Mutable Variables

In C++, mutable variables are variables that can be modified even within constant functions, useful for managing internal state flexibly.

Syntax

To declare a variable as mutable, the mutable keyword needs to be placed before the variable type:

mutable type name;

• type: The type of the variable (e.g., int, char).
• name: The name of the variable.

Example

In the example below, accessCount is marked mutable, allowing it to be modified within the constant displayData() function:

#include <iostream>
#include <string>

class Data {
public:
  Data(std::string value) : data(value), accessCount(0) {}

  void displayData() const {
    ++accessCount; // Modification allowed due to 'mutable'
    std::cout << "Data: " << data << ", Access count: " << accessCount << std::endl;
  }

private:
  std::string data;
  mutable int accessCount; // Can be modified in constant methods
};

int main() {
  Data d("Sample");
  d.displayData();
  d.displayData();
  return 0;
}

The above code produces the following output:

Data: Sample, Access count: 1
Data: Sample, Access count: 2

Here, even though displayData() is a constant member function, accessCount can be incremented due to its mutable declaration.

Codebyte Example

The following codebyte example demonstrates the usage of mutable variables:

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