FAQ 14.04: Does const allow the compiler to generate more efficient code?

Occasionally, but that's not the purpose of const. The purpose of const is correctness, not optimization. That is, const helps the compiler find bugs, but it does not (normally) help the compiler generate more efficient code.

Declaring the constness of a parameter or variable is just another form of type safety; therefore, const correctness can be considered an extension of C++'s type system. Type safety provides some degree of semantic integrity by promising that, for instance, something declared as a string cannot be used as an int. However, const correctness guarantees even tighter semantic correctness by making sure that data that is not intended to be changed cannot be changed. With const correctness, it is easier to reason about the correctness of the software. This is helpful during software inspection.