叮叮当当的包含优先级是怎么回事？

Question

My command：

/usr/bin/c++ -fPIC -I/Users/me/project/include -I/usr/local/include/opencv 
-I/usr/local/include -I/opt/local/include -std=c++11 -O3 -M -c 
/Users/me/project/src/program.cpp | grep opencv

program.cpp有：

#include "opencv2/core/core.hpp"
#include "opencv2/ml/ml.hpp"

输出：

  /opt/local/include/opencv2/core/core.hpp 
  /opt/local/include/opencv2/core/types_c.h /usr/include/assert.h 
  /usr/include/math.h /opt/local/include/opencv2/core/version.hpp 
  /opt/local/include/opencv2/core/operations.hpp 
  /opt/local/include/opencv2/core/mat.hpp 
  /opt/local/include/opencv2/objdetect/objdetect.hpp 
  /opt/local/include/opencv2/ml/ml.hpp 

但是，存在：/usr/local/include/opencv2/core/core.hpp 和/usr/local/include/opencv2/ml/ml.hpp 。

使用-v标志，clang告诉我：

ignoring duplicate directory "/usr/local/include"
  as it is a non-system directory that duplicates a system directory
#include "..." search starts here:
#include <...> search starts here:
 /Users/me/project/include
 /usr/local/include/opencv
 /opt/local/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1
 /usr/local/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/5.0/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
 /usr/include
 /System/Library/Frameworks (framework directory)
 /Library/Frameworks (framework directory)

为什么 clang 包含来自 /opt/local/include 而不是 /usr/local/include ，尽管/usr/local/include在命令的-I目录中排在第一位？为什么/usr/local/include被推下优先级列表。

Answer 1

您可以通过以下方式检查#include的默认搜索路径：

gcc -Wp,-v -E -

（将 -v 标志提供给预处理器）。

您的目录（以 -I 给出）按照您给出的顺序在标准列表之前搜索。

你明确地给出了/usr/local/include，gcc忽略了你的指令，因为它无论如何都会在以后添加（作为系统目录）;因此目录的搜索顺序错误。如果您真的想自己控制搜索的目录，请使用-nostdinc并将它们全部提供。这是极其脆弱的。

拥有两组具有相同名称的头文件是一个非常糟糕的主意（正如您发现的那样）。没有办法收拾那个烂摊子？