Question

What’s wrong with this code below and how to fix it.

#include<iostream>
using namespace std;

template<typename Func1, typename Func2>
class guard{
public:
    guard(Func1 first, Func2 last) : last(last){
        first();
    }
    ~guard(){
        last();
    }
private:
    Func2& last;
};

template<typename Func1, typename Func2>
guard<Func1, Func2> make_guard(Func1 first, Func2 last){
    return guard<Func1, Func2>(first, last);
}

void first(){
    cout << "first" << endl;
}

void last(){
    cout << "last" << endl;
}

int main(){
    {
        first(); // ok
        last(); // ok
        auto g = make_guard(first, last);
        first(); //exception: Access violation
        last(); //exception: Access violation
    }
    first(); // ok
    last(); // ok
    cin.get();
}

The function first() and last() can’t be called before variable g expired. Compiled at VC++ 2012 and got the same problem both in debug and release mode.

Answer 1

Your guard keeps a reference, but it takes a value. The reference becomes invalid as soon as guard‘s constructor end, as it refers to the last parameter taken by the constructor and not to the parameter passed to make_guard.

Once you access an invalid reference you have undefined behavior, and all bets are off.