C++ Memory Management: What are the differences between constant objects?

Q: What are the differences between constant objects?

A: To make any object constant, one uses the keyword 'const'. However, depending on the actual place you insert this keyword, it can have different meanings for the associated object:
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• Reference to object -> Can change the referenced object
  
  
  Code:

  ---

  foo instance;
foo& reference_to_instance = instance;

  ---

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• Reference to constant object -> Can not change the referenced object
  
  
  Code:

  ---

  foo instance;
const foo& constant_reference_to_instance = instance;

  ---

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• Pointer to object -> Can change the adress it is pointing to as well as the object
  
  
  Code:

  ---

  foo instance;
foo* pointer_to_instance = &instance;

  ---

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• Pointer to constant object -> Can change the adress it is pointing to but not the object
  
  
  Code:

  ---

  foo instance;
const foo* pointer_to_constant_instance = &instance;

  ---

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• Constant pointer -> Can change the object but no the address it is pointing to
  
  
  Code:

  ---

  foo instance;
foo* const constant_pointer_to_instance = &instance;

  ---

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• Constant pointer to constant object -> Can neither change the address nor the object
  
  
  Code:

  ---

  foo instance;
const foo* const constant_pointer_to_constant_instance = &instance;

  ---

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