What is sizeof() in C++?

Key takeaways:

  • sizeof() is an operator in C++ that returns the size of its operand in bytes.

  • It can be applied to variables, types, structures, or classes.

  • The return type of sizeof() is size_t, an unsigned integer type.

  • Useful for determining the size of arrays, user-defined types, and pointers (size of the pointer itself, not the pointed data).

  • For arrays, dividing the total size by the element size gives the number of elements.

The sizeof() operator in C++ is used to calculate the size of a parameter passed to it in bytes. The passed parameter can be a variable, structure, class, or a datatype. With the use of sizeof() in programs, developers can determine the amount of memory occupied within the system by different objects, variables, etc.

The sizeof() operator in C++ returns the size of its operand in bytes. The operand can be a variable, a type (like int, double), a user-defined type (like classes or structures), or expressions. sizeof() is not a function but an operator in C++.

Syntax for sizeof()

Here is the syntax of the sizeof() function in C++:

sizeof(parameter)

The sizeof() operator returns a value of type size_t, which is an unsigned integer type that represents the size of any object in bytes.

Use cases for using sizeof()

Here are a few use cases of the sizeof() function:

  • We can use sizeof() to find the length of an array.

  • sizeof() can be used to determine the size of user-defined data structures, like structs or classes.

  • sizeof() can be used to determine the size of a pointer itself, but not the size of the memory it points to. In other words, sizeof(ptr) gives the size of the pointer (usually 4 or 8 bytes, depending on the architecture), not the size of the data being pointed to.

Code example

Here is a comprehensive example to understand the sizeof() function:

#include <iostream>
using namespace std;
struct Employee{
  int Id;
  string Name;
  bool Gender;
  void size_of_each_variable()
  {
    cout<<"Size of Id (int): "<<sizeof(Id)<<endl;
    cout<<"Size of Name (string): "<<sizeof(Name)<<endl;
    cout<<"Size of Gender (bool: "<<sizeof(Gender)<<endl;
  }
};
int main() {
  Employee E;
  size_t e_size;
  e_size = sizeof(E);
  cout<<"Size of Employee (User defined datatype): "<<e_size<<endl;
  E.size_of_each_variable();
  return 0;
}

Code explanation

Let’s try to understand what’s happening in the code:

  • Lines 4–7: Create a class Employee with int, bool and string variables.

  • Lines 11–13: Here we call the sizeof() function.

  • Lines 18–19: Declare e_size variable, a special type of integer(size_t), and storing the size of the object E.

  • Lines 20–21: Print the total size of the object E and the size of each of its member variables by calling its function size_of_each_variable().

Conclusion

In conclusion, sizeof() is a powerful tool for memory-related operations in C++, enabling developers to calculate the size of various data types and objects, ensuring efficient memory management and program optimization.

Frequently asked questions

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What is the difference between `sizeof()` and `size()` in C++?

  • sizeof(): Returns the size of a type or object in bytes.
  • size(): Returns the number of elements in containers like std::vector or std::string.

What type does `sizeof()` return in C++?

sizeof() returns size_t, an unsigned integer type.

What is the alternative to `sizeof()` in C++?

Use size() for containers or std::size() (C++17) for raw arrays to get the number of elements.

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