Type-safe printf for C++

This is my attempt at writing a type-safe printf for C++.

It is currently minimal. It handles formats, like %s, and type-checks these. However it does not handle all the other features of printf, such as width and precision. There is nothing preventing this, I just haven't done it yet.

This isn't runtime type-safe. This is another feature that printf ought to have, to support localization. That is, it should statically type-check the canonical string; and then at runtime ensure that the string taken from the message catalog is also correct. This is readily doable by making the real formatting function a variadic template function; I just haven't done it.

Type safety can eliminate some of the oddities of printf. For example there's no reason to differentiate between the different integer widths -- %d can just handle all of them. Likewise %s could handle char *, char16_t *, std::string, and so forth. With some changes to make the implementation more template-based, this could be user-extensible as well.

See It Work

From this source (see test.cc):

int x;
...
safe_printf("%s -> %d", x, "hi bob");

We get this from gcc:

In file included from test.cc:1:0:
test.cc: In function ‘int main()’:
safe-printf.hh:199:5: error: non-constant condition for static assertion
     static_assert(decltype(getWrapper(__VA_ARGS__))::checkFormat(format), \
     ^
test.cc:10:3: note: in expansion of macro ‘safe_printf’
   safe_printf("%s -> %d", x, "hi bob");
   ^
test.cc:10:3:   in constexpr expansion of ‘safe_printf::impl::checkFormatWrapper<T>::checkFormat<{int, const char*}>(safe_printf::impl::constExprStr("%s -> %d"))’
safe-printf.hh:183:45:   in constexpr expansion of ‘safe_printf::impl::checkFormat<int, const char*, {}>(str)’
safe-printf.hh:172:45:   in constexpr expansion of ‘safe_printf::impl::checkFormatPercent<int, const char*, {}>(str.safe_printf::impl::constExprStr::tail())’
safe-printf.hh:148:27:   in constexpr expansion of ‘safe_printf::impl::checkOneFormat<int>(((int)str.safe_printf::impl::constExprStr::first()))’
safe-printf.hh:87:58: error: expression ‘<throw-expression>’ is not a constant-expression
      : throw std::logic_error("argument to %s not string"))
                                                          ^

C++ Thoughts

GCC comes with format-checking code. With C++11, this can be replaced with metaprogramming. However, it's pretty difficult.

C++ compilers need to come with a debugger for template and constexpr metaprogramming. Even logging-style debugging would be better than what there is now; but of course as programs grow, real interactive debugging will be needed.

One concrete thing that could be done immediately would be to emit the argument values to constexpr functions when printing an error.

That said, we're also going to need a way for constexpr programs to control the errors that are emitted more directly.

There are weird corners of the language which need to be filled. See below for a few specifics.

Implementation

This mostly uses constexpr but there are also some other fun hacks:

Check out the way that checkFormatWrapper works to extract the parameter pack. Maybe this is well-known, I don't know, but I thought it was cool.
If template<char... Chars> operator "" _mumble worked for string constants, this feature could be implemented using templates. This would be better in many ways, but unfortunately this can't be done yet. Let's hope for C++17.
A macro is needed because I couldn't think of another way to make the format be both statically checked and passed to a non-constexpr function. I'd love to hear a solution.
Note that at the end I had to use a GNU extension, because the __VA_ARGS__ botch still has not been fixed. I guess I could fix this with an indirection, but I couldn't be bothered.

pmachata/typesafe-printf

Type-safe printf for C++

See It Work

C++ Thoughts

Implementation