c++ - std::is_same result with lvalue and rvalue reference -

-

i playing std::is_same utility function in combination rvalue , lvalue reference , came across weird behavior.

consider function template checks type of variable t.

i using vs 2013 :

struct test {};  template < class t> void h(t && t) {      cout << " type &&: " << std::is_same<decltype(t), t &&>::value << endl;     cout << " type &: " << std::is_same<decltype(t), t &>::value << endl; }

i observe following outputs :

h(test()); // type && : 1  type & : 0

which normal test() temporary object, , universal reference in parameter of h resolves r value reference (&& && = &&)

but consider :

test mytest; h(mytest); // type && : 1  type & : 1 !!!

same result if write :

test &mytest = test(): h(mytest); // type && : 1  type & : 1 !!!

and same :

test &&mytest = test(): h(mytest); // type && : 1  type & : 1 !!!

am missing ? looks complete mess me :) features rvalue reference / decltype not supported in vs 2013?

thanks help

romain

romain, easy enough. let's condider following case:

test mytest; h(mytest); // type && : 1  type & : 1 !!!

inside h, t test&, , t of type test& &&, is, test&. when test, std::is_same<decltype(t), t &&> true, since test& && test&, , t type test&.

std::is_same<decltype(t), t &> true, since t type test&, , t& test&&, test&.

careful application of reference collapsing rules helps here.

a bit on why inside h() t of type test&. reason is type of t match actual argument. t can not test, since test&& not bind(match) lvalue of type test (as argument type). however, test& && will, because of reference collapsing rules.


Comments

Post a Comment

Popular posts from this blog

ruby - Trying to change last to "x"s to 23 -

-
i have been playing around irb , have string looks this: irb(main):072:0> puts ["[\"4354 5432 5432 xxxxx\", \"6547 6547 8543 xxxxx\", \"2344 6543 6674 xxxxx\", \"2346 6236 7543 xxxxx\", \"1273 5585 5587 xxxxx\"]"] => nil irb(main):073:0> what want gsub last 2 "x" s of each 17 digit combination, 23 . what i've tried far (i've been using rubular): a.gsub(/\d[x]/,"23") <= grabs "x" s i'm close a.gsub(/\w[x{2}]/,"23") <= grabs "x" s grabs two digits each combination. is there easier way i'm not understanding? a.gsub(/\d{4} \d{4} \d{4} xxx\kxx/, '23') \d{4} = 4 digits what \k drop matched far. can use don't replace entire number, still able validate have in desired format.
Read more

jquery - Clone last and append item to closest class -

-
this bare-bones code on page: <div class="list-container"> <div class="single-list-item"> <button>remove</button> </div> <div class="single-list-item"> <button>remove</button> </div> <div class="single-list-item"> <button>remove</button> </div> </div> <p><button class="add-list-item">add</button></p> <div class="list-container"> <div class="single-list-item"> <button>remove</button> </div> <div class="single-list-item"> <button>remove</button> </div> <div class="single-list-item"> <button>remove</button> </div> </div> <p><button class="add-list-item">add</button></p> <di
Read more

c - Unrecognised emulation mode: elf_i386 on MinGW32 -

-
i'm trying make kernel, , cannot link c output assembly. ld . i'm getting error: unrecognized emulation mode: elf_i386 i'm using windows 10 professional mingw32 , msys. code using: link.ld /* * link.ld */ output_format(elf32-i386) entry(start) sections { . = 0x100000; .text : { *(.text) } .data : { *(.data) } .bss : { *(.bss) } } kernel.c /* * kernel.c */ void kmain(void) { const char *str = "my first kernel"; char *vidptr = (char*)0xb8000; //video mem begins here. unsigned int = 0; unsigned int j = 0; /* loops clears screen * there 25 lines each of 80 columns; each element takes 2 bytes */ while(j < 80 * 25 * 2) { /* blank character */ vidptr[j] = ' '; /* attribute-byte - light grey on black screen */ vidptr[j+1] = 0x07; j = j + 2; } j = 0; /* loop writes string video memory */ while(str[j] != '\0') { /
Read more