C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
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Section 18:
[18.14] Does const_cast mean lost optimization opportunities?

In theory, yes; in practice, no.

Even if the language outlawed const_cast, the only way to avoid flushing the register cache across a const member function call would be to solve the aliasing problem (i.e., to prove that there are no non-const pointers that point to the object). This can happen only in rare cases (when the object is constructed in the scope of the const member function invocation, and when all the non-const member function invocations between the object's construction and the const member function invocation are statically bound, and when every one of these invocations is also inlined, and when the constructor itself is inlined, and when any member functions the constructor calls are inline).