lnt-make-member-function-const

When a member function doesn’t modify the object's state, annotate it with the const keyword. This guidance comes from the C++ Core Guideline Con.2.

Example

The linter flags the following code twice because getValue() and getRadius() don't modify the object's state:

class MyClass
{ 
public: 

  int getValue() { return value; }  // Flagged: ‘getValue’ doesn't modify the object's state. 
  void setValue(int newValue) { value = newValue; }  // OK: ‘setValue’ modifies the object's state.   

private: 

  int value = 42; 
}; 

double getRadius()
{   // Flagged: ‘getRadius’ doesn't modify the object's state. 
    return radius; 
} 

How to fix the issue

Mark member functions const when they don't modify the object's state. This lets readers of the code and the compiler know that the function is safe to call on a const object.

In the following example, const has been added to getValue() and getRadius():

class MyClass
{ 
public: 

  int getValue() const { return value; }  // Added const 
  void setValue(int newValue) { value = newValue; }  // OK: ‘setValue’ modifies the object's state.   

private: 

  int value = 42; 

}; 

double getRadius() const // added const
{   // ‘getRadius’ doesn't modify the object's state. 
    return radius; 
}  

The editor can make this change for you. Place the cursor on the flagged symbol and choose Show potential fixes and then Make member const:

The cursor is on the line int getValue() and **Show potential fixes** appeared and was chosen. Now **Make member const** is visible and it shows the get value function with const added to it. You can now choose **Make member const** to make the change.

Make this change for all flagged member functions.

Remarks

Introduced in Visual Studio 2022 17.8, this check focuses on const usage for member functions in C++ code. The C++ Core Guidelines recommends marking member functions as const when they don't modify the object's state.

The current implementation of this check allows you to add const to member functions after their declaration. It's a good practice to declare member functions as const from the beginning if they don't modify the object's state.

See also

IntelliSense code linter for C++ overview