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.

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) ;
}

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

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"
      }
    }
  }
  ...
}

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"
    }
  }
}

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);
}

Answer 6

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

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

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