dump_float: truncation from ptrdiff_t to long

[ftillier]

Building with VisualStudio 2017, using /W4 and /Wx, I get the following:
1>json.hpp(6946): warning C4244: '=': conversion from 'std::_Array_iterator<_Ty,64>::difference_type' to 'long', possible loss of data

On Windows, long is always 32-bits. Declaring 'len' as std::ptrdiff_t fixes the issue.

[ftillier]

index a9a5067..e7b23a5 100644
--- a/src/json.hpp.re2c
+++ b/src/json.hpp.re2c
@@ -6927,7 +6927,7 @@ class basic_json
             static constexpr auto d = std::numeric_limits<number_float_t>::digits10;
 
             // the actual conversion
-            long len = snprintf(number_buffer.data(), number_buffer.size(),
+            std::ptrdiff_t len = snprintf(number_buffer.data(), number_buffer.size(),
                                 "%.*g", d, x);
 
             // negative value indicates an error

[gregmarr]

According to the VS 2015 docs, the return type is int. The VS2017 docs say the same thing. I wonder if they changed the return type in VS 2017 without updating the docs.

[ftillier]

Sorry for the lack of context. The compiler error comes from line 6946:
len = (end - number_buffer.begin());

The return type of snprintf is still int, which gets promoted to long just fine. It's the assignment, which uses std::array::difference_type (in this case std::ptrdiff_t) on the right hand side, which could truncate since long is smaller than ptrdiff_t.

[gregmarr]

Thanks for the clarification, that makes more sense. I just assumed that the error was on the code line that you posted, and that it was line 6946 in the version of the header that you were using.

[ftillier]

I don't have a system on which I can run re2c, so I hesitate to issue a PR for this - is the patch above sufficient?

[TurpentineDistillery]

@ftillier, you don't need re2c. You can do git checkout src/json.hpp.re2c && git diff src/json.hpp | patch src/json.hpp.re2c
@nlohmann, perhaps mention the above hack in contribution guidelines?

WRT the fix: const auto len = snprintf(... ?

[nlohmann]

@TurpentineDistillery Good point!

[nlohmann]

@ftillier Should be fixed now. Could you please validate?

[ftillier]

@nlohmann, validated as fixed. Thanks!