Cpp: DRY for error check using preprocessor

#cpp

#exception

#preprocessor

Introduction

In order to deal with many checks conditions, the below code snippet is rapid to implement and is DRY.

Use case

Suppose to parse a json string: how many different cases to control ?!
<br>Let's suppose a method like the below:

void JsonParser::parsing_json(const json&obj) {
 auto it = obj.cbegin();
 int position = 0;

 while(it != obj.cend()) {
 if (it->value() != "{")
 throw my_exception("Error at position {}", position);

 //...code...and other check
 if (!it->find("}")) 
 throw my_exception("Error at position {}", position);

 //...code...and other check
 if (!it->find("[")) 
 throw my_exception("Error at position {}", position);

 //...code...
 } 
}

Using inner preprocessor the code will turn to:

void JsonParser::parsing_json(const json&obj) {
#define JSON_CHECK(x) if (x) throw position;

 auto it = obj.cbegin();
 int position = 0;

 try {
 while(it != obj.cend()) {
 JSON_CHECK(it->value() != "{"])

 //...code...and other check
 JSON_CHECK(!it->find("}")) 

 //...code...and other check
 JSON_CHECK(!it->find("[")) 

 //...code... 
 } 
 } 
 catch (int pos) {
 //...customize your exception here
 throw my_exception("Error at position {}", pos);
 }

#undef JSON_CHECK 
}

Note

Preprocessor are not always the best solution, but each case is different and you should consider your specific case before accept the statement "preprocessor must be avoided" ^__^
<br>Enjoy

#cpp

#exception

#preprocessor