boost :: combine（）是否与方法的输出一起工作？

Question

In the test case below, I use boost::combine to iterate on output of a function getPoints() .

Expected Output

I expect (1, 2, 3) printed 6 times; since I effectively zip two lists -

([point, point, point], [point, point, point]).

Actual Output

The output is surprising to me, and wrong. The first two lines are off suggesting memory corruption?

(0, 0, 3)          // <-- wrong!
(52246144, 0, 3)   // <-- wrong! memory corruption?
(1, 2, 3)
(1, 2, 3)
(1, 2, 3)
(1, 2, 3)

This can also be verified online here, http://cpp.sh/622h4 .

Is this a bug?

Code below -

#include <iostream>
#include <vector>

#include <boost/range/combine.hpp>


struct Point {
    int x, y, z;
};

const std::vector<Point> getPoints() {
    // There is only one Point in the entire code, which is (1, 2, 3).
    const Point point = {1, 2, 3};
    // Return a vectore of 3 copies of the point (1, 2, 3).
    return {point, point, point};
}


int main() {
    // Zip over two copies of 3-tuples of {1, 2, 3}.
    for (const auto& zipped : boost::combine(getPoints(), getPoints())) {
        auto p1 = zipped.get<0>();
        auto p2 = zipped.get<1>();
        // Expected output is (1, 2, 3), six times.
        std::cout << "(" << p1.x << ", " << p1.y << ", " << p1.z << ")" << std::endl;
        std::cout << "(" << p2.x << ", " << p2.y << ", " << p2.z << ")" << std::endl;
    }

    return 0;
}

Answer 1

You have undefined behavior here as you access a dangling reference. This can be fixed by

const auto points1 = getPoints();
const auto points2 = getPoints();

for (const auto& zipped : boost::combine(points1, points2)) {
    // ...
}

Rvalue references are always problematic when dealing with range libraries. Obviously, a range algorithm like boost::combine doesn't copy the argument. And it creates a new proxy range object, which makes it impossible to extend the lifetime of the temporary range passed in.

Contrary, a range-based for loop for(const auto& item: getPoints()) {...} expands to

{
    auto && __range = getPoints();
    for (auto __begin = begin_expr, __end = end_expr; __begin != __end; ++__begin) {

        range_declaration = *__begin;
        loop_statement
    }
} 

where the lifetime of getPoints() is extended by binding it to an rvalue reference. Imagine a function template combine as

template<class Rng>
auto combine(Rng&& rng) {
    auto && == range; // Nice try but doesn't help

    // ...

    return someProxyRange;
}

This function template can't do anything about extending the lifetime of rng , as it acts in a different scope than rng , which comes from the client side. In a range based for loop, this is different. The scope of the temporary (eg getPoints() ) and the forwarding reference auto&& __range are at the same scope, hence the lifetime can be extended.