Address of a dereferenced InputIterator? The case of istream_iterator

问题

I have a legacy code in which the interface is defined for pointer. I am trying to adapt some functions to take iterators, e.g. forward iterators.

Is one allowed to take the address of the element dereferenced by InputIterator such as istream_iterator?

The result is a temporary and has to be is somewhere in memory for the life of the call, but I am not sure.

The following example uses double, but the type can be more complicated (large).

#include<iostream>
#include<iterator>
#include<sstream>

void f_legacy(double const* t){
    std::cout << *t << std::endl;
};

void f(std::istream_iterator<double> it){
    f_legacy(std::addressof(*it)); // OK????
    // to avoid a copy:     auto v = *it; f_legacy(std::addressof(v));
}

int main(){
    double d = 5.;
    std::istringstream iss("1 2 3");
    std::istream_iterator<double> it(iss);
    f_legacy(&d);
    f(it);
}

回答1:

It is always legal to take the address of an lvalue.

Since istream_iterator::operator* returns a reference, it must be returning a reference to an object that survives beyond the function call, and until the iterator is advanced. So your code is well-defined.

来源：https://stackoverflow.com/questions/49267066/address-of-a-dereferenced-inputiterator-the-case-of-istream-iterator