I want to fill std::vector (or some other STL container):
class Foo {
public:
Foo(int _n, const Bar &_m);
private:
std::vector<Foo> fooes_;
}
1.Good looking ctor, expensive performance
std::vector<Foo> get_vector(int _n, const Bar &_m) {
std::vector<Foo> ret;
... // filling ret depending from arguments
return ret;
}
Foo::Foo(int _n, const Bar &_m) : fooes_(get_vector(_n, _m) {}
2. Better performance, worse looking ctor
void fill_vector(int _n, const Bar &_m, std::vector<Foo> &_ret) {
... // filling ret depending from arguments
}
Foo::Foo(int _n, const Bar &_m) { fill_vector(_n, _m, fooes_); }
Is it possible to rewrite get_vector
function from 1st example with C++0x (move semantics features and so on) to avoid redundant copying and constructor calls?
If you're using a C++0x-compatible compiler and standard library, you get better performance from the first example without doing anything. The return value of get_vector(_n, _m)
is a temporary, and the move constructor for std::vector
(a constructor taking an rvalue reference) will automatically be called with no further work on your part.
In general, non-library writers won't need to use rvalue references directly; you'll just reap a decent chunk of the benefits automatically.