在宏中使用`else` [英] usage of `else` in macros

问题描述

我看到以下代码：

  #define QL_REQUIRE（condition，message）\ 
 if ！（condition））{\ 
 std :: ostringstream _ql_msg_stream; \ 
 _ql_msg_stream<<信息; \ 
 throw QuantLib :: Error（__ FILE __，__ LINE__，\ 
 BOOST_CURRENT_FUNCTION，_ql_msg_stream.str（））; \ 
} else 
  

这是我们假设使用它的方式。

  void testingMacros1（）{
 double x = 0.0; 
 QL_REQUIRE（x！= 0，Zero number！）; 
} 
  

我假设 else 到最后有一些特殊的用法。

问题> else 这个宏定义？

谢谢

解决方案

条件。它需要条件为 true ，否则它将抛出异常。如果是真的，你可以把大括号放在正常的如果语句之后。

这个：

  QL_REQUIRE（x！= 0，x不能为0）
 {
y = 100 / x; //除以0是坏的
} 
  

条件中的宏订阅，它失败，它将打印给定的消息。如果它不失败，你的大括号或单行形成else语句。当看看整个事情时，逻辑只是有点反转。当使用它，它就像一个 if ，但是当它把它，如果和

pre> assert（x！= 0&&x must not be 0）;
y = 100 / x; // divide by 0 is bad


I have seen the following code:

#define QL_REQUIRE(condition,message) \
if (!(condition)) { \
    std::ostringstream _ql_msg_stream; \
    _ql_msg_stream << message; \
    throw QuantLib::Error(__FILE__,__LINE__, \
                          BOOST_CURRENT_FUNCTION,_ql_msg_stream.str()); \
 } else 

This is how we suppose to use it.

void testingMacros1 (){
   double x =0.0;
   QL_REQUIRE (x!=0 ," Zero number !");
}

I assume the else in the end has some special usage.

Question> What is the usage of else appended in the end of this macros definition?

Thank you

解决方案

The macro checks the condition. It needs the condition to be true, or else it will throw an exception. If it's true, you'd put braces after like a normal if statement.

You'd use it like this:

QL_REQUIRE (x != 0, "x must not be 0")
{
    y = 100 / x; //dividing by 0 is bad
}

The macro subs in the condition, and if it fails, it will print the given message. If it doesn't fail, your braces or one-liner form the else statement. The logic is just a bit reversed when looking at the whole thing. When using it, it's like an if, but when subbing it in, the if and the else sort of get reversed roles.

It's sort of like saying this:

assert (x != 0 && "x must not be 0");
y = 100 / x; //dividing by 0 is bad

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