I wanted to verify the if the following optimizations work as expected:
- RVO
- Named RVO
- Copy elision when passing an argument by value
So I wrote this little program:
#include <algorithm>
#include <cstddef>
#include <iostream>
#include <vector>
struct Foo {
Foo(std::size_t length, char value) : data(length, value) { }
Foo(const Foo & rhs) : data(rhs.data) { std::cout << "*** COPY ***" << std::endl; }
Foo & operator= (Foo rhs) {
std::cout << "*** ASSIGNMENT ***" << std::endl;
std::swap(data, rhs.data); // probably expensive, ignore this please
return *this;
}
~Foo() { }
std::vector<char> data;
};
Foo TestRVO() { return Foo(512, 'r'); }
Foo TestNamedRVO() { Foo result(512, 'n'); return result; }
void PassByValue(Foo inFoo) {}
int main()
{
std::cout << "
Test RVO: " << std::endl;
Foo rvo = TestRVO();
std::cout << "
Test named RVO: " << std::endl;
Foo named_rvo = TestNamedRVO();
std::cout << "
Test PassByValue: " << std::endl;
Foo foo(512, 'a');
PassByValue(foo);
std::cout << "
Test assignment: " << std::endl;
Foo f(512, 'f');
Foo g(512, 'g');
f = g;
}
And I compiled it with optimizations enabled:
$ g++ -o test -O3 main.cpp ; ./test
This is output:
Test RVO:
Test named RVO:
Test PassByValue:
*** COPY ***
Test assignment:
*** COPY ***
*** ASSIGNMENT ***
According to the output RVO and named RVO work as expected. However, copy elision is not performed for the assignment operator and when calling PassByValue
.
Is copy elision not allowed on user defined copy-constructors? (I know that RVO is explicitly allowed by the standard but I don't know about copy elision when passing by value.) Is there a way to verify copy elision without defining copy constructors?
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