Memset in C++

Memset() is a C++ function that copies a single character for a specified number of times to the given bytes of memory. It is useful for filling a number of bytes with a given value starting from a specific memory location. It is defined in <cstring> header file.

Syntax of std::memset() in C++

memset(str,ch, n);

Parameters

• str[]: Pointer to the object to copy the character.
• ch: The character to copy. It can be a character, a normal value as well a Boolean value.
• num: Number of bytes to copy.

Return Value

• The memset() function returns str, the pointer to the destination string.

std::memset() 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 behaviour is undefined. 

Examples of memset()

Example 1: Demonstrating the simple use of std::memset() function for Array Initialization

// C++ program to demonstrate memset
#include <cstring>
#include <iostream>
using namespace std;

// Driver Code
int main()
{
    char str[] = "geeksforgeeks";
    // setting each byte of memory of str to 't' using
    // memset
    memset(str, 't', sizeof(str));
  
    cout << str;
  
    return 0;
}

tttttttttttttt

Time Complexity: O(N) [For traverse from begin to end of the object]
Auxiliary Space Complexity: O(1)

Note: 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.

Example 2: Program to show how to use memset to set 0 for integral data type.

// C++ Program to demonstrate that we can use memset() to
// set all values as 0 or -1 for integral data types also
#include <bits/stdc++.h>
using namespace std;

// Driver Code
int main()
{
    int a[5];

    // all elements of A are zero
    // Mainly all bits are set as 0
    memset(a, 0, sizeof(a));
    for (int i = 0; i < 5; i++)
        cout << a[i] << " ";
    cout << endl;

    // all elements of A are -1
    // Mainly all bits are set 1. Note that
    // the 2's complement representaion of -1
    // has all 1s
    memset(a, -1, sizeof(a));
    for (int i = 0; i < 5; i++)
        cout << a[i] << " ";
    cout << endl;

    // Would not work as we are setting word
    // by word (8 bits) and then accessing
    // integers (32 bits). A better way would be
    // to run a loop to set. Or use std::fill() 
    // (C++ only)
    memset(a, 5, sizeof(a)); // WRONG
    for (int i = 0; i < 5; i++)
        cout << a[i] << " ";
}

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

Example 3: Program to show how to use memset for integral data type.

// C++ Program to demonstrate that we can use memset() to
// set all values as boolean data types also
#include <bits/stdc++.h>
using namespace std;

int main()
{
    bool prime[5];
    memset(prime, true, sizeof(prime));
    // If you print without using boolalpha it will print
    // like this
    for (int i = 0; i < 5; i++)
        cout << prime[i] << " ";
    cout << "\n";
    // If you use boolalpha it will print like this
    for (int i = 0; i < 5; i++)
        cout << boolalpha << prime[i] << " ";

    return 0;
}

// This code is contributed by Susobhan Akhuli

1 1 1 1 1 
true true true true true 

Note: For boolean, the object must be of bool type for C++. Eg. bool arr[n];

Advantages of std::memset( ) function

1. It is a one-line piece of code, making it highly compact, fast and improving readability overall.
   
2. The C++ memset() function aids the programmer in solving the misalignment issue. There are instances where you discover that the processor is having trouble with data alignment, which results in a programming mistake. The memcpy() and memcmp() methods in C++ are the best options in this situation.

Limitations of std::memset() in C++

1. memset operates at the byte level, and its use is not type-safe. It treats the memory block as a sequence of bytes.
   
2. For non-POD (Plain Old Data) types in C++, such as classes with constructors, memset should be used with caution. Initializing such objects using memset can bypass constructors and lead to undefined behaviour. Instead, constructors should be used for initialization.

Memset() is an old function inherited from C. It is not very flexible when filling memory with different data types and value. The std::fill function is better alternative available in STL Algorithm library.

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Article Tags :

• C++
• CPP-Functions
• CPP-Library
• cpp-memory-management
• cpp-strings-library

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