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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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