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// Copyright (C) 2008-2018 Lorenzo Caminiti
// Distributed under the Boost Software License, Version 1.0 (see accompanying
// file LICENSE_1_0.txt or a copy at http://www.boost.org/LICENSE_1_0.txt).
// See: http://www.boost.org/doc/libs/release/libs/contract/doc/html/index.html
#include <boost/contract.hpp>
#include <cassert>
//[friend_invariant
template<typename T>
class positive {
public:
void invariant() const {
BOOST_CONTRACT_ASSERT(value() > 0);
}
// Can be considered an extension of enclosing class' public interface...
friend void swap(positive& object, T& value) {
boost::contract::old_ptr<T> old_object_value =
BOOST_CONTRACT_OLDOF(object.value());
boost::contract::old_ptr<T> old_value = BOOST_CONTRACT_OLDOF(value);
// ...so it can be made to check invariants via `public_function`.
boost::contract::check c = boost::contract::public_function(&object)
.precondition([&] {
BOOST_CONTRACT_ASSERT(value > 0);
})
.postcondition([&] {
BOOST_CONTRACT_ASSERT(object.value() == *old_value);
BOOST_CONTRACT_ASSERT(value == *old_object_value);
})
;
T saved = object.value_;
object.value_ = value;
value = saved;
}
private:
T value_;
/* ... */
//]
public:
// Could program contracts for following too.
explicit positive(T const& value) : value_(value) {}
T value() const { return value_; }
};
int main() {
positive<int> i(123);
int x = 456;
swap(i, x);
assert(i.value() == 456);
assert(x == 123);
return 0;
}
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