json.hpp:5746:32: error: 'to_string' is not a member of 'std'

[Ingener74]

Hello.
On Android i have this messages

• json.hpp:5746:32: error: 'to_string' is not a member of 'std'
• json.hpp:6911:36: error: 'strtold' is not a member of 'std'

android ndk have no to_strign realization

[gregmarr]

The first one works fine for us, we use the c++-shared library.

We had to add this function to our codebase to handle the second one.
inline long double strtold(const char _nptr, char *_endptr)
{
return strtod(nptr, endptr);
}

[nlohmann]

Hey @Ingener74, could you please tell me which compiler you are using?

[Ingener74]

Hello @nlohmann, i am using arm-linux-androideabi-4.9 toolchain for compiling

[nlohmann]

Could this maybe solve your problem: http://stackoverflow.com/a/27589053 ?

[Ingener74]

Thanks @nlohmann, i known this solution :) but that about strtold?
I wanted to ask whether it will be added to your library? for android

[nlohmann]

Ok, I'll have a look at strtold. It would be easier to check on OSX - is there an OSX version of the compiler you use?

[Ingener74]

On Mac OS X i use compiler

Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/c++/4.2.1
Apple LLVM version 7.0.0 (clang-700.0.72)
Target: x86_64-apple-darwin14.5.0
Thread model: posix

but on mac os everythink compiles ok

[nlohmann]

I meant the compiler to create code for Android. Is there a way to cross-compile from OSX?

[gregmarr]

You need the android NDK.

[gregmarr]

Yes, it is available for OS X.

[Ingener74]

Hello @nlohmann i agree with @gregmarr, you need Android NDK for compiling
https://developer.android.com/ndk/downloads/index.html

[ZahlGraf]

Hi,

for a current project I compiled this library with Android NDK r10e, (it includes gcc 4.9) together with gnustl-shared library.
The errors mentioned by @Ingener74 are still in the json-lib, thus I created a small patch-file for that.

On android, it adds the function std::to_string (common workaround implementation, found on stackoverflow) and std::strtold (workaround implementation suggested by @gregmarr).

Hopefully this patch is usefull for anyone who want to use this library on Android. Maybe linking from the main-README to those compatibility patches would be a good idea.

Best regards,
ZG

From 092170941c6a69cbc3a113b098940b4c03b448c2 Mon Sep 17 00:00:00 2001
From: Andre Netzeband <andre@netzeband.eu>
Date: Sat, 28 Nov 2015 13:08:39 +0100
Subject: [PATCH] Added better android support.

Added Android NDK support.
---
 src/json.hpp | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/src/json.hpp b/src/json.hpp
index 1e3cd11..e878295 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -70,6 +70,25 @@ Class @ref nlohmann::basic_json is a good entry point for the documentation.
     using ssize_t = SSIZE_T;
 #endif

+#ifdef __ANDROID__
+   // Workaround for Android NDK builds (version r10e) that does not support std::to_string and std::strold so far
+   namespace std
+   {
+       template <typename T>
+       std::string to_string(T Value)
+       {
+           std::ostringstream TempStream;
+           TempStream << Value;
+           return TempStream.str();
+       }
+       
+       inline long double strtold(const char * str, char ** str_end)
+       {
+           return strtod(str, str_end);
+       }       
+   }
+#endif
+
 /*!
 @brief namespace for Niels Lohmann
 @see https://github.com/nlohmann
-- 
2.5.0.windows.1

BTW: Unit-Tests are running fine on Android Emulator with these changes.

0001-Added-better-android-support.patch.txt

[nlohmann]

Thanks! I'm on holiday right now and shall have a look then!

[nlohmann]

Thanks @ZahlGraf! I added your patch to the code. @Ingener74, can you please check if this fixes your original issue?