When is `(type)value` Different from `type(value)` in C ?

Clarifying the Distinction Between (type)value and type(value)

In C , the expressions (type)value and type(value) might appear interchangeable, but there is a subtle difference to consider. Specifically, the operator (type) can be used to cast a value to the specified type, whereas type(value) is equivalent to a constructor invocation only when a list of values is provided.

Per the C standard, a simple-type-specifier (type) followed by a parenthesized expression-list creates a value of the specified type based on the given expression list. If there is only a single expression, both (type)value and type(value) function as cast expressions.

However, a comma-separated list of values in the expression introduces a distinction. In this case, type(value) becomes equivalent to a constructor invocation. A temporary variable is implicitly declared, and the expression effectively initializes that variable using the provided values. The result is the rvalue of the temporary variable.

While most types can be used interchangeably in both (type)value and type(value) forms, certain type names may not work for the latter syntax. For example, using the type name char * in type(value) will result in a compilation error, while (type)value works fine. In such cases, using a typedef to define an alternative type name can allow for the use of type(value) syntax.

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