Why is this variable messed up after a function call - c++

Question

#include <iostream>
using namespace std;
template<class KeyT, class ValueT>
struct KeyValuePair {
const KeyT &key_;
const ValueT &value_;
KeyValuePair() {
cout << "KeyValuePair() constructor" << endl;
}
KeyValuePair( const KeyValuePair<KeyT, ValueT> &other) {
cout << "KeyvaluePiar copy constructor" << endl;
}
KeyValuePair(KeyT key, ValueT value) : key_(key), value_(value) {
cout << "KeyValuePair(KeyT, ValueT) constructor" << " key_: " << key_ << " value_ " << value_ << endl;
}
~KeyValuePair() {}
};
struct foo {
int i;
};
void dump(const KeyValuePair<int, foo*> &kp) {
//printf("dump printf key: %d, value: %p\n", kp.key_, kp.value_);
cout << "dump cout key_: " << kp.key_ << " value_: " << kp.value_ << " i: " << (kp.value_)->i << "\n";
}
int main() {
cout << "test kv\n";
foo *ptr = new foo();
ptr->i = 3000;
printf("address of ptr: %p\n", ptr);
dump(KeyValuePair<int, foo*>(10, ptr));
return 0;
}
Run it with
g++ -g -std=c++11 -fPIC -O0 -o main main.cc && ./main
on a Linux machine.
In the above c++ example code gives the following result
test kv
address of ptr: 0x18a1010
KeyValuePair(KeyT, ValueT) constructor key_: 10 value_ 0x18a1010
dump cout key_: 10 value_: 0x7fffae060070 i: -1375338428
It seems that KeyValuePair's value_ is messed up after calling dump function, anyone knows the reason? It seems to be related to reference and pointers.

Answer 1

Your member variable is a reference:
const KeyT &key_;
Your constructor, on the other hand, passes by value:
KeyValuePair(KeyT key, ValueT value)
That means you are storing a reference to a temporary variable that will get destroyed almost immediately.
One solution would be to pass by reference in your constructor:
KeyValuePair(KeyT& key, ValueT& value)
which is better, but not perfect, since you pass a int literal 10 into the function.
If you really just need a pair, the best solution is probably to use std::pair.