Remco Remco - 4 years ago 72
C++ Question

How do I extend the lifetime of a temporary in a ranged for expression?

I'm getting dangling references while using a ranged-for loop. Consider the following C++14 expression (full example program below):

for(auto& wheel: Bike().wheels_reference())
wheel.inflate();


It's output is:

Wheel()
Wheel()
Bike()
~Bike() with 0 inflated wheels.
~Wheel()
~Wheel()
Wheel::inflate()
Wheel::inflate()


Obviously something is going very wrong. Wheels are accessed beyond their lifetime and the result is 0, not the expected 2.

An easy fix is to introduce a variable for
Bike
in
main
. However, I do not control the code in
main
or
Wheel
. I can only change the struct
Bike
.

Is there any way to fix this example by only changing
Bike
?

A successful solution solution will either fail at compile time, or count 2 inflated tires and not touch any objects beyond their lifetimes.

Appendix: compile ready source



#include <cstdlib>
#include <iostream>
#include <array>
#include <algorithm>
using std::cout;
using std::endl;

struct Wheel
{
Wheel() { cout << " Wheel()" << endl; }
~Wheel() { cout << "~Wheel()" << endl; }
void inflate() { inflated = true; cout << " Wheel::inflate()" << endl; }
bool inflated = false;
};

struct Bike
{
Bike() { cout << " Bike()" << endl; }
~Bike() {
cout << "~Bike() with " << std::count_if(wheels.begin(), wheels.end(),
[](auto& w) { return w.inflated; }) << " inflated wheels." << endl;
}
std::array<Wheel, 2>& wheels_reference() { return wheels; }
std::array<Wheel, 2> wheels{Wheel(), Wheel()};
};

int main()
{
for(auto& wheel: Bike().wheels_reference())
wheel.inflate();
return EXIT_SUCCESS;
}

Answer Source

Delete the rvalue overload of wheels_reference.

std::array<Wheel, 2>& wheels_reference() & { return wheels; }
std::array<Wheel, 2>& wheels_reference() && = delete;

That way you won't return a reference to a member of a temporary.

Your example usage of the Bike class

for(auto& wheel: Bike().wheels_reference())
    wheel.inflate();

Will then refuse to compile with (clang 3.4 output):

test.cpp:31:29: error: call to deleted member function 'wheels_reference'
    for(auto& wheel: Bike().wheels_reference())
                     ~~~~~~~^~~~~~~~~~~~~~~~
test.cpp:24:27: note: candidate function has been explicitly deleted
    std::array<Wheel, 2>& wheels_reference() && = delete;
                          ^
test.cpp:23:27: note: candidate function not viable: no known conversion from 'Bike' to 'Bike' for object argument
    std::array<Wheel, 2>& wheels_reference() & { return wheels; }

If the lifetime of the temporary is manually extended things work.

Bike&& bike = Bike();
for(auto& wheel: bike.wheels_reference())
    wheel.inflate();
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