Use wasm-bindgen to *.cpp files.
# include "WasmBindgen.h"
WasmBindgenImport(alert, Unit, ((String, message)))
__attribute__((import_module("__wbindgen_placeholder__")))
extern "C" void alert(const char* message, size_t messageLength);
int main() {
alert("Hei!", sizeof("Hei!"));
}