Re: [Boost-users] [variant] Efficiency

Subject: Re: [Boost-users] [variant] Efficiency
From: Igor R (boost.lists_at_[hidden])
Date: 2009-04-21 10:25:50

> there is great disadvantage of variant in performance terms. One can't
> create instance of variant with proper type inside without calling
> copy costructor.

I think this depends on your compiler/optimizer. The following test
code was compiled with MSVC9, release mode (/O2 /Ob2 /Oi /Ot /Oy /GL),
see comments inside the code:

int main()
{
        struct A
        {
                      A() : a_(1)
                      {}
          int a_;
        };
        struct B
        {
                      B() : b_(2.0)
                      {}
          double b_;
        };
        struct Visitor : boost::static_visitor<>
        {
                void operator()(const A &a) const
                {	std::cout << a.a_; }
                void operator()(const B &b) const
                {	std::cout << b.b_; }

                void operator()(int i) const
                {	std::cout << i; }
                void operator()(const std::string &s) const
                {	std::cout << s; }
        };

        B b;
        boost::variant<A, B> v(b);
                    // the previous construction compiled into 1
instruction (even a call to the constructor was optimized out):
                    // movsd       xmm0,mmword ptr
[__real_at_4000000000000000 (41FD68h)]

        boost::apply_visitor(Visitor(), v);

        std::string s = "abcd";

        boost::variant<int, std::string> v1(s);
                    // the previous construction compiled into a
direct call to variant::make_initializer_node::initialize(),
                    // which in turn calls basic_string::assing() - that's all.

        boost::apply_visitor(Visitor(), v1);
}