Re: Why does not RVO work in this case?

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On Mar 13, 2013 10:12 AM, "Henrik Mannerström" wrote:
>
> Hello,
>
> Below is my trivial test case that I think should exhibit return value
> optimization. However, when I run it I get the following result:
> Address of a in main ? ?0x7fff676bb130
> Address of a in func ? ?0x7fff676ba190
> 5
>
> I compiled with:
> g++ -std=gnu++0x -Wall -Wextra -pedantic -O2 test1.cc
>
> My compiler is:
> > g++ --version
> g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
> Copyright (C) 2011 Free Software Foundation, Inc.
>
> I guess the return values are not optimized away. How should I enable RVO?

The object in main() already exists when f() is called, so it's
address cannot be used as the return address for the function.

RVO doesn't work for assignment, only construction.

> BR,
> Henrik
>
>
>
> #include <iostream>
> #include <array>
>
> class MT {
> public:
> ? MT(int a = 0) {
> ? ? data[0] = a;
> ? }
>
> ? int val() {
> ? ? return data[0];
> ? }
>
> private:
> ? std::array<int, 1000> data;
> };
>
> MT func() {
> ? MT a(5);
> ? std::cout << "Address of a in func\t" << &a << std::endl;
> ? return a;
> }
>
> int main() {
> ? MT a;
> ? std::cout << "Address of a in main\t" << &a << std::endl;
> ? a = func();
> ? std::cout << a.val() << std::endl;
> ? return 0;
> }
>