The defined? Operator in Ruby

The defined? Operator

To test whether the given expression has its operand defined or not, the defined? Operator is used. It is a special unary operator. Generally, when the expression is not defined, it raises an error. But, using this operator, it returns NIL if the expression or method used on the right side of the defined? is undefined.

Defined? operator also returns NIL if the expression uses YIELD or SUPER in the wrong context, i.e., when there are no such blocks to yield to or no superclass method to invoke. It is of very low precedence.

Syntax:

# Compute f(x), however only if f and x are both defined
y = f(x) if defined? f(x)

If the given expression is correct, it returns a string. The various values it can return-

Example:

c =  7
Width = 500
puts ("The defined? operator in Ruby")
puts defined? c                       # "local-variable"
puts defined? 77                    # "expression"
puts defined? N                      # nil
puts defined? puts                 # "method"
puts defined? Width              # "constant"
puts defined? $&                    # nil
puts defined? $_                     # "global-variable"
puts defined? Math::PI           # "constant"
puts defined?( l = 22 )            # "assignment"
puts defined? 72.abs              # "method"

Output:

The defined? operator in Ruby
local-variable
expression

method
constant

global-variable
constant
assignment
method

Frequently Asked Questions

Is the expression passed to the defined? operator evaluated?

No, the expression is not evaluated. It is only used it check if it can run without errors.

What does it mean by non-associative?

The operator can’t be used multiple times in an expression using the parentheses.

Why does the unary operator uses +@ and -@?

It uses this symbol to avoid ambiguity with the binary operator having the same symbol.

Know more about Unary operator overloading in c++ in detail here.

Conclusion

In this article on Ruby, we have extensively discussed the defined? operator. We started with a brief introduction of the operators, their important terms, how to use them, and the defined? operator along with an example.

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