Syntax understanding issue

Question

Please help me to understand the line in main function. What does that means

#include<iostream>

struct ABC
    {
      private:
       int A,
       float B;

      public :
       ABC() {};
       ~ABC(){};

    };

    int main()
    {
       ABC A={};     // What does this statement means?
      return 0;
    }

This is not valid C++; `main` must have return type `int` in C++. — Kerrek SB, Apr 06 '17 at 09:32
@KerrekSB actually most compiler will accept that anyway, not important for this question — John London, Apr 06 '17 at 09:33
Consult your book. If it doesn't cover it, get a better book. — StoryTeller - Unslander Monica, Apr 06 '17 at 09:36
i understand that A is the object for ABC but why this ={};, I didn't understand this assignment part — Mukesh, Apr 06 '17 at 09:37
http://stackoverflow.com/questions/15954208/when-initializing-a-struct-with-syntax-whats-happening-under-the-hood — user7825910, Apr 06 '17 at 09:39
Possible duplicate of [When initializing a struct with ={} syntax, what's happening under the hood?](http://stackoverflow.com/questions/15954208/when-initializing-a-struct-with-syntax-whats-happening-under-the-hood) — , Apr 06 '17 at 10:40

John London · Answer 1 · 2017-04-06T09:56:55.040

2

ABC A={} is the same as invoking constructor with no parameter. In your case it is ABC A = ABC();

Lets consider this class (public constructors with 0/1/2 params)

class Hello {
private:
  int x;
  float y;
public:
  Hello() {}
  Hello(int _x) : x(_x) {}
  Hello(int _x, float _y) : x(_x), y(_y) {}
};

Hello h = {} == Hello h = Hello() == just Hello h

Hello h = {1} == Hello h = Hello(1) == Hello h; h.x = 1;

Hello h = {1, 2} == Hello h = Hello(1, 2) == Hello h; h.x = 1; h.y = 2;

edited Apr 06 '17 at 09:56

answered Apr 06 '17 at 09:36

John London

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if ABC A={} and ABC A both are same then why do we write ABC A ={} ? What is the point of using it? – Mukesh Apr 06 '17 at 09:39
`ABC A={1, 2}` is not the same as `ABC A; A.A = 1; A.B = 2;`. If you have no constructor accepting them, it won't compile! it's the same as `ABC A(1,2)` (also applies to {1} version) – pergy Apr 06 '17 at 09:42
@pergy You sure? struct S { int x; float y; }; S a = {1, 2}; works for G++ 6.3.0 – John London Apr 06 '17 at 09:44
@JohnLondon I was talking about this exact case: private members, only default constructor – pergy Apr 06 '17 at 09:47
Ah sorry, didn't see the members are under`private, my bad. Tweak my answer a bit. – John London Apr 06 '17 at 09:48

score 1 · Accepted Answer · answered Apr 06 '17 at 10:03

1

ABC A={};

This is called copy-list-initialization since c++11. In this particular case it's the same as calling: ABC A; or ABC A{} or ABC A() etc. All it does is initializes an object calling a constructor.

answered Apr 06 '17 at 10:03

nikitablack

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score 0 · Answer 3 · answered Apr 06 '17 at 10:08

In your case ABC A={}; will call default constructor, but... if your class didn't have default constructor (provided by you) = {} initialization would do "a magic" - it would initialize all struct/class members with "zero/default values", i.e.: 0 for numeric types, 0.0 for floating point types, nullptr for pointers... which is much more then default default constructor does.

score 0 · Answer 4 · answered Apr 06 '17 at 11:37

0

In C++ it has no difference irrespective of {};

But if it was like C style structure like below, then ABC A={}; will reset its members A and B to zero.

struct ABC
{
    int A;
    float B;
};

answered Apr 06 '17 at 11:37

TuneFanta

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Syntax understanding issue

4 Answers4