c++ - Is it better to pass-by-value and then move-construct or to pass-by-reference and then copy-construct -

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• why copy move? 4 answers

in c++11 in constructor, better do:

class {   a(std::string str) : mstr(std::move(str)) {}   std::string mstr;   } 

or do

class {   a(const std::string& str) : mstr(str) {}   std::string mstr;   } 

in cases can copy elision occur when rvalue passed in constructor?

you should "pass value , move construct" if type's move constructor cheap, , use "pass reference , copy construct" otherwise.

for lvalues

pass value , move construct

you doing 1 copy followed 1 move.

pass reference , copy construct

you doing 1 copy

for rvalues

pass value , move construct

in best case (when rvalue temporary expression) doing no copy/no move, followed move.

in normal case (when rvalue not temporary expression) doing move followed move.

pass reference , copy construct

you doing 1 copy