cial">)</span> <span class="keyword">const</span><span class="special">;</span></pre>Return a (constant) reference to the old value that was copied. <p>Contract assertions should not change the state of the program so the old value copy is returned as <code class="computeroutput">const</code> (see  <a class="link" href="../../boost_contract/contract_programming_overview.html#boost_contract.contract_programming_overview.constant_correctness" title="Constant-Correctness">
Constant Correctness</a>). </p>
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