Alignof operator in C++

C++ alignof() Operator

The alignof operator is the operator that returns the memory alignment for a given variable type. This alignment tells how the data is arranged and accessed in memory. It returns the value in bytes and is the part of the <type_traits> header file in C++. which is mainly used for low-level programming like memory management or hardware requirements. 

Syntax

Here is the following syntax of alignof operator, which returns memory alignment required for a given data type in bytes:

alignof(data_type)

Example Demonstrating Usage of alignof() Operator

Here is the following example code of using alignof() operator for built-in data types in C++, which is returning memory alignment in bytes.

#include <iostream>
using namespace std;
int main(){
   cout<<"Memory alignment of char: "<<alignof(char)<< endl;
   cout<<"Memory alignment of int: "<<alignof(int)<<endl;
   cout<<"Memory alignment of float: "<<alignof(float)<< endl;
   cout<<"Memory alignment of double: "<<alignof(double)<< endl;
   cout<<"Memory alignment of pointer: "<<alignof(int*)<< endl;
   return 0;
}

Output

Memory alignment of char: 1
Memory alignment of int: 4
Memory alignment of float: 4
Memory alignment of double: 8
Memory alignment of pointer: 8

Using alignof() Operator with Arrays and User-Defined Structures

In the following example, we are checking the memory alignment of user-defined data types like structures and arrays in C++.

#include <iostream>
using namespace std;
// a user-defined structure with members of different types
struct basic {
   int i;
   float f;
   char s;
};
// an empty structure
struct Empty {
};
int main(){
   cout<<"Alignment of character array of 10 elements: "<<alignof(char[10])<<endl;
   cout<<"Alignment of integer array of 10 elements: "<<alignof(int[10])<<endl;
   cout<<"Alignment of float array of 10 elements: "<<alignof(float[10])<<endl;
   cout<<"Alignment of class basic: "<<alignof(basic)<<endl;
   cout<<"Alignment of Empty class: "<<alignof(Empty)<<endl;
   return 0;
}

Output

Alignment of character array of 10 elements: 1
Alignment of integer array of 10 elements: 4
Alignment of float array of 10 elements: 4
Alignment of class basic: 4
Alignment of Empty class: 1

C++ sizeof vs alignof Operators 

The sizeof operator returns the size of a variable or data type, telling how much memory it occupies whereas the alignof operator returns the alignment required for a given data-type, telling the boundary on which it stores in memory (for CPU efficiency).

Example

Here is the following example code showcasing the difference between sizeof and alignof operator in C++.

#include <iostream>
using namespace std;

// a structure with possibility padding
struct my_struct {
    char a;     // 1 byte
    int b;      // 4 bytes (here compiler adds padding after 'a')
};

int main() {
    // Single structure
    cout << "Size of my_struct: " << sizeof(my_struct) << " bytes" << endl;
    cout << "Alignment of my_struct: " << alignof(my_struct) << " bytes" << endl;

    // Array of structures
    my_struct arr[5]; 
    cout << "Size of my_struct[5]: " << sizeof(arr) << " bytes" << endl;
    cout << "Alignment of my_struct[5]: " << alignof(decltype(arr)) << " bytes" << endl;

    return 0;
}

Output

Size of my_struct: 8 bytes
Alignment of my_struct: 4 bytes
Size of my_struct[5]: 40 bytes
Alignment of my_struct[5]: 4 bytes

Explanation

Inside my_struct, the members are char and int, which require 1 and 4 bytes respectively. So, the actual size is 1 + 4 = 5 bytes, but because of alignment the compiler adds 3 bytes of padding between char and int, making the total size equal to 8 bytes. 

Why is 3 bytes of padding added?

Because char has a 1-byte alignment (which can be placed anywhere) but for int, it should be started from the position of multiple of 4 (as 4-byte alignment).
Therefore after char (1 byte) to align int, the compiler adds 3 bytes of padding more.
whereas alignof returns the alignment required, as in this case (comparing char vs int) int is the most strictly aligned member (as it strictly needs 4-byte alignment) therefore alignof operator will return 4 byte, saying that given struct must be aligned on a 4-byte boundary.

Similarly, inside an array of 5 structures, each structure is of 8 bytes (because of padding), therefore total size of total memory occupies 5 × 8 = 40 bytes.
Whereas alignof operator will return alignment strictly followed by struct, which will again equal to int (4 bytes) (when comparing both int and char).

Comparison Table