clang, std::function and -fno-rtti

Question

I am using Clang 5 on Windows via clang-cl and have run into a problem trying to turn off runtime type information (-fno-rtti) when using std::function.

Here's my example that won't compile:

#include <functional>

void foo(std::function<void()> ra2)
{
}

int main()
{
    auto bar = []()
    {
    };

    foo(bar);

    return EXIT_SUCCESS;
}

Command line:

clang-cl test.cpp -Xclang -fno-rtti

The error is:

C:\Program Files (x86)\Microsoft Visual Studio\Preview\Community\VC\Tools\MSVC\14.13.26128\include\functional(435,11):  error:
      cannot use typeid with -fno-rtti
                return (typeid(_Callable));
                        ^

This surprises me, is there a way to use std::function with lambdas in Clang without RTTI? The docs say that only target and target_type on std::function should need RTTI. I can roll my own version of std::function, but it seems a shame to have to do so.

It works fine if I use MSVC with compiler flag /GR-.

Answer 1

This is a bug fixed in Clang 13.

---

The Clang MSVC driver fails to define _HAS_STATIC_RTTI to 0, which is used by the MSVC standard library to enable no-RTTI code.

As a workaround you can manually define it globally using compiler flags or defining it before including any standard library headers.