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This topic is about D – Contract Programming.

Contract programming in D programming is focused on providing a simple and understandable means of error handling. Contract programming in D are implemented by three types of code blocks −

• body block
• in block
• out block

Body Block in D

Body block contains the actual functionality code of execution. The in and out blocks are optional while the body block is mandatory. A simple syntax is shown below.

return_type function_name(function_params) 
in { 
   // in block 
} 

out (result) { 
   // in block 
}
 
body { 
   // actual function block 
}

In Block for Pre Conditions in D

In block is for simple pre conditions that verify whether the input parameters are acceptable and in range that can be handled by the code. A benefit of an in block is that all of the entry conditions can be kept together and separate from the actual body of the function. A simple precondition for validating password for its minimum length is shown below.

import std.stdio; 
import std.string;
  
bool isValid(string password) 
in { 
   assert(password.length>=5); 
}
 
body { 
   // other conditions 
   return true; 
}
  
void main() { 
   writeln(isValid("password")); 
}

When the above code is compiled and executed, it reads the file created in previous section and produces the following result −

true 

Out Blocks for Post Conditions in D

The out block takes care of the return values from the function. It validates the return value is in expected range. A simple example containing both in and out is shown below that converts months, year to a combined decimal age form.

import std.stdio;
import std.string;

double getAge(double months,double years) 
in { 
   assert(months >= 0); 
   assert(months <= 12); 
}
 
out (result) { 
   assert(result>=years); 
} 

body { 
   return years + months/12; 
} 
 
void main () { 
   writeln(getAge(10,12)); 
} 

When the above code is compiled and executed, it reads the file created in previous section and produces the following result −

12.8333

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