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• size of a bit-field member of a structure
• size of an array (except in the case of variable length arrays)
• value of an enumeration constant
• value of a case constant

If any of these are required, then an integer constant (which would be an rvalue) must be used.

const-qualified objects allows the programmer to take the address of the object.

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Also, const-qualified integers cannot be used in locations where an integer constant is required, such as the value of a case constant.

const:

• operates at run time
• consumes memory (though this is not too important)
• can't use in compile-time constant expression
• uses consistent syntax
• can create pointers to
• does type checking

Enumeration Constants

An enumeration constant is a member of an enumeration. Enumeration constant can be used to represent an integer constant expression that has a value representable as an int. Unlike const-qualified objects, enumeration constants do not require that storage is allocated for the value so it is not possible to take the address of an enumeration constant.

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• operates at compile time
• consumes no memory (though this is not too important)
• can use in compile-time constant expression
• uses different syntax; can make mistake with ;
• can't create pointers to
• no type checking

Summary

const:

• operates at run time
• consumes memory (though this is not too important)
• can't use in compile-time constant expression
• uses consistent syntax
• can create pointers to
• does type checking

If any of these are required, then an integer constant (which would be an rvalue) must be usedThe following table summarizes some of the differences between const-qualified objects, enumeration constants, and object-like macro definitions.

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