Notes on memset

1. memset

  void * memset ( void *pMemoryBlock, int byte, size_t numOfByte );

2. Typical using senarios

• Bytewise memory initialization
• Zero a block of memory
• Zero a POD struct

3. Test

#include<vector>
#include<string>
#include<algorithm>
#include<cstring>

using namespace std;

struct C {
 char a;
 short *b;
 int c[10];
};

struct CPP {
 string a;
 CPP& operator=(const CPP &r);
};

CPP& CPP::operator=(const CPP &r) {
 if (this != &r) {
  (*this).a = r.a;
 }
 return *this;
}

char achar[3];
int aint[3];

C s;
C as[3];

CPP c;
CPP ac[3];

int main (){
 // 1. Initialize a block of memory using a given byte
 memset(achar, 'a', 3);
 
 // 2. Quickly zero a block of memory
 memset(achar, 0, 3);
 memset(achar, 0, sizeof achar);
 
 memset(aint, 0, 3 * sizeof(int));
 memset(aint, 0, sizeof aint);
 
 memset(as, 0, 3 * sizeof as[0]); 
 memset(as, 0, 3 * sizeof (struct C));
 
 // 3. Reset POD struct
 memset(&s, 0, sizeof(struct C));
 
 // Caution 1. memset works byte by byte only
 memset(aint, 1, 3 * sizeof(int)); // aint : 16843009, 16843009, 16843009
 
 // Caution 2. Be careful with pointer in struct
 s.b = new short;
 memset(&s, 0, sizeof s); // address of b : 0
 
 // Caution 3. POD only
 //memset(&c, 0, sizeof(CPP)); // crash!!!
 
 // Either define a reseting member function, or
 // use fill() and '=' overriding
 c.a = "N/A";
 fill(ac, ac+3, c);  // ac : "N/A", "N/A", "N/A"
 
 return 0;
}

 (code test on g++ 4.6.2)