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Design by Contract in D

The D programming language has native support for Design by Contract — built right into its syntax from day one.

Using the in, out, and invariant blocks, D allows you to express preconditions, postconditions, and invariants directly in your code. These contracts are enforced at runtime, and their use is encouraged as part of D’s overall focus on correctness and expressiveness.

How Contracts Work in D​

Preconditions and Postconditions​

Use in and out blocks inside functions:

int deposit(int currentBalance, int amount)
in {
    assert(amount > 0); // Precondition
}
out(result) {
    assert(result == currentBalance + amount); // Postcondition
}
body {
    return currentBalance + amount;
}
  • in block: runs before the function body — must be satisfied to proceed
  • out(result) block: runs after — validates the result
  • body: the actual function logic

Invariants​

You can define class invariants with the invariant block:

class Account {
    int balance;

    this(int initial) {
        balance = initial;
    }

    void deposit(int amount)
    in {
        assert(amount > 0);
    }
    out {
        assert(balance >= 0);
    }
    body {
        balance += amount;
    }

    invariant {
        assert(balance >= 0);
    }
}
  • The invariant block is checked before and after any public method call
  • It enforces object-level correctness

Runtime Control​

Contracts in D are compiled in or out depending on build flags:

  • Use -release to disable them (e.g., in production)
  • Keep them on during development for full safety

Why D Matters​

D’s contract support is:

  • First-class and part of the language
  • Readable and idiomatic — no decorators or boilerplate
  • Designed to help catch bugs where they happen, not after

It strikes a great balance between systems-level performance and high-level expressiveness, making it a strong choice for correctness-minded developers.

Resources​