Android ndk std::to_string support

Question

I'm using android NDK r9d and toolchain 4.8 but I'm not able to use std::to_string function, compiler throws this error:

 error: 'to_string' is not a member of 'std'

Is this function not supported on android ndk? I try APP_CPPFLAGS := -std=c++11 with no luck.

Balliol

Answer 1

You can try LOCAL_CFLAGS := -std=c++11, but note that not all C++11 APIs are available with the NDK's gnustl. Full C++14 support is available with libc++ (APP_STL := c++_shared).

The alternative is to implement it yourself.

#include <string>
#include <sstream>

template <typename T>
std::string to_string(T value)
{
    std::ostringstream os ;
    os << value ;
    return os.str() ;
}

int main()
{
    std::string perfect = to_string(5) ;
}

Deutschland

I make some tries with no luck (LOCAL_CPPFLAGS += -std=c++11, LOCAL_CPPFLAGS += -std=gnu+11 etc.), and also I found some answers (#17951314) which explains that ndk not supports new c++11 string function, so finally I implement to_string method as you suggests. Thanks :). – Balliol

I have tried it but gives error on std::ostringstream. The issue might be about Android Studio 2.2 alpha 6. Is there any easy way to convert? – Haze

Extremely helpful all for compilers that don't support C++11. BTW: Is there a way (a macro definition) to check if C++11 is present, to skip this? – Pallbearer

I just found the char template specializations missing, you need 3 of them, for: char, signed char, unsigned char – Pallbearer

I have used the same function but application get crash for some devices with below log libloglib-jni.so 0xdd7bdacf std::basic_ostringstream<char, std::char_traits<char>, std::allocator<char> >::~basic_ostringstream() (basic_ios.h:276) 36 libart.so 0xf4832812 (Missing) 37 libloglib-jni.so 0xdd7a5851 std::string patch::to_string<int>(int const&) can you please provide any solution if you have. – Wirra

I've updated the answer to account for modern NDKs, and also clarified the situations where the C++11 APIs were missing. – Fantasm

Better to pass value by const reference: std::string to_string(const T& value) – Dreamworld

Answer 2

With NDK r9+ you can use llvm-libc++ which offers full support for cpp11.

In your Application.mk you have to add:

APP_STL:=c++_static 

or

APP_STL:=c++_shared

Rushton

using cocos2d-x this worked to get around the std::to_string compile issue. However it caused other build problems down stream: "error: 'pthread_key_t' does not name a type static pthread_key_t s_threadKey" In case some tries this. I was never able to solve that. As mentioned here #22165064 changing compilers led to other compile issues... The accepted answer got around the issue in the short term... – Radiogram

@Hunter I didn't mention that on my question however I was using cocos2d, and the accepted answer is simply an clean and as you said got around the issue so this is why I did accept it :) – Balliol

Yes I too went with the accepted answer, I was not able to figure out exactly how to use this approach correctly. I was missing other threading libs. Probably could have done it if I spent more time... I just wanted that specific error here so if I ever run into this again I will have a hit right away :) – Radiogram

I was looking for this one for hours, so thank you thursdaysDove! – Doriedorin

I also get other errors down stream with this answer. – Totter

@thursdaysDove, +1. Working fine when the flag added to the application.mk file. As expected i am able to compile from the terminal window using ndk-build. But i am not able to do this from eclipse (enable with android). There i am getting error on the to_string function. Do you have any idea which file need to be included? – Loriannlorianna

This breaks ABI compatibility with any static libraries compiled (e.g. third party libs). So you'll have to rebuild them again. Would be nice to avoid that. – Finstad

Answer 3

Gradle

If you looking for solution for Gradle build system. Look at this answer.

Short answer.

Add the string

arguments "-DANDROID_STL=c++_shared"

in your build.gradle. Like

android {
  ...
  defaultConfig {
    ...
    externalNativeBuild {
      cmake {
        ...
        arguments "-DANDROID_STL=c++_shared"
      }
    }
  }
  ...
}

Whooper

Actually this is the only valid answer ,given that today's native development for Android is mostly done with Android Studio which uses Gradle. – Disoperation

Answer 4

Experimental Gradle Plugin

If you're looking for a solution for the Experimental Gradle plugin, this worked for me...

Tested with com.android.tools.build:gradle-experimental:0.9.1

model {
  ...
  android {
    ...
    ndk {
      ...
      stl = "c++_shared"
    }
  }
}

Hit

Answer 5

I could not use c++_static, it gave some errors about undefined exceptions. So I returned to the gnustl_static.

But in NDK sources, in sources/cxx-stl/llvm-libc++/src/string.cpp, I found implementation of to_string(int) and tried to copy it to my code. After some corrections it worked.

So the final piece of code I had:

#include <string>
#include <algorithm>

using namespace std;


template<typename S, typename P, typename V >
inline
S
as_string(P sprintf_like, S s, const typename S::value_type* fmt, V a)
{
    typedef typename S::size_type size_type;
    size_type available = s.size();
    while (true)
    {
        int status = sprintf_like(&s[0], available + 1, fmt, a);
        if ( status >= 0 )
        {
            size_type used = static_cast<size_type>(status);
            if ( used <= available )
            {
                s.resize( used );
                break;
            }
            available = used; // Assume this is advice of how much space we need.
        }
        else
            available = available * 2 + 1;
        s.resize(available);
    }
    return s;
}

template <class S, class V, bool = is_floating_point<V>::value>
struct initial_string;

template <class V, bool b>
struct initial_string<string, V, b>
{
    string
    operator()() const
    {
        string s;
        s.resize(s.capacity());
        return s;
    }
};

template <class V>
struct initial_string<wstring, V, false>
{
    wstring
    operator()() const
    {
        const size_t n = (numeric_limits<unsigned long long>::digits / 3)
          + ((numeric_limits<unsigned long long>::digits % 3) != 0)
          + 1;
        wstring s(n, wchar_t());
        s.resize(s.capacity());
        return s;
    }
};

template <class V>
struct initial_string<wstring, V, true>
{
    wstring
    operator()() const
    {
        wstring s(20, wchar_t());
        s.resize(s.capacity());
        return s;
    }
};

string to_string(int val)
{
    return as_string(snprintf, initial_string<string, int>()(), "%d", val);
}

Armond

Answer 6

For Android Studio, add this in build.gradle (Mobile App)

externalNativeBuild {
    cmake {
        cppFlags "-std=c++11"

        arguments "-DANDROID_STL=c++_static"
    }
}

Vamp