编译Assimp时出现多个-fpermissive错误，求解决方法

问题场景 #

通过git clone https://github.com/assimp/assimp.git下载Assimp源码，执行CMake时添加-DASSIMP_WARNINGS_AS_ERRORS=OFF参数规避stb_i.h中未使用函数的编译问题，但执行sudo make时出现以下编译错误：

/home/kshaugnnessy/Downloads/engine/assimp/code/PostProcessing/JoinVerticesProcess.cpp:226:13: error: specialization of ‘template<class _Tp> struct std::hash’ in different namespace [-fpermissive]
 struct std::hash<Vertex> {
             ^
In file included from /usr/include/c++/5/bits/basic_string.h:5471:0,
                 from /usr/include/c++/5/string:52,
                 from /usr/include/c++/5/bits/locale_classes.h:40,
                 from /usr/include/c++/5/bits/ios_base.h:41,
                 from /usr/include/c++/5/ios:42,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iterator:64,
                 from /home/kshaugnnessy/Downloads/engine/assimp/contrib/utf8cpp/source/utf8/core.h:31,
                 from /home/kshaugnnessy/Downloads/engine/assimp/contrib/utf8cpp/source/utf8/checked.h:31,
                 from /home/kshaugnnessy/Downloads/engine/assimp/include/../contrib/utf8cpp/source/utf8.h:31,
                 from /home/kshaugnnessy/Downloads/engine/assimp/include/assimp/types.h:79,
                 from /home/kshaugnnessy/Downloads/engine/assimp/include/assimp/Importer.hpp:60,
                 from /home/kshaugnnessy/Downloads/engine/assimp/include/assimp/GenericProperty.h:52,
                 from /home/kshaugnnessy/Downloads/engine/assimp/code/Common/BaseProcess.h:46,
                 from /home/kshaugnnessy/Downloads/engine/assimp/code/PostProcessing/JoinVerticesProcess.h:48,
                 from /home/kshaugnnessy/Downloads/engine/assimp/code/PostProcessing/JoinVerticesProcess.cpp:48:
/usr/include/c++/5/bits/functional_hash.h:58:12: error:   from definition of ‘template<class _Tp> struct std::hash’ [-fpermissive]
     struct hash;
            ^
/home/kshaugnnessy/Downloads/engine/assimp/code/PostProcessing/JoinVerticesProcess.cpp:235:13: error: specialization of ‘template<class _Tp> struct std::equal_to’ in different namespace [-fpermissive]
 struct std::equal_to<Vertex> {
             ^
In file included from /usr/include/c++/5/string:48:0,
                 from /usr/include/c++/5/bits/locale_classes.h:40,
                 from /usr/include/c++/5/bits/ios_base.h:41,
                 from /usr/include/c++/5/ios:42,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iterator:64,
                 from /home/kshaugnnessy/Downloads/engine/assimp/contrib/utf8cpp/source/utf8/core.h:31,
                 from /home/kshaugnnessy/Downloads/engine/assimp/contrib/utf8cpp/source/utf8/checked.h:31,
                 from /home/kshaugnnessy/Downloads/engine/assimp/include/../contrib/utf8cpp/source/utf8.h:31,
                 from /home/kshaugnnessy/Downloads/engine/assimp/include/assimp/types.h:79,
                 from /home/kshaugnnessy/Downloads/engine/assimp/include/assimp/Importer.hpp:60,
                 from /home/kshaugnnessy/Downloads/engine/assimp/include/assimp/GenericProperty.h:52,
                 from /home/kshaugnnessy/Downloads/engine/assimp/code/Common/BaseProcess.h:46,
                 from /home/kshaugnnessy/Downloads/engine/assimp/code/PostProcessing/JoinVerticesProcess.h:48,
                 from /home/kshaugnnessy/Downloads/engine/assimp/code/PostProcessing/JoinVerticesProcess.cpp:48:
/usr/include/c++/5/bits/stl_function.h:332:12: error:   from definition of ‘template<class _Tp> struct std::equal_to’ [-fpermissive]
     struct equal_to;
            ^
code/CMakeFiles/assimp.dir/build.make:747: recipe for target 'code/CMakeFiles/assimp.dir/PostProcessing/JoinVerticesProcess.cpp.o' failed
make[2]: *** [code/CMakeFiles/assimp.dir/PostProcessing/JoinVerticesProcess.cpp.o] Error 1
CMakeFiles/Makefile2:203: recipe for target 'code/CMakeFiles/assimp.dir/all' failed
make[1]: *** [code/CMakeFiles/assimp.dir/all] Error 2
Makefile:135: recipe for target 'all' failed
make: *** [all] Error 2

错误原因 #

C++标准规定：特化std命名空间中的模板时，必须将特化代码放在std命名空间内部。当前Assimp源码直接在全局命名空间下写struct std::hash<Vertex>，这种写法不符合标准，GCC 5及以上版本会严格检查并报错。

解决方法 #

方法一：修改源码中的模板特化写法 #

1. 打开文件/home/kshaugnnessy/Downloads/engine/assimp/code/PostProcessing/JoinVerticesProcess.cpp
2. 找到错误对应的两处模板特化代码（行226和235），将原有代码：

   struct std::hash<Vertex> {
       // 原实现逻辑
   };
   
   struct std::equal_to<Vertex> {
       // 原实现逻辑
   };
   

   修改为：

   namespace std {
   template<> struct hash<Vertex> {
       size_t operator()(const Vertex& v) const {
           // 保留原有的哈希计算逻辑
       }
   };
   
   template<> struct equal_to<Vertex> {
       bool operator()(const Vertex& a, const Vertex& b) const {
           // 保留原有的顶点比较逻辑
       }
   };
   } // namespace std
   

3. 保存文件后，重新执行sudo make即可完成编译。

方法二：切换到Assimp稳定分支 #

如果不想修改源码，可以切换到Assimp的稳定版本分支（比如v5.2.x），这类分支通常已经修复了旧编译器的兼容性问题：

cd assimp
git checkout v5.2.5 # 可替换为最新稳定版本号
rm -rf build && mkdir build && cd build
cmake -DASSIMP_WARNINGS_AS_ERRORS=OFF ..
sudo make

内容的提问来源于stack exchange，提问作者Kevin Shaughnessy