Try block and exception handling

1. exception handling
1)throw expression
The error detection section uses this expression to indicate that an unhandled error has been encountered ,throw An exception condition was raised .
2)try block
The error handling section uses it to handle exceptions .try Statement block to try Keyword start , And with one or more catch End of clause . stay try The exception thrown by the code executing in the block , Usually by one of them catch Clause processing .catch Clause is also called processing code .
2) A set of exception classes defined by the standard library , Used in throw And corresponding catch Pass relevant error messages between .

2.throw expression
The system goes through throw The expression throws an exception .throw The expression consists of the keyword throw And a trailing expression .

if(!item1.same_isbn(item2))
{
    
 throw runtime_error("Data must refer to same ISBN");
}
std:cout << item1+item2 << ::endl;

This code check ISBN Whether the objects are different , If it's not the same , Stop the execution of the program , And transfer control to the processing code that handles this error .
runtime_error Type is one of the standard library exception classes , stay stdexcept The header file defines .

3.try block

try{
    
 program-statements
}catch（exception-specifier）{
    
handler-statements
}catch（exception-specifier）{
    
handler-statements
}

With keywords try Start , This is followed by a statement sequence block enclosed in curly braces .try The block is followed by one or more catch Clause .
Every catch The clause consists of three parts ： keyword catch, Declaration of a single type or object in parentheses —— Called exception specifier , And a block of statements usually enclosed in curly braces .
try In the sentence program-statement Form the normal logic of the program .

while（cin >> item1 >> item2）{
    
// Perform adding two Sales_items object 
// If the addition fails ,throw One runtime_error expression 
}catch（runtime_error err）{
    
 // Remind users ISBN Must match and 
 cout << err.what()
      <<“\n Try Again?Enter y or n” <<endl;
      char c;
      cin >> c;
      if(cin && c == 'n')
         break;
}