Re: [Boost-users] [variant] Efficiency

In common case it is not compiller solution due to complexity of constructors/destructors of holded class. variant v( B() ); this code MUST call constructor of class B two times. First time when B is created, second time when just created instance of B copyied to variant. Valid solution is use type-traits. Something like: variant v( Trait<B>()) where Trait is empty class with typedefs On Tue, Apr 21, 2009 at 6:25 PM, Igor R wrote:

...

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 v(b);                    // the previous construction compiled into 1 instruction (even a call to the constructor was optimized out):                    // movsd       xmm0,mmword ptr [__real@4000000000000000 (41FD68h)]

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

       std::string s = "abcd";

       boost::variant 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); } _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users