I'm doing some experimenting with y-combinator-like lambda wrapping (although they're not actually strictly-speaking y-combinators, I know), and I've encountered a very odd problem. My code operates exactly as I'd anticipate in a Debug configuration (with Optimizations turned off), but skips large (and important!) bits in Release (with it set to Optimizations (Favor Speed) (/Ox)).
Please note, the insides of the lambda functions are basically irrelevant, they're just to be sure that it can recursion correctly etc.
// main.cpp
#include <iostream>
#include <string>
#define uint unsigned int
// Defines a y-combinator-style thing to do recursive things. Includes a system where the lambda can declare itself to be obsolete.
// Yes, it's hacky and ugly. Don't worry about it, this is all just testing functionality.
template <class F>
class YCombinator {
public:
F m_f; // the lambda will be stored here
bool m_selfDestructing = false; //!< Whether the combinator will self-destruct should its lambda mark itself as no longer useful.
bool m_selfDestructTrigger = false; //!< Whether the combinator's lambda has marked itself as no longer useful.
// a forwarding operator:
template <class... Args>
decltype(auto) evaluate(Args&&... args) {
// Avoid storing return if we can,
if (!m_selfDestructing) {
// Pass itself to m_f, then the arguments.
return m_f(*this, std::forward<Args>(args)...);
}
else {
// Pass itself to m_f, then the arguments.
auto r = m_f(*this, std::forward<Args>(args)...);
// self-destruct if necessary, allowing lamdas to delete themselves if they know they're no longer useful.
if (m_selfDestructTrigger) {
delete this;
}
return r;
}
}
};
template <class F> YCombinator(F, bool sd)->YCombinator<F>;
// Tests some instances.
int main() {
// Most basic test
auto a = YCombinator{
[](auto & self, uint in)->uint{
uint out = in;
for (uint i = 1u; i < in; ++i) {
out += self.evaluate(i);
}
return out;
},
false
};
// Same as a, but checks it works as a pointer.
auto b = new YCombinator{
[](auto & self, uint in)->uint {
uint out = in;
for (uint i = 0u; i < in; ++i) {
out += self.evaluate(i);
}
return out;
},
false
};
// c elided for simplicity
// Checks the self-deletion mechanism
auto d = new YCombinator{
[&a, b](auto & self, uint in)->uint {
std::cout << "Running d(" << in << ") [SD-" << self.m_selfDestructing << "]..." << std::endl;
uint outA = a.evaluate(in);
uint outB = b->evaluate(in);
if (outA == outB)
std::cout << "d(" << in << ") [SD-" << self.m_selfDestructing << "] confirmed both a and b produced the same output of " << outA << "." << std::endl;
self.m_selfDestructTrigger = true;
return outA;
},
true
};
uint resultA = a.evaluate(4u);
std::cout << "Final result: a(4) = " << resultA << "." << std::endl << std::endl;
uint resultB = (*b).evaluate(5u);
std::cout << "Final result: b(5) = " << resultB << "." << std::endl << std::endl;
uint resultD = d->evaluate(2u);
std::cout << "Final result: d(2) = " << resultD << "." << std::endl << std::endl;
resultD = d->evaluate(2u);
std::cout << "Final result: d(2) = " << resultD << "." << std::endl << std::endl;
}
What should happen is that the first evaluation of d works fine, sets d.m_selfDestructTrigger, and causes itself to be deleted. And then the second evaluation of d should crash, because d no longer really exists. Which is exactly what happens in the Debug configuration. (Note: As @largest_prime_is_463035818 points out below, it shouldn't crash so much as encounter undefined behaviour.)
But in the Release configuration, as far as I can tell, all of the code in evaluate gets skipped entirely, and the execution jumps straight to the lambda. Obviously, break-points in optimised code are a little suspect, but that appears to be what's happening. I've tried rebuilding the project, but no dice; VS seems pretty adamant about it.
Am I crazy? Is there something I've missed? Or is this an actual bug in VS (or even the compiler)? Any assistance in determining if this is a code issue or a tool issue would be greatly appreciated.
Note: I'm on VS2019 16.8.3, using the /std:c++ latest featureset.
