This stackoverflow post deals with your question directly. The wikipedia page is also useful. However, in this context a key difference between Julia and C++ is that while emulation of multiple dispatch in C++ involves some kind of virtual function/runtime dispatch, in Julia the dispatch is generally handled at compile time. In such cases, Julia will be more performant than C++ would be with a similar programming paradigm.
However, if you write more conventional C++, then Julia’s and C++'s performance should be quite similar. There are cases where limitations in one give an advantage to the other, but for many tasks they are pretty equivalent.