to_string is not a member of std

2016-02-23 [2451] technology android ndk cpp cocos2d-x

现象

cocos2d-x 2.2.6 项目的源码中使用了 std::to_string() 方法，使用 NDK r9d 编译的时候，报如下错误：

1error: 'to_string' is not a member of 'std'

Application.mk 文件的部分内容如下：

1APP_STL := gnustl_static
2NDK_TOOLCHAIN_VERSION := 4.8
3APP_CPPFLAGS := -frtti -std=c++11 -fexceptions -Wno-error=format-security -Wno-literal-suffix -Wno-deprecated-declarations -fsigned-char -Os $(CPPFLAGS)

换用 NDK 10e ，错误依然。

快速解决

想快速解决这个问题，可以自己写一个 std::to_string() 替换标准库中的。

首先写一个 stdtostring.h 文件：

 1#ifndef STDTOSTRING_H
 2#define STDTOSTRING_H
 3#include <string>
 4#include <sstream>
 5
 6using namespace std;
 7namespace std
 8{
 9    template < typename T > std::string to_string( const T& n )
10    {
11        std::ostringstream stm ;
12        stm << n ;
13        return stm.str() ;
14    }
15}
16#endif

然后在需要使用 std::to_stirng() 方法的源文件中包含它：

1#if CC_TARGET_PLATFORM == CC_PLATFORM_ANDROID
2#include "stdtostring.h"
3#endif

追根溯源

Why gnustl_static 中介绍了采用 c++_static 和 clang 编译器来实现 std::to_string() 支持，于是我尝试了下：

1# APP_STL := gnustl_static
2# NDK_TOOLCHAIN_VERSION := 4.8
3APP_STL := c++_static
4NDK_TOOLCHAIN_VERSION := clang

根据 C++ Library Support 的说明，c++_static 对应的是 The LLVM libc++ runtime (static) ，因此应该采用 clang 的工具链进行编译。

结果是： std::to_string() 编译正常，但出现下面的错误：

 1In file included from src/Utilities.cpp:1:0:
 2android-ndk-r9d/sources/cxx-stl/llvm-libc++/libcxx/include/cmath:1498:46: error: expected unqualified-id before 'float'
 3inline _LIBCPP_INLINE_VISIBILITY float       remainder(float __x, float __y) _NOEXCEPT             {return remainderf(__x, __y);}
 4                                             ^
 5android-ndk-r9d/sources/cxx-stl/llvm-libc++/libcxx/include/cmath:1498:46: error: expected ')' before 'float'
 6android-ndk-r9d/sources/cxx-stl/llvm-libc++/libcxx/include/cmath:1499:46: error: expected unqualified-id before 'long'
 7inline _LIBCPP_INLINE_VISIBILITY long double remainder(long double __x, long double __y) _NOEXCEPT {return remainderl(__x, __y);}
 8                                             ^
 9android-ndk-r9d/sources/cxx-stl/llvm-libc++/libcxx/include/cmath:1499:46: error: expected ')' before 'long'
10android-ndk-r9d/sources/cxx-stl/llvm-libc++/libcxx/include/cmath:1509:1: error: expected ')' before '__x'
11remainder(_A1 __x, _A2 __y) _NOEXCEPT
12^
13android-ndk-r9d/sources/cxx-stl/llvm-libc++/libcxx/include/cmath:1559:46: error: expected unqualified-id before 'float'
14inline _LIBCPP_INLINE_VISIBILITY float       round(float __x) _NOEXCEPT       {return roundf(__x);}
15                                             ^
16android-ndk-r9d/sources/cxx-stl/llvm-libc++/libcxx/include/cmath:1559:46: error: expected ')' before 'float'
17android-ndk-r9d/sources/cxx-stl/llvm-libc++/libcxx/include/cmath:1560:46: error: expected unqualified-id before 'long'
18inline _LIBCPP_INLINE_VISIBILITY long double round(long double __x) _NOEXCEPT {return roundl(__x);}
19                                             ^
20android-ndk-r9d/sources/cxx-stl/llvm-libc++/libcxx/include/cmath:1560:46: error: expected ')' before 'long'
21android-ndk-r9d/sources/cxx-stl/llvm-libc++/libcxx/include/cmath:1565:1: error: expected ')' before '__x'
22round(_A1 __x) _NOEXCEPT {return round((double)__x);}

minggo 在 Cocos can't compile Android with APP_STL := c++_static 中提到：

A prebuilt version can only support one type of c++ lib. Did you mean we should provide two kinds of prebuilt libcocos2d that using two different c++ libs?

I don't know if cocos2d can compile with c++_static or not. I don't remember if there is a good reason to choose one instead of other one. But cocos2d-x uses gnustl_static from very early version, so we know it works well. You can replace it with c++_static if you like, but it is not fully tested.

因此我怀疑在 cocos2d-x 的旧版本中，c++_static 是没有经过完整测试的。

也有人说 在新版本（>3.7）中， gnustl_static 会导致 lambda 和 std::to_string 不能使用，已经切换到使用 c++_static 和 clange 组合。

I never made it to work with gnustl_static and gcc with all the c++11 features. I was getting crashes in simple lambdas, as mentioned std::to_string was missing and much more, so i've switched to c++_static and clang long time ago.

同样也有人做了证明 。

既然目前我们依然在使用旧版本，就不折腾了。

全文完