C ++连接__FILE__和__LINE__宏？ [英] C++ concatenating __FILE__ and __LINE__ macros?

问题描述

我想要我的例外消息包含关于异常位置的信息。

所以我想有一些类似的东西。

  #define LOCATION __FILE__：__LINE__ 
 
 throw std :: exception（std :: string（ABCD。 ）; 
  

这个定义显然是不正确的。如何实现这一点？

解决方案

您需要双宏键技术（在两个级别展开该宏）：

  #define S1（x）#x 
 #define S2（x）S1（x）
 #define LOCATION __FILE__：S2（__ LINE__）
  

为什么？

$ b b

简短答案：

您需要在两个级别中展开 __ LINE __

p> 长回答：

首先，使用操作符＃ em 宏，它必须后跟一个宏参数， __ LINE __ 不是参数，否则编译器会抱怨它是一个杂散运算符。

其次， __ LINE __ 本身是一个宏并且包含当前行号，应该将其扩展为使用＃，否则，您将得到__ LINE __而不是数字。

宏 S2（__ LINE __）将 __ LINE __ 扩展到行号，然后将行号传递 #x 。

I want my exception messages to contain info about the exception location.

So I would like to have some like thing this.

#define LOCATION __FILE__ " : " __LINE__

throw std::exception(std::string("ABCD. ") + LOCATION);

That define is obviously incorrect. How to achieve this?

解决方案

You need double-macro-stringy technique (Expand that macro in two levels):

#define S1(x) #x
#define S2(x) S1(x)
#define LOCATION __FILE__ " : " S2(__LINE__)

Why?

Short answer:

You need expand __LINE__ in two levels, before passing it to #x.

 

Long answer:

First of all, using operator # in a function-like macro, it must followed by a macro parameter and __LINE__ is not a parameter, otherwise compiler complains it's a stray operator.

Second, __LINE__ itself is a macro and contains current line number, it should be expanded to the number before using it with #, otherwise, you will get string "__LINE__" instead of a number.

Macro S2(__LINE__) expands __LINE__ to a line number, then we pass the line number to #x.

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