问题

重新封装画板内核库后，所有的混编文件（.mm）都被打包到了库中，外面应用层代码全部都是OC文件（.m）。这时候编译工程会出现如下连接错误：

Undefined symbols for architecture arm64:
“vtable for cxxabiv1::vmi_class_type_info”, referenced from:
…
NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
“std::1::basic_string<char, std::1::char_traits, std::1::allocator >::assign(char const*)”, referenced from:
…
“_cxa_pure_virtual”, referenced from:
…
“std::1::basic_string<char, std::1::char_traits, std::1::allocator >::operator=(std::1::basic_string, std::1::allocator > const&)”, referenced from:
…
“std::1::vector_base_common::throw_out_of_range() const”, referenced from:
…
“std::1::basic_string<char, std::1::char_traits, std::1::allocator >::basic_string(std::1::basic_string, std::1::allocator > const&)”, referenced from:
…
“std::terminate()”, referenced from:
…
“_cxa_guard_release”, referenced from:
…
“_cxa_guard_abort”, referenced from:
…
“std::1::basic_string, std::1::allocator >::init(char const*, unsigned long)”, referenced from:
…
“std::1::basic_string<char, std::1::char_traits, std::1::allocator >::reserve(unsigned long)”, referenced from:
…
“_gxx_personality_v0”, referenced from:
…
…
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

探索

通过网上搜索到如下解决方案

1. 将Build Settings->Apple LLVM7.1-Language中的Compile Sources As修改为According To File Tyle
2. 将Build Settings->Apple LLVM7.1-Language中的Compile Sources As修改为Objective-C++

以上第二个方法对我不适用，修改为全部按Objective-C++方式编译源文件会出问题。第一个解决方案的话也有问题，由于项目中没有了混编文件（.mm），那么Xcode根本不会调用Objective-C++编译器，所以连接问题还是存在。

解决

将项目中的某个OC文件（.m）强行改为后缀为mm，使其成为混编文件。
这种方法也适用于Swift工程，如果Swift也要引用C＋＋的混编库，那么可以增加一个空的混编文件（.mm），该文件不用实现，只是为了欺骗IDE，调出相应编译器完成连接。