usage of `else` in macros

Question

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

Answer 1

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