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Visibility is a major feature of Object-Oriented Programming (OOP) allowing architects and engineers to design rules covering the visibility of an Object's state variables and operations to other objects within the application or system.

for instance to prevent a certain variable to be modified from outside the class. The default visibility is public, which means that the class members can be accessed from anywhere. This means that declaring the visibility is optional, since it will just fall back to public if there is no access modifier. For backwards compatibility, the old way of declaring a class variable, where you would prefix the variable name with the “var” keyword (this is from PHP 4 and should not be used anymore) will also default to public visibility.

visibility of a class member variable or member function determines from where that variable can be accessed/modified, or from where the member function can be called.

Note: Visibility is generally implemented by adding labels to the member variables or member operations.

See the following on C++ and visibility¹⁾.

// C++ implementation to show
// Visibility modes
  
#include <bits/stdc++.h>
using namespace std;
  
class BaseClass 
{ //===== Public
  public:
    int x;
      
  //===== Protected
  protected:
    int y;
      
  //===== Private
  private:
    int z;

}; // End BaseClass
  
// DerivedClass inherits from BaseClass
class DerivedClass : public BaseClass 
{
}; // End DerivedClass
  
// main function
int main()
{
    DerivedClass derivedClass;
  
    // x is labeled public, therefore it's value is displayed in the standard output stream
    cout << derivedClass.x << endl;
  
    // y is labeled protected, therefore the compiler displays a visibility error
    cout << derivedClass.y << endl;
  
    // z is labeled private within BaseClass, therefore the compiler displays a visibility error
    cout << derivedClass.z << endl;
 
}; // End main

Note: Often the coding styles specify the attributes with a particular label be listed together (i.e., all public state variables listed together, etc.) and they are listed alphabetically within the group. The Labels are listed from least restrictive to most restrictive (i.e., public, protected, and private)