Re: [Boost-users] [variant] Efficiency

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); }