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perfect_forwarding.cpp
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perfect_forwarding.cpp
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#include <iostream>
#include <memory>
struct A {
int value;
A()
: value(0) {
}
A(const A& /* rhs */) {
std::cout << "copying (ctor)\n";
}
A(A&& /* rhs */) {
}
A& operator=(const A& /* rhs */) {
std::cout << "copying (operator)\n";
return *this;
}
A& operator=(A&& /* rhs */) {
return *this;
}
};
void bar(A&&) {
std::cout << "A&&\n";
}
void bar(A&) {
std::cout << "A&\n";
}
template <typename T>
void foo(T&& t) {
bar(std::forward<T>(t));
}
void baz(A&& t) { // t is lvalue
bar(t); // no way to perfect forward (call corresponding bar overload)
}
template <typename T, typename A>
inline
std::shared_ptr<T>
factory(A&& a) {
return std::shared_ptr<T>(new T(std::forward<A>(a)));
}
template <typename T, typename A>
inline
std::shared_ptr<T>
factory_noforward(A&& a) {
return std::shared_ptr<T>(new T(a));
}
struct B {
B(int&) {
std::cout << "B(int&)\n";
}
B(int&&) {
std::cout << "B(int&&)\n";
}
};
int make_int() {
return 0;
}
int main() {
A x; // lvalue
bar(x); // calls bar(A&)
bar(A()); // calls bar(A&&)
foo(x); // x is deduced as A& (lvalue ref) // bar(A&) is called
foo(A()); // A() is deduced as A // bar(A&&) is called
// baz(x); // error: invalid initialization of reference of type 'A&&' from expression of type 'A'
baz(std::move(x)); // bar(A&) is called
baz(A()); // bar(A&) is called
int i = 0;
factory<B>(i); // call B&
factory<B>(make_int()); // call B&&
factory_noforward<B>(i); // call B&
factory_noforward<B>(make_int()); // call B&
}