typeof in C23

Introduction

Among other things, C23 added the typeof and typeof_unqual keywords that return the type of an expression:

int x;        // int (obviously)
typeof(x) y;  // also int

Note: for this article, I’ll simply use typeof to mean either typeof or typeof_unqual rather than always repeating “or typeof_unqual” unless otherwise stated.

The expression is not evaluated. Like sizeof, typeof is a compile-time operator. typeof standardizes the long-existing gcc extension.

typeof is sort-of like decltype in C++. Why isn’t typeof called decltype in C also? (Or why isn’t decltype in C++ called typeof?) The short answer is that C++ has references and C doesn’t — and that affects the type deduced. There’s also the long answer.

But wait! Since C23 also added auto, why is typeof needed?

• Variables declared with typeof don’t need initializers.
• typeof can clarify complicated declarations.

Declarations without Initializers

Using auto always requires an initializer since the compiler deduces the type from the type of the initializer expression; using typeof does not:

double f(void);

auto x = f();   // double
typeof(f()) y;  // double, but without initializer

Depending on what you’re doing, you may not want to initialize upon declaration, for example if a variable is initialized only conditionally or you need to defer initialization.

Of course you still can use an initializer with typeof if you want to guarantee a type based on something other than the initializer:

typeof(f()) y = g();  // type is what f() returns, not g()

Clarifying Complicated Declarations

C in infamous for complicated declarations. Normally, complicated declarations can be simplified via typedef. However, typeof can also be used since it can also take a type instead of an expression:

int *p1, *q1;                   // both pointer to int
typeof(int*) p2, q2;            // same

// array 4 of pointer to function (int) returning double
double (*a1[4])(int);
typeof(double(int)) *a2[4];     // same

// function (void) returning ...
// ... pointer to function (int) returning double
double (*f1(void))(int);
typeof(double(int))* f2(void);  // same

While the typeof declarations are longer, they’re much clearer.

The ability of typeof to take either an expression or a type parallels the ability of sizeof to do the same.

You can even declare macros to create an entirely alternate declaration syntax:

#define ARRAY_OF(T, N)  typeof(T[N])
#define POINTER_TO(T)   typeof(T*)

ARRAY_OF(POINTER_TO(char const), 4) pc;  // char const *pc[4]

typeof_unqual

The difference between typeof and typeof_unqual is that the latter removes all top-level qualifiers (_Atomic, const, restrict, and volatile):

extern int i;
extern int const ci;
extern int *pi;
extern int const *pci;
extern int *const cpi;
extern int const *const cpci;

typeof       (i)    i2;      // int
typeof_unqual(i)    i2u;     // int
typeof       (ci)   ci2;     // int const
typeof_unqual(ci)   ci2u;    // int
typeof       (pi)   pi2;     // int *
typeof_unqual(pci)  pci2u;   // int const*
typeof       (cpi)  cpi2;    // int *const
typeof_unqual(cpi)  cpi2u;   // int *
typeof       (cpci) cpci2;   // int const *const
typeof_unqual(cpci) cpci2u;  // int const *

Conclusion

typeof is complementary to auto as an addition to C for writing type-agnostic code or simplifying complicated declarations.