better method for initializing array

Question

Looks a newbie question, but this function is called many times, to be honest thousands of time per-second, so an optimization is CRUCIAL in here. What would be the best method?

struct CHOLDELEM
{
    DWORD dwColor[3], dwItemId[3];
    int nPos[3], nLength[3];
    CItemElem* pItem[3];
    CHOLDELEM()
    {
        for( int i=0; i<=3; i++ )
        {
            dwColor[i] = dwItemId[i] = 0;
            nPos[i] = nLength[i] = 0;
            pItem[i] = NULL;
        }
    }
};

or with memsets?

memset( dwColor, 0, sizeof( dwColor ) );

or another method.

ooks a newbie question, but this function is called many times, to be honest thousands of time per-second, so an optimization is CRUCIAL in here. What would be the best method? -- I strongly doubt this. Being called often != should optimize - please profile to demonstrate that this strip of code is actually slowing your program down. — djechlin
– djechlin, Commented Jul 30, 2012 at 17:50
The only thing that can tell you the answer for sure is your profiler. — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Jul 30, 2012 at 17:50
You could speed it up slightly by not writing beyond the end of each array. — Mike Seymour
– Mike Seymour, Commented Jul 30, 2012 at 17:52
DWORD dwColor[3]; declares an array with 3 valid indexes, namely 0, 1 and 2. There is no index 3. Trying to access dwColor[3] is forbidden by the language; it yields undefined behavior (read: anything could happen). Thus, your loop condition should be i < 3, not i <= 3. — fredoverflow
– fredoverflow, Commented Jul 30, 2012 at 18:10

AnT stands with Russia · Accepted Answer · 2012-07-30 17:55:02Z

4

As long as you are interested in zero initialization only, you can simply do

CHOLDELEM() : dwColor(), dwItemId(), nPos(), nLength(), pItem()
  {}

(no C++11 necessary).

However, you might want to take a look at the code your compiler generates for it. If it is not optimal somehow, then a better idea might be to keep your struct a POD (no constructor) and initialize it "from outside" when you declare objects of that type

CHOLDELEM c = {};

answered Jul 30, 2012 at 17:55

AnT stands with Russia

323k44 gold badges548 silver badges793 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Some programmer dude · Accepted Answer · 2012-07-30 17:52:14Z

1

If your compiler can handle C++11 initializers, then you can set the array values in the constructor initializer list:

CHOLDELEM() :
    dwColor{0}, dwItemId{0}, nPos{0}, nLength{0}, pItem{nullptr}
    { }

Then the compiler will generate (pretty optimal) code to handle that.

answered Jul 30, 2012 at 17:52

Some programmer dude

411k36 gold badges420 silver badges655 bronze badges

Comments

fredoverflow · Accepted Answer · 2012-07-30 18:04:34Z

1

I would probably use the memset approach, but I would definitely want to make sure that it doesn't break when more complex data members are added to CHOLDELEM:

#include <type_traits>

// ...

    CHOLDELEM()
    {
        static_assert(std::is_trivially_copyable<CHOLDELEM>::value,
                      "It is no longer safe to use memset!");
        memset(this, 0, sizeof *this);
    }

By the way, CHOLDELEM is a terrible name. Why don't you rename it to ElementHolder or something?

answered Jul 30, 2012 at 18:04

fredoverflow

265k101 gold badges403 silver badges680 bronze badges

2 Comments

Vinícius Over a year ago

all structs god knows why start with a C and caps lock on this source, I'm just following...

fredoverflow Over a year ago

What do you mean "all structs"? All structs your teacher uses?

Collectives™ on Stack Overflow

better method for initializing array

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related