C + + return value optimization Rvo

return value optimization is a compiler-based technique that accelerates the execution of source code by transforming the creation of source code and objects. RVO = return value optimization.

Test platform: STM32F103VG + Keil 5.15

Background:
We have a macaddress::toarray.

byte Const {        return (byte*) &Value;}

Because the package needs to return the object of the byte array class ByteArray, there are

Const {        return ByteArray ((byte6);}

Calling code

ByteArray bs = mac. ToArray (); Bs. CopyTo (General_reg. SHAR);

According to my shallow knowledge of C + +, when I return within ToArray, I will produce a copy of the object to the temporary object.
Then the equals sign at the caller produces a copy construct.

In fact, compile burn-write debugging, view disassembly

   358: ByteArray bs = Mac. ToArray ();0x0800595c4629      MOVr1,r50x0800595e A804ADDr0,sp, #0x100x08005960 f000fe92 BL. WMacAddress:: ToArray (0x08006688)359: BS. CopyTo (General_reg. SHAR);     the: 0x080059642300MOVS R3, #0x000x08005966 461AMOVr2,r30x08005968 F1040109ADDr1,r4, #0x090x0800596C A804ADDr0,sp, #0x100x0800596E f002fb8f BL. WArray:: CopyTo (0x08008090)

Directly allocates memory, which is used by incoming toarray. After the toarray, it was not seen that the second copy structure was assumed.
Here's a look at ToArray's disassembly.

0x08006688 B570PUSH{r4-r6,lr}0x0800668a4605      MOVr5,r00x0800668c0C      MOVR4,R1481: Return ByteArray (byte*) &value,6);0x0800668e2206MOVS R2, #0x060x08006690 F1040108ADDR1,R4, #0x080x080066944628      MOVr0,r50x08006696 f7fffdeb BL. W _zn9bytearrayc2epkhi (0x08006270) 0x0800669a4605      MOVr5,r0482:} 0x0800669c BD70POP{R4-R6,PC}

Oh, my God! There is only one constructor in this, not a guess, to construct a local variable and then return and copy it.
Also, the memory address of this constructor is the one that is passed in externally.

This is the C + + Rvo, return value optimization technology, did not expect MDK also support.

The acquisition of this skill has allowed me to raise my C + + level from 30% to 40%

C + + return value optimization Rvo