memset in C++

Converts the value ch to unsigned char and copies it into each of the first n characters of the object pointed to by str[]. If the object is not trivially-copyable (e.g., scalar, array, or a C-compatible struct), the behavior is undefined. If n is greater than the size of the object pointed to by str, the behavior is undefined.

Template

void* memset( void* str, int ch, size_t n);
Parameters
str[] : Pointer to the object to copy the character.
ch : The character to copy.
n : Number of bytes to copy.
Return value :
The memset() function returns str, the pointer to the destination string.
// CPP program to illustrate memset
#include <cstring>
#include <iostream>

using namespace std;

int main()
{
    char str[] = "geeksforgeeks";
    memset(str, 't', sizeof(str));
    cout << str;
    return 0;
}

Output:

tttttttttttttt

We can use memset() to set all values as 0 or -1 for integral data types also. It will not work if we use it to set as other values. The reason is simple, memset works byte by byte.

#include <bits/stdc++.h>
using namespace std;
 
int main()
{
    int a[5];
 
    // all elements of A are zero
    memset(a, 0, sizeof(a));
    for (int i = 0; i < 5; i++)
        cout << a[i] << " ";
    cout << endl;
 
    // all elements of A are -1
    memset(a, -1, sizeof(a));
    for (int i = 0; i < 5; i++)
        cout << a[i] << " ";
    cout << endl;
 
    // Would not work
    memset(a, 5, sizeof(a)); // WRONG
    for (int i = 0; i < 5; i++)
        cout << a[i] << " ";
}

Output:

0 0 0 0 0 
-1 -1 -1 -1 -1 
84215045 84215045 84215045 84215045 84215045

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