Better error messages

[foonathan]

If you pass in a type that cannot be formatted, the error messages aren't.. exactly nice.

There are strategies to provide better error messages, namely a fallback functions that gets chosen as last resort with a static_assert() upon instantiation (I wrote a blog post if you are not aware of such techniques).

It would be nice if you implement something like this.

[vitaut]

Thanks for the suggestions. I agree that it would be nice to improve error reporting in this case. The only problem is that the most general template-based overload is already taken for std::ostream support here:

fmt/fmt/ostream.h

Line 72 in 0d5ef5c

template <typename Char, typename ArgFormatter, typename T>

Great blog post BTW!

[foonathan]

Hm, maybe disable it via expression SFINAE (on compilers that support it...)?
And then add a fallback.

[xaxxon]

Is this the "not exactly nice" error message?

/usr/local/include/fmt/format.h:1182:5: error: no matching function for call to 'format'
format(static_cast<Formatter>(formatter),
^~~~~~
/usr/local/include/fmt/format.h:1273:22: note: in instantiation of function template specialization
'fmt::internal::MakeValue<fmt::BasicFormatter<char, fmt::ArgFormatter > >::format_custom_arg' requested here
custom.format = &format_custom_arg;
^
/usr/local/include/fmt/format.h:2081:20: note: in instantiation of function template specialization
'fmt::internal::MakeValue<fmt::BasicFormatter<char, fmt::ArgFormatter > >::MakeValue' requested here
Value result = MakeValue(value);
^
/usr/local/include/fmt/format.h:3436:1: note: in instantiation of function template specialization 'fmt::internal::ArgArray<1,
true>::make<fmt::BasicFormatter<char, fmt::ArgFormatter >, MapCoordinate>' requested here
FMT_VARIADIC(std::string, format, CStringRef)
^
/usr/local/include/fmt/format.h:3408:3: note: expanded from macro 'FMT_VARIADIC'
FMT_VARIADIC_(char, ReturnType, func, return func, VA_ARGS)
^
/usr/local/include/fmt/format.h:3342:26: note: expanded from macro 'FMT_VARIADIC_'
ArgArray::template makefmt::BasicFormatter(args)...};
^
/Users/xaxxon/apb/include/utility.h:122:16: note: in instantiation of function template specialization 'fmt::format' requested here
reason(fmt::format(reason, std::forward(ts)...))
^
/Users/xaxxon/apb/src/map.cpp:52:9: note: in instantiation of function template specialization 'ApbException::ApbException<const MapCoordinate &>'
requested here
throw ApbException("Position not in map {}", position);
^
/usr/local/include/fmt/format.h:3436:27: note: candidate function [with Args = <const char *, MapCoordinate>] not viable: no known conversion from
'fmt::BasicFormatter<char, fmt::ArgFormatter >' to 'CStringRef' (aka 'BasicCStringRef') for 1st argument
FMT_VARIADIC(std::string, format, CStringRef)
^
/usr/local/include/fmt/format.h:3408:35: note: expanded from macro 'FMT_VARIADIC'
FMT_VARIADIC_(char, ReturnType, func, return func, VA_ARGS)
^
/usr/local/include/fmt/format.h:3338:14: note: expanded from macro 'FMT_VARIADIC_'
ReturnType func(FMT_FOR_EACH(FMT_ADD_ARG_NAME, VA_ARGS),
^
/usr/local/include/fmt/format.h:3437:30: note: candidate function [with Args = <const char *, MapCoordinate>] not viable: no known conversion from
'fmt::BasicFormatter<char, fmt::ArgFormatter >' to 'WCStringRef' (aka 'BasicCStringRef<wchar_t>') for 1st argument
FMT_VARIADIC_W(std::wstring, format, WCStringRef)
^
/usr/local/include/fmt/format.h:3411:38: note: expanded from macro 'FMT_VARIADIC_W'
FMT_VARIADIC_(wchar_t, ReturnType, func, return func, VA_ARGS)
^
/usr/local/include/fmt/format.h:3338:14: note: expanded from macro 'FMT_VARIADIC_'
ReturnType func(FMT_FOR_EACH(FMT_ADD_ARG_NAME, VA_ARGS),
^
/usr/local/include/fmt/format.h:3147:21: note: candidate function not viable: requires 2 arguments, but 3 were provided
inline std::wstring format(WCStringRef format_str, ArgList args) {
^
/usr/local/include/fmt/format.h:3141:20: note: candidate function not viable: requires 2 arguments, but 3 were provided
inline std::string format(CStringRef format_str, ArgList args) {
^

[vitaut]

@xaxxon Yes, looks like it.

[vitaut]

Improved error reporting is implemented in 31e7e80. Now it will give:

...
fmt/format.h:1156:43: error: static assertion failed: Cannot format argument. To 
enable the use of ostream operator<< include fmt/ostream.h. Otherwise provide 
an overload of format_arg.

[xaxxon]

That's awesome.

On Sun, Aug 14, 2016 at 7:31 AM, Victor Zverovich notifications@github.com
wrote:

Improved error reporting is implemented in 31e7e80
31e7e80.
Now it will give:

...
fmt/format.h:1156:43: error: static assertion failed: Cannot format argument. To
enable the use of ostream operator<< include fmt/ostream.h. Otherwise provide
an overload of format_arg.

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[courtney-g]

I'm trying to use fmt::hex and am seeing this error. I tried adding #include "fmt/ostream.h" and then get:

external/fmtlib/fmt/ostream.h:82:10: error: invalid operands to binary expression ('std::basic_ostream' and 'const fmt::IntFormatSpec<unsigned int, fmt::TypeSpec<'x'>, char>')
output << value;

...

Suggestions?

[vitaut]

It looks that you are trying to use fmt::hex with iostreams while it should be used with a writer. If this is not the case please post the code that causes the error.