c++ - Pass by reference, constant reference, rvalue-reference, or constant rvalue-reference? -

i learning passing reference, , here test did:

#include <iostream>  using namespace std;  int = 0;  //if uncommented, compiler gives ambiguous definition error. //void paramcheck (string s) { //  cout << ++i << ". param var.\n"; //}  void paramcheck (const string& s) {     cout << ++i << ". param const ref.\n"; }  void paramcheck (string& s) {     cout << ++i  << ". param non-const ref.\n"; }  void paramcheck (const string&& s) {     cout << ++i  << ". param const rvalue-reference.\n"; }  void paramcheck (string&& s) {     cout << ++i  << ". param non-const rvalue-reference.\n"; }   int main(int argc, char **argv) {     //function call test     paramcheck("");      paramcheck(string{""});      string s3{""};     paramcheck(s3);      const string s4{""};     paramcheck(s4);      //illegal     //string& s{""};     //paramcheck(s);      const string& s5{s3};     paramcheck(s5);      string&& s6{""};     paramcheck(s6);      //illegal     //const string&& s{s1};     //onstfp(s);      //reference test     string = s3;     = "a changed s3";     cout << s3;      {     string& b = s3;     b = "b changed after assigning s3\n";     cout << "s3 " <<s3;      b = s4;     b = "b changed after assigning s4\n";     cout << "s3 " <<s3;     cout << "s4 " <<s4;     }      cin.get();     return 0; } 

and here result get:

1. param non-const rvalue-reference. 2. param non-const rvalue-reference. 3. param non-const ref. 4. param const ref. 5. param const ref. 6. param non-const ref. s3 b changed after assigning s3 s3 b changed after assigning s4 s4 

my question is:

1. if pass constant expression, triggers non-constant rvalue-reference? under condition trigger constant rvalue-reference (and why s6 not trigging it?)

2. why non-constant reference , constant rvalue-reference illegal?

3. i expected cannot change s3, why b in inner scope can change s3? if assigning new object s3 b assigning new reference, why when assign s4 , s3 got changed , s4 empty afterwards?

sorry asking many questions... increase points when questions answered :) reference brings confusion pointer whole new level.

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i don't know how increase point... wait 2 days till eligible bounty choose answer.

first code

paramcheck(""); //constructs temporary. temporaries bind `string&&` paramcheck(string{""}); //constructs temporary. temporaries bind `string&&` string s3{""}; paramcheck(s3); //passes reference existing string: `string&` const string s4{""}; paramcheck(s4); //passes reference existing string+const: `const string&` //illegal //string& s{""}; //cannot assign temporary non-const l-reference                  //what s refer when temporary "dies"?                  //`const string&` have worked though //paramcheck(s); //passes reference existing string+const: `const string&` const string& s5{s3}; //s5 s3, `const`.  paramcheck(s5); //passes reference existing string+const: `const string&` string&& s6{""}; //r-references extend life of temporaries. paramcheck(s6); //passes reference existing strong: `string&` //const string&& s{s1}; //temporaries can extended `t&&` or `const t&` only.  //reference test string = s3; //a _copy_ of s3 = "a changed s3"; //so changing copy doesn't effect origional. cout << s3; //s3 still blank, hasn't changed.  { string& b = s3; //b isn't "reference" `s3`".  `b` _is_ `s3`. b = "b changed after assigning s3\n"; //since `b` `s3`, changes `s3`. cout << "s3 " <<s3;  b = s4; //`b` _is_ `s3`, changed `s3` again. b = "b changed after assigning s4\n"; cout << "s3 " <<s3; cout << "s4 " <<s4; //s4 still blank, hasn't changed. } 

then questions:

if pass constant expression, triggers non-constant rvalue-reference? under condition trigger constant rvalue-reference (and why s6 not trigging it?)

existing objects pass string& or const string& depending on if they're const or not. can copied in string. temporaries pass string&&, can copied in string. there are ways trigger const string&&, there's no reason ever, doesn't matter. they're shown here.

why non-constant reference , constant rvalue-reference illegal?

the standard says const string& , string&& extend lives of temporaries, though i'm not why didn't mention string& , const string&&.

i expected cannot change s3, why b in inner scope can change s3? if assigning new object s3 b assigning new reference, why when assign s4 , s3 got changed , s4 empty afterwards?

you initialized b reference s3. not copy, reference. means b refers s3 forever, no matter what. when typed b = "b changed after assigning s3\n";, that's same s3 = "b changed after assigning s3\n";. when typed b = s4;, that's same s3 = s4. that's reference is. cannot "reseated".