c++ - References not giving expected result -
if run below code, outputs,
key: 3, value: 4 key: 32767, value: 1971892984 i created example mistake, know kv class should not storing references in member variables. having trouble explaining result.
i have expected key_ , value_ alias i , j on stack. should still valid when passed cout << first since haven't been taken off stack yet. can't guess reason why output different. implementation-specific detail i'm seeing? or there deeper here understand?
#include <iostream> using namespace std; template <typename k, typename v> class kv { public: kv(): key_(), value_() {} kv(k& key, v& value): key_(key), value_(value) {} k key() const { return key_; } v value() const { return value_; } private: k& key_; v& value_; }; template <typename t, typename u> ostream& operator<<(std::ostream &strm, const kv<t, u> &a) { cout << "key: " << a.key() << ", value: " << a.value() << endl; strm << "key: " << a.key() << ", value: " << a.value() << endl; } template <typename t, class u> kv<t, u> make_kv(t t, u u) { kv<t,u> kv(t, u); return kv; } int main() { int = 3, j = 4; kv<int, int> first = make_kv(i, j); cout << first; return 0; }
you have dangling references, , therefore undefined behaviour. reason references store in make_kv references local objects, scope function itself:
template <typename t, class u> kv<t, u> make_kv(t t, u u) { kv<t,u> kv(t, u); return kv; } // t, u end here!!! you fix problem modifying make_kv take references:
template <typename t, class u> kv<t, u> make_kv(t& t, u& u) { kv<t,u> kv(t, u); return kv; } note: must make sure understand copy , assignment semantics of types hold references. might idea make references const limit scope of unintended behaviour.
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