Typeof Operator in C
Is Typeof in C, Really an Operator? I'm Thinking Because There Is No Polymorphism in C, That There Is Nothing to Do at Run-Time. That Is, the Answer to Typeof...
Is typeof in C, really an operator?
I'm thinking because there is no polymorphism in C, that there is nothing to do at run-time. That is, the answer to typeof is known at compile-time. (I can't think of a use of typeof that would not be known at compile time.) So it appears to be more of a compile-time directive, than an operator.
Does typeof use any (processor) run-time (in GCC)?
5 Answers
Since typeof is a compiler extension, there is not really a definition for it, but in the tradition of C it would be an operator, e.g sizeof and _Alignof are also seen as an operators.
And you are mistaken, C has dynamic types that are only determined at run time: variable modified (VM) types.
size_t n = strtoull(argv[1], 0, 0);
double A[n][n];
typeof(A) B;
can only be determined at run time.
Add on in 2021: there are good chances that typeof with similar rules as for sizeof will make it into C23.
It's a GNU extension. In a nutshell it's a convenient way to declare an object having the same type as another. For example:
int x; /* Plain old int variable. */
typeof(x) y; /* Same type as x. Plain old int variable. */
It works entirely at compile-time and it's primarily used in macros. One famous example of macro relying on typeof is container_of.
It is a C extension from the GCC compiler , see
It's not exactly an operator, rather a keyword. And no, it doesn't do any runtime-magic.
One case in which you will need to use this extension is to get the pointer of an anonymous structs. You can check this this question for an example.
David Miller brings 15 years of experience in global economics, personal finance strategy, and market dynamics. He specializes in turning complex economic trends into actionable insights for everyday readers.
