References

[andrea9940]

On Friday, 20 September 2013 at 09:44:51 UTC, Namespace wrote:

> This prints 'ref' if you change func(A a) to func(const A a) 
> the match of const ref isn't prefered over A a because const 
> need an implicit conversion.
Thanks for the tip.

> a + b is an rvalue and will moved to func(A a). This is even 
> faster than ref. ;)
> A() is also moved.
Mmm... here is the assembly generated (with -release -O), it 
seems to me that the struct is always copied.

; fun(a + b)
LEA EAX,[EBP-604]
PUSH EAX
LEA EAX,[EBP-404]
PUSH EAX
LEA EAX,[EBP-804]
CALL 00402010             ;opAdd()
MOV EBX,EAX               ;pointer to the stack
MOV ECX,80
ADD EBX,1FC
004020C3: PUSH DWORD PTR DS:[EBX] ; copy !
SUB EBX,4                         ; copy !
LOOP SHORT 004020C3               ; copy !
CALL 00402048             ; fun(const A a)

;fun(A())
PUSH 80
MOV EAX,OFFSET 00422930
PUSH EAX
CALL 00405DB0
MOV DWORD PTR SS:[EBP-4],EAX
MOV ECX,DWORD PTR SS:[EBP-4]
XOR EAX,EAX
MOV DWORD PTR DS:[ECX],EAX
MOV EAX,DWORD PTR SS:[EBP-4]
MOV DWORD PTR DS:[EAX+4],0
; a lot of movs later ...
MOV EAX,DWORD PTR SS:[EBP-4]
MOV DWORD PTR DS:[EAX+1FC],0
MOV ESI,DWORD PTR SS:[EBP-4]
LEA EDI,[EBP-204]
MOV ECX,80
REP MOVS DWORD PTR ES:[EDI],DWORD PTR DS:[ESI]
ADD ESP,8
; ^ a lot of istructions to set a struct to zero imo
LEA EBX,[EBP-8]
MOV ECX,80
0040271B: PUSH DWORD PTR DS:[EBX]    ; copy !
SUB EBX,4                            ; copy !
LOOP SHORT 0040271B                  ; copy !
CALL 00402048                 ;fun(const A a)

; fun(a)
LEA EAX,[EBP-804]  ;load the reference
CALL 00402060      ; fun(const ref A a)