动态#include基于宏定义 [英] Dynamic #include based on macro definition

问题描述

我在写一个C ++应用程序，其中我想让开发人员选择在编译时为特定问题使用什么算法。这两种算法都实现为实现公共接口的C ++类，并且是互相替换的。他们都有一个.h和一个.cpp文件，并驻留在一个子目录（让我们称之为 impl / ）。

在我的Makefile中，我有这样的：

  ... 
 IMPL = default 
 ... 
 binary：... impl / $（IMPL）.o 
 ... 
 impl /％。o：impl /％。 h impl /％。h 
 ... 
％o：％.cpp ... 
 $（CXX）$（CXXFLAGS）-DIMPL = $（IMPL）-c -o $ @ $ *。cpp 
  

这个想法是用户应该能够输入 make binary IMPL = thatimpl 。

在任何想要使用用户选择的算法的文件中，

  IImpl o = new IMPL（）; 
  

但是，这需要包括所选实现的头文件。不幸的是，C ++需要 #include 后跟一个string， ; libfile> 。您也可以此处的建议使用宏，但它需要宏的参数为文字字符串。如果我使用：

  #define QUOTEME（M）#M 
 #define INCLUDE_FILE （M）QUOTEME（impl / ## M ##。h）
 #include INCLUDE_FILE（IMPL）
  

$ b b

编译器将尝试包含文本字符串 impl / IMPL.h ，而不是将 IMPL 无论是传递给 make 然后到编译器。

任何指向我如何实现这将是非常欢迎！

解决方案

您只需要添加一个额外的间接层，因为预处理器的工作方式。这应该：

  #define QUOTEME（x）QUOTEME_1（x）
＃定义QUOTEME_1（x）#x 
 #define INCLUDE_FILE（x）QUOTEME（impl / xh）
 
 #include INCLUDE_FILE（IMPL）
  pre> 
 
 
  直播示例  
 
I'm writing a C++ application in which I want to let the developer choose what algorithm to use for a particular problem at compile time. Both algorithms are implemented as C++ classes that implement a common interface, and are drop-in replacements for each other. They both have a .h and a .cpp file, and reside in a subdirectory (let's call it impl/).

In my Makefile, I have something along the lines of this:
...
IMPL = default
...
binary: ... impl/$(IMPL).o
...
impl/%.o: impl/%.cpp impl-interface.h impl/%.h
...
%o: %.cpp ...
    $(CXX) $(CXXFLAGS) -DIMPL=$(IMPL) -c -o $@ $*.cpp
The idea is that the user should be able to type make binary IMPL=thatimpl.

In whatever files wants to use the algorithm the user has chosen, I then do:
IImpl o = new IMPL();
However, this requires me to include the header file for the chosen implementation. Unfortunately, C++ requires #include to be followed by either a "string", a <libfile>. You can also use a macro as suggested here, but it requires the argument to the macro to be a literal string. If I use:
#define QUOTEME(M)       #M
#define INCLUDE_FILE(M)  QUOTEME(impl/##M##.h)
#include INCLUDE_FILE(IMPL)
The compiler will try to include the literal string impl/IMPL.h, rather than expanding IMPL to whatever was passed to make and then to the compiler.

Any pointers on how I might achieve this would be very welcome!
解决方案
You just need to add an extra layer of indirection, because of the way the preprocessor works. This should do:
#define QUOTEME(x) QUOTEME_1(x)
#define QUOTEME_1(x) #x
#define INCLUDE_FILE(x) QUOTEME(impl/x.h)

#include INCLUDE_FILE(IMPL)
Live example

                        
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