Re: We do not use C++ exceptions

Peter Dimov <>
Fri, 6 Feb 2009 02:01:48 CST
On Feb 5, 4:06 pm, Thant Tessman <> wrote:

Marsh Ray wrote:

On Feb 3, 12:15 am, Thant Tessman <> wrote:

Design by contract is nothing but a band-aid for the lack of a good type

// Contract: Function returns true iff 'v' represents
// the SHA-1 hash of the specified string.
bool checkHash(const std::string & str, const vector<uint8_t> & v);

Oh man, I'd love to see that contract expressed in a type system.

This is not a contract by my understanding. If it were, someone using
checkHash is expected to only pass in 'v' such that it represents the
SHA-1 hash of 'str'. And checkHash is only allowed to return true.

Your understanding of design by contract is flawed. The function does
have a contract, despite having no preconditions. The existence of a
precondition as such is not necessary for a contract to be in place;
without a precondition, the caller has no obligations, only the callee
does. You can remove the assertable preconditions and replace them
with a guarantee that states that a specific exception will be thrown,
and this would still be a contract. You can also transform
preconditions into invariants, starting from:

// pre: v.size() == 20
// returns: true iff v is the SHA-1 hash of str
bool checkHash(const std::string & str, const vector<uint8_t> & v);

and replacing it with

// returns: true iff v is the SHA-1 hash of str
bool checkHash(const std::string & str, const SHA1 & v);

where SHA1 is

class SHA1
    vector<uint8_t> data_;

and SHA1::data_.size() == 20 is an invariant of SHA1.

In the first case, checkHash is allowed to assert( v.size() == 20 );
in the second, it is allowed to assert( v.invariant() ). In both
cases, there asserts Should Never Fire but sometimes will, likely
because of a bug in the code that produces SHA-1 hashes that has not
been caught by the test suite.

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