Compile-time manipulation of std::source_location::current()

Question

I want to write a function that can obtain caller's function name, or at least its length, at compile time. I think obtaining std::source_location::current() is close to what I want and currently I'm able to do this:

consteval auto funcNameLength(const std::source_location &Loc = std::source_location::current()) {
  const char *Name = Loc.function_name();
  std::size_t Length = 0;
  while (Name[Length] != '\0')
    ++Length;
  return Length;
}

int main() {
  constexpr auto L = funcNameLength(); // L == 4
}

or this (with a constexpr strlen):

consteval auto
funcNameLength(std::size_t Length = constexpr_strlen(
                   std::source_location::current().function_name())) {
  return Length;
}

int main() { constexpr auto L = funcNameLength(); }

However, that is not enough for me. I need to use the length as a constant expression to do something before returning to caller, e.g.

consteval auto
doSomething(std::size_t Length = constexpr_strlen(
                   std::source_location::current().function_name())) {
  int A[Length]; // Error: Length is not a constant expression.
}

int main() {
  doSomething();
}

Is this achievable?

function arguments can't be part of constexpr even if function is constexpr. Why you need A and why it need to be C-array? What are you trying to accomplish? Explaining your solution to this goal doesn't look like a good approach. — Marek R
– Marek R, Commented Aug 5 at 15:37
I am really curious about your use case. Shipping code with extra debug information is usually a sign that some other part of the development process is missing. E.g. unit tests, with enough unit tests and say exceptions/error handling in your code you shouldn't need it. My experience is that code with unit testing hardly requires any logging/debugging helpers in code just normal error handling/reporting. And logging based on explicit customer requirements (e.g. for traceability, who did what when). So yes I am with @MarekR here — Pepijn Kramer
– Pepijn Kramer, Commented Aug 5 at 17:02
@PepijnKramer Actually I'm working on a Clang-based tool. I've written a simple wrapper for Clang's diagnostic engine and it would be nice if I can print the name of the checker that initiates the current diagnostic. Of course this can be done from source_location::current, but Clang seems to suggest against runtime strings in its related interfaces (see issue #116169) and I feel their attitude somewhat unclear on this. That's why I keep trying to generate and manipulate compile-time strings. — GKxx
– GKxx, Commented Aug 5 at 18:01

Jarod42 · Accepted Answer · 2025-08-05 16:42:37Z

Function parameters are not constant expression, even in consteval function.

You have to pass argument as template.

Since C++20, you might have structural types as template parameter, so

constexpr std::size_t constexpr_strlen(const char* s)
{
    std::size_t i = 0;
    while (s[i]) ++i;
    return i;
}

template <std::size_t N = 42> // Some hard coded limit for non literal string
struct LiteralString
{
    consteval LiteralString(const char *s) { std::copy(s, s + constexpr_strlen(s), &data[0]); }
    consteval LiteralString(const char (&s)[N]) { std::copy(s, s + N, &data[0]); }

    static constexpr std::size_t size = N;
    std::array<char, N> data{};
};

Then you might do something like:

template <LiteralString S = std::source_location::current().function_name()>
void foo() {
    constexpr std::size_t len = constexpr_strlen(S.data.data());
    // ...
}

int main() {
    foo();   // empty string for clang/msvc :(
    foo<>(); // empty string for clang/msvc :(
}

Unfortunately, only gcc feeds current source_location in template parameter Demo.

MACRO might be a workaround Demo

#define FOO() foo<std::source_location::current().function_name()>()

int main() {
    foo();   // empty string for clang/msvc :(
    foo<>(); // empty string for clang/msvc :(

    // "int main()" or "int __cdecl main(void)"
    foo<std::source_location::current().function_name()>();
    FOO();
}

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