Trying to understand the purpose of memset

[hashbrown]

I'm looking at Matrix addition samples and in one of them I noticed the use of memset in the Identity matrix function:

// Class Matrix4x4
void Matrix4x4::Identity()
{
	memset(this, 0, sizeof(Matrix4x4)); // Why?

	m[0][0] = m[1][1] = m[2][2] = m[3][3] = 1.0f;
}

The simplest example of memset I found was at MSDN:

   char buffer[] = "This is a test of the memset function";

   printf( "Before: %s\n", buffer );
   memset( buffer, '*', 4 );
   printf( "After:  %s\n", buffer );

I understand what happened, and what happens if I change the last argument of memset, more characters in the buffer will get replaced.

What I don't understand is the purpose of memset, what are the benefits of using it, and most importantly how is it being used in this Identity Matrix function.

I especially don't understand the use of this. I know it's saving to this (current class), but where? The MSDN buffer sample was easy, I'm lost when saving to a class object.

I'm sure my understanding of memset is very incomplete too.

[48bpfSEw]

http://www.cplusplus.com/reference/cstring/memset/?kw=memset

It is a trick no 7 to initialize all attributes of the class, but I would prefer not to do so.

void Matrix4x4::Identity() {
	memset(this, 0, sizeof(Matrix4x4));
}

I would rather initialize them on a clear way:

  
  this->attributename1 = "";   // if it's a string;
  this->attributename2 = 0.0; // if it's a double
  etc.

[hashbrown]

Oh that's all it is? I thought there was some performance gain. Thanks.

One last question, how do I know what memset is going to initialize when I throw in a class like that, how does it choose what to initialize?

I'm guessing that's what the third argument is for.

[48bpfSEw]

this points to the memory where your current instance of the class is.
sizeof(Matrix4x4) returns the size of the structure. that is all what memset needs to start its job: overwritting the memory from the position "this" with the amount delivert from sizeof with a specific value e.g. 0


it is the same like :

  for (int i=0; i<iSize; i++) 
    (*((char*)this)+i)) = 0;
  

isn't it ugly? ^^

typeconversion is a special topic in c++
don't convert your types like I did above!

[hashbrown]

Perfect, now I got it, thanks @Necip.

It's actually a very convenient tool, at least for Vectors, Matrices, etc. One line and everything is instantiated at 0.