What does this do ???

Bob Langelaan

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Mar 15, 2016, 5:06:11 PM3/15/16

to

This code compiles on MS VS 2015:

#include <array>
int main()
{
std::array<bool, 100> bArray;
bArray = { false };
}

What does the last line of code do?

Thanks,
Bob

Vir Campestris

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Mar 15, 2016, 5:16:18 PM3/15/16

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Initialises the first element of the array with false.

http://en.cppreference.com/w/cpp/container/array

and follow the links to

http://en.cppreference.com/w/cpp/language/aggregate_initialization

HTH
Andy

Marcel Mueller

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Mar 15, 2016, 5:17:14 PM3/15/16

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Assign a list of one boolean to the array. Due to the fixed nature of
std::array this will in fact clear all elements.

Marcel

Bob Langelaan

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Mar 15, 2016, 5:23:12 PM3/15/16

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Marcel,

Are you saying that the last statement assigns all 100 bool values in bArray to false? I would have thought a loop of some type would have been necessary to accomplish this? Can you expand on the last statement you made? Thanks,
Bob

Bob Langelaan

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Mar 15, 2016, 5:36:59 PM3/15/16

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Ok, I modified code to:

#include <array>
#include <iostream>

int main()
{
std::array<bool, 100> bArray;

bArray = { true };
for (auto x : bArray)
{
std::cout << x << " ";
}
}

And I have figured out what is happening by varying whether true or false is assigned or statement is commented out.

If assigned false, all 100 bools become false; if assigned true, only the first element becomes true, the remainder become false; if commented out the 100 bools have seemingly random values (memory in bArray is not initialized in other words).

Marcel Mueller

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Mar 15, 2016, 5:49:14 PM3/15/16

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On 15.03.16 22.22, Bob Langelaan wrote:
>> Assign a list of one boolean to the array. Due to the fixed nature of
>> std::array this will in fact clear all elements.
>>

> Are you saying that the last statement assigns all 100 bool values in bArray to false?

Exactly.

> I would have thought a loop of some type would have been necessary to accomplish this? Can you expand on the last statement you made? Thanks,

It is a ->"list initializer".

And similar to C array assignments trailing elements are initialized to
zero i.e. false.

Marcel

Öö Tiib

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Mar 16, 2016, 3:24:14 AM3/16/16

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Yes, it is aggregate so when initialized it will be default-initialized
(so falses) to end and when not initialized then it will be uninitialized.

Bob Langelaan

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Mar 16, 2016, 1:23:51 PM3/16/16

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But in my mind an entity can only be "initialized" when it is defined. Therefore the statement:

bArray = { false };

does not initialize bArray because bArray was previously defined.

I am assuming that what is happening that the { false } value on the right of the assignment statement is being converted to a temporary std::array<bool, 100> object which is initialized to { false }. This temporary object is then assigned to bArray.

Is my assumption correct?

Thanks,
Bob

Öö Tiib

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Mar 16, 2016, 7:04:00 PM3/16/16

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Yes, perhaps bad wording of mine.

> Therefore the statement:
>
> bArray = { false };
>
> does not initialize bArray because bArray was previously defined.
>
> I am assuming that what is happening that the { false } value on the right of the assignment statement is being converted to a temporary std::array<bool, 100> object which is initialized to { false }. This temporary object is then assigned to bArray.
>
> Is my assumption correct?

Yes, the '{ false }' is of magic C++11 type 'std::initializer_list'.
Converted from it is a temporary object of type 'std::array<bool, 100>'
The "conversion" is same as initialization as aggregate and then it
is assigned to 'bArray'. Actually compiler likely optimizes all the
temporaries out.

Bob Langelaan

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Mar 16, 2016, 7:14:15 PM3/16/16

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Thanks! I think I have it now :)