is there any specific case where pass-by-value is preferred over pass-by-const-reference in C++?

Question

I read that they are conceptually equal. In practice, is there any occasion that

foo(T t) 

is preferred over

foo(const T& t)

? and why?

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Thanks for the answers so far, please note I am not asking the difference between by-ref and by-val.

Actually I was interested in the difference between by-const-ref and by-val.

I used to hold the oipinion that by-const-ref can replace by-value in call cases since even Herb Sutter and Bjarne said they are conceptually equal, and "by ref"(be it const) implies being faster. until recently, I read somewhere that by-val may be better optimized in some cases.

Then when and how?

Answer 1

Built-in types and small objects (such as STL iterators) should normally be passed by value.

This is partly to increase the compiler's opportunities for optimisation. It's surprisingly hard for the compiler to know if a reference parameter is aliasing another parameter or global - it may have to reread the state of the object from memory a number of times through the function, to be sure the value hasn't changed.

This is the reason for C99's restrict keyword (the same issue but with pointers).