alignof operator in C++

The alignof operator was introduced in C++11 to determine the alignment requirement of a type in memory. It returns the number of bytes by which objects of a particular type must be aligned for efficient memory access.

• Returns the alignment requirement of a type in bytes
• The returned value is typically of type std::size_t
• Works with built-in types, arrays, references, classes, and structures

#include <iostream>
using namespace std;

struct Geeks {
    int i;
    float f;
    char s;
};

struct Empty {
};

// driver code
int main()
{
    cout << "Alignment of char: " << alignof(char) << endl;
    cout << "Alignment of pointer: " << alignof(int*) << endl;
    cout << "Alignment of float: " << alignof(float) << endl;
    cout << "Alignment of class Geeks: " << alignof(Geeks) << endl;
    cout << "Alignment of Empty class: " << alignof(Empty) << endl;

    return 0;
}

Alignment of char: 1
Alignment of pointer: 8
Alignment of float: 4
Alignment of class Geeks: 4
Alignment of Empty class: 1

Explanation

• The alignof operator returns the alignment requirement of a type in bytes.
• char, float, and int* have alignment requirements of 1, 4, and 8 bytes respectively (on most 64-bit systems).
• For the Geeks structure, the compiler uses the largest alignment requirement among its members, which is 4 bytes.
• Therefore, alignof(Geeks) returns 4.

Syntax

alignof(type)

Parameters: The type can be:

• A complete type (such as int, float, or a user-defined type)
• An array type, in which case the alignment requirement of the element type is returned
• A reference type, in which case the alignment requirement of the referenced type is returned

Return Value: The alignof operator returns the alignment requirement of the specified type in bytes. The returned value is of type std::size_t.

Difference Between alignof and sizeof

The alignof operator returns the alignment requirement of a type, while the sizeof operator returns the amount of memory occupied by a type or object.

• alignof determines how data should be aligned in memory.
• sizeof determines how many bytes a type occupies.
• For fundamental types, both values are often the same.
• For structures and classes, the values can differ due to padding and alignment requirements.

Example:

struct S {
    int a;
    double b;
};

// alignof(S) == 8

• The alignment requirement of S is 8 bytes.
• This is because double has the strictest alignment requirement among the structure members.

Example: alignof vs sizeof

#include <iostream>
using namespace std;

struct Geeks {
    int i;
    float f;
    char s;
};

int main()
{

    cout << "alignment of Geeks : " << alignof(Geeks) << '\n';
    cout << "sizeof of Geeks : " << sizeof(Geeks) << '\n';
    cout << "alignment of int : " << alignof(int) << '\n';
    cout << "sizeof of int     : " << sizeof(int) << '\n';
}

alignment of Geeks : 4
sizeof of Geeks : 12
alignment of int : 4
sizeof of int     : 4

Explanation

• alignof(Geeks) returns 4, which is the alignment requirement of the structure.
• sizeof(Geeks) returns 12, which is the total memory occupied by the structure.
• alignof(int) and sizeof(int) both return 4.
• This demonstrates that alignment requirements and memory size are not always the same.