<Effective C++>读书摘要--Designs and Declarations<二>

<Item 20> Prefer pass-by-reference-to-const to pass-by-value

1、By default, C++ passes objects to and from functions by value (a characteristic it inherits from C). Unless you specify otherwise, function parameters are initialized with copies of the actual arguments, and function callers get back a copy of the value returned by the function. These copies are produced by the objects‘ copy constructors. This can make pass-by-value an expensive operation. 

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