Question

Is there a way to print the value of a constexpr or #defined value at compile time? I want the equivalent of std::cout <<, or some way to do something like

constexpr int PI_INT = 4;
static_assert(PI_INT == 3,
              const_str_join("PI_INT must be 3, not ", const_int_to_str(PI_INT)));

Edit: I can do some basic compile-time printing with constexprs, at least on gcc by doing something like

template <int v>
struct display_non_zero_int_value;

template <>
struct display_non_zero_int_value<0> { static constexpr bool foo = true; };

static constexpr int v = 1;

static_assert(v == 0 && display_non_zero_int_value<v>::foo, "v == 0");

which gives me error: incomplete type ‘display_non_zero_int_value<1>’ used in nested name specifier static_assert(v == 0 && display_non_zero_int_value<v>::foo, "v == 0");. (icpc, on the other hand, is less helpful, and just says error: incomplete type is not allowed) Is there a way to write a macro that can generalize this so that I can do something like

constexpr int PI_INT = 4;
PRINT_VALUE(PI_INT)

and get an error message that involves 4, somehow?

Answer 1

Quoting the grammar given for declarations in §7/1 [dcl.dcl]:

static_assert-declaration:

static_assert ( constant-expression , string-literal ) ;

The standard says it has to be a string literal, so you’re out of luck; you can’t use a constexpr function to construct your error message.

You can, however, use whatever preprocessor magic you like to generate a string literal to go in there. If PI_INT is a #define instead of a constexpr int, you could use something like this:

#define PI_INT 4
#define pi_err_str_(x) #x
#define pi_err_str(x) pi_err_str_(x)
#define pi_int_err "PI_INT must be 3, not " pi_err_str(PI_INT)

static_assert(PI_INT == 3, pi_int_err);

output:

error: static assertion failed: "PI_INT must be 3, not 4"

---

Edit in response to comment by OP and updated question

Is there a way to write a macro that can generalize this so that I can do something like … and get an error message that involves 4, somehow?

Sure, a bit of preprocessor magic can generalize that, assuming you’re happy to be reliant on compiler-specific error message behaviour:

#define strcat_(x, y) x ## y
#define strcat(x, y) strcat_(x, y)
#define PRINT_VALUE(x) template <int> struct strcat(strcat(value_of_, x), _is); static_assert(strcat(strcat(value_of_, x), _is)<x>::x, "");

constexpr int PI_INT = 4;
PRINT_VALUE(PI_INT)

stackoverflow/13465334.cpp:20:1: error: incomplete type ‘value_of_PI_INT_is<4>’ used in nested name specifier

As for other compilers, I don’t know what you can do offhand, but you may want to look at a copy of boost’s static_assert.hpp to see if any of the tricks employed there can be used to get an evaluated template arg printed.