A function declared with the **`const` keyword** is a constant function. Constant functions are popular due to their ability to prevent accidental changes to the object’s value.
## Syntax


The syntax of the `const` function is as follows:
```
int getValue() const 
```

## Code
Following is the sample code for the `const` function.
In the code below, `getValue()` is the constant function, `s` is a `const` object, and `s1` is a non- `const` object:

#include<iostream>
using namespace std;
class sample {
   int val;
   public:
   sample(int x = 0) {
      val = x;
   }
   int getValue() const {
      return val;
   }
};
int main() {
   const sample s(20);
   sample s1(2);
   cout << "The value using object d : " << s.getValue();
   cout << "\nThe value using object d1 : " << s1.getValue();
   return 0;
}

`const` member functions have the following use cases: 
* A `const` function can be called by either a `const` or non-`const` object.
* Only a non-`const` object can call a non-`const` function; a `const` object cannot call it.


#include<iostream>
using namespace std;
class sample {
   int val;
   
   public:
   sample(int x = 0) {
      val = x;
   }
   int getValue() const {
      return val;
   }
   int getValue1() {
      return val;
   }
};

int main() {
   const sample s(20);
   sample s1(2);
   
   cout << "The const function called by const object s : " << s.getValue();
   cout << "\nThe const function called by non-const object s1 : " << s1.getValue();
   cout << "\nThe non-const function called by non-const object s1 : " << s1.getValue1();
   //cout << "\nThe non-const function called by const object s : " << s.getValue1(); //Gives compiler error because
   return 0;
}

## Explanation
In the code above, the function `getvalue()` is a `const` member function, whereas  `getvalue1()` is a non-`const` member function. Moreover, `s` is a `const`, whereas `s1` is a non-`const` object. 

* There is no error when `s` calls `getValue()`, but when `s` calls  `getValue1()`, the compiler gives an error, as shown below. 
* There is no error when `s1` calls either `getValue()`or `getValue1()`.


> **Note**:  We can uncomment the code in line 25 in the `main` function to check the following compiler error due to the constant object calling a non-constant member function.

What are const functions in C++?

The `const` function

Syntax

Code

`const` function use cases

Explanation