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A constant method differes from all other methods by the fact that it is not allowed to make any changes to data members of the class it belongs to. Consider e.g. a method that basically prints out the data - it won't alter it. So that's a ideal candidate for making it 'const' and thus allow not only the compiler to make some optimizations but also make clear to others who might derive from that class that altering any data when overriding this method has to be avoided.
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A constant method differes from all other methods by the fact that it is not allowed to make any changes to data members of the class it belongs to. Consider e.g. a method that basically prints out the data - it won't alter it. So that's a ideal candidate for making it 'const' and thus allow not only the compiler to make some optimizations but also make clear to others who might derive from that class that altering any data when overriding this method has to be avoided.

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19. Why we need constant method? What is the difference of defining constant method of usual method?
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19. Why we need constant method? What is the difference of defining constant method of usual method?

C++ is an intermediate-level general-purpose programming language, not to be confused with C or C#. It was developed as a set of extensions to the C programming language to improve type-safety and add support for automatic resource management, object-orientation, generic programming, and exception handling, among other features.