vector element, accessed through raw pointer, unexpectedly changes value - c++

Question

Unexpected behavior of the following program (as short as I could make it):
#include <vector>
#include <iostream>
class Entry;
class CachedPayload {
public:
CachedPayload(const Entry* const o) : owner_(o) {}
void info(const char* where);
private:
const Entry* const owner_;
// meaningful payload not shown in this minimal example
};
class Entry {
public:
Entry(int i) : cached(this), my_i(i) {}
void showEntry(const char* where) const {
std::cout << where << ": this=" << this << ", i=" << my_i << std::endl; }
CachedPayload cached;
private:
int my_i;
};
void CachedPayload::info(const char* where) { owner_->showEntry(where); }
class CachingVector {
public:
CachingVector() {
resize();
data_.at(0).cached.info("GET");
}
void resize() {
data_.push_back(Entry(97));
data_.at(0).cached.info("RSZ");
}
private:
std::vector<Entry> data_;
};
int main()
{
CachingVector vec;
}
Outcome:
$ clang++-6.0 h.cpp && a.out
RSZ: this=0x7ffe1dc52dc8, i=97
GET: this=0x7ffe1dc52dc8, i=4206609
$ g++ h.cpp && a.out
RSZ: this=0x7ffc5e977040, i=97
GET: this=0x7ffc5e977040, i=-578764228
Why is the value of vec.data_[0].my_i, when accessed through vec.data_[0].cached.owner_, overwritten in a nonsensical, compiler-dependent way?
When I merge the two-line implementation of CachingVector::resize() into the constructor CachingVector(), then i=97 stays uncorrupted.

Answer 1

You are storing a pointer to a temporary in your vector (specifically in CachedPayload::owner_). That's the cause of the problem. Once the temporary has been destroyed you have a dangling pointer.
I added this to your code
~Entry() { std::cout << "destroyed " << this << std::endl; }
and this
CachingVector() {
data_.reserve(1); // prevent reallocation
resize();
data_.at(0).cached.info("GET");
}
New output is
destroyed 00000000001AF588
RSZ: this=00000000001AF588, i=97
GET: this=00000000001AF588, i=-858993460
As you can see you are accessing an object which has been destroyed, which is undefined behaviour of course.
To fix this I think you just need to define a copy constructor and assignment operator for Entry that make sure cached.owner_ always has the correct value.
Entry(const Entry& rhs) : cached(this), my_i(rhs.my_i) {}
Entry& operator=(const Entry& rhs) { my_i = rhs.my_i; return *this; }