameter and when it's short for this->x, but I don't know what it is because it is simply rude to do this kind of thing.

Second, defining a default can really be useful when there are rarely used trailing arguments. However, it is bad practice to define a default argument in a virtual/override/final function. The reason is that if different versions of the function specify different defaults, which one will be used is very confusing and error prone.

Southmountain 29-May-21 18:14pm

thank you for sharing your understanding with me. now I am more confident to use this feature!

What is the meaning of this constructor definition in C++?

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What is the meaning of this constructor definition in C++?

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