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// declval example
#include <utility> // std::declval
#include <iostream> // std::cout
struct A { // abstract class
virtual int value() = 0;
};
class B : public A { // class with specific constructor
int val_;
public:
B(int i,int j):val_(i*j){}
int value() {return val_;}
};
int main() {
decltype(std::declval<A>().value()) a; // int a
decltype(std::declval<B>().value()) b; // int b
decltype(B(0,0).value()) c; // same as above (known constructor)
a = b = B(10,2).value();
std::cout << a << '\n';
return 0;
}
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