std::format pointer error C3615: consteval function 'std::_Compile_time_parse_format_specs' cannot result in a constant expression

Question

How to fix (stdC++20 mode VS2022)

#include <format>
#include <string>

auto dump(int *p)
{
    std::string resultstring = std::format(" p points to address {:p}", p);

resulting in:

error C3615: consteval function 'std::_Compile_time_parse_format_specs' cannot result in a constant expression

I simplified the example to use a int pointer instead of a class pointer. — GeePokey, Oct 23 '22 at 02:44

score 3 · Accepted Answer · answered Oct 23 '22 at 02:16

3

Cast the pointer to void like this:

std::string resultstring = std::format("{:p}", (void *)p);

The problem is not with the format string itself, but rather that the templated type checking on the variable arguments FAILS for any non-void pointer types, generally with a confusing error in the header .

answered Oct 23 '22 at 02:16

GeePokey

159
7

good answer because to tell you the truth, I had no idea what was going on other than the pointer was the problem (in my similar format code within a template) – ycomp Feb 06 '23 at 05:29

score 1 · Answer 2 · answered Oct 24 '22 at 16:06

1

Formatting of non-void pointers is disallowed by default. You should wrap them in fmt::ptr or cast to void*:

std::string resultstring = std::format("{:p}", fmt::ptr(p));

answered Oct 24 '22 at 16:06

vitaut

49,672
25
199
336

fmt::ptr() looks nice - and I would prefer that over a normal cast, but it is not in the standard library - only available if [fmt library](https://fmt.dev/latest/index.html) is installed, so I am accepting the (void *) answser. – GeePokey Oct 31 '22 at 03:32

std::format pointer error C3615: consteval function 'std::_Compile_time_parse_format_specs' cannot result in a constant expression

2 Answers2