int i=int(); what happens in D?

Question

Consider following program (See live demo here)

import std.stdio;
void main()
{
   int i=int();
   writefln("i is %d",i);
}

In language like C++ the statement int i=int(); is known as value initialization. For the type int, value initialization basically ends up being zero initialization. If I am not wrong C++ standard guarantees that it gives me zero always. But is D language contain feature like value initialization as we have in C++? Is it necessarily give me a zero on all D compilers & on every environment where I compile & run above D program?

score 7 · Accepted Answer · answered Oct 24 '15 at 18:50

7

When a variable is declared in D, it is always set to its 'default initializer', which can be manually accessed as T.init where T is the type (ex. int.init). The default initializer for integer types is 0, for booleans false, and for floating-point numbers NaN.

So you don't even need to assign to a variable for default initialization; just declaring it is fine. int i; will always be zero until set to something else.

Default initialization can be explicitly disabled by initializing the variable to void; for example, int i = void;.

answered Oct 24 '15 at 18:50

Colonel Thirty Two

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But http://www.tutorialspoint.com/d_programming/d_programming_variables.htm says that "For definition without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables is undefined.". & in my program variable is having automatic storage duration. – Destructor Oct 25 '15 at 05:20

score 2 · Answer 2 · answered Oct 24 '15 at 17:36

2

In D any value is initialized, it will here always give the default value for an integer: 0. It is way safer than in C++ where initialization occurs only if it can be done at no cost.

answered Oct 24 '15 at 17:36

cym13

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Only if it can be done at no cost? Tell that to `std::vector`. – chris Oct 24 '15 at 18:10
@chris What type from C++ has to do with D? – sigod Oct 24 '15 at 18:29
1

Note: you can use `int i = void;` when you want uninitialized value. – sigod Oct 24 '15 at 18:30
@sigod, I gave a counterexample to the statement in the answer: *It is way safer than in C++ where initialization occurs only if it can be done at no cost.* – chris Oct 24 '15 at 18:37
@chris I will admit my ignorance of std::vector internals, C++ is complex and I merely wanted to give a rule of thumb :) – cym13 Oct 25 '15 at 19:21

int i=int(); what happens in D?

2 Answers2