I was trying to implement max-queue

But a more general version with arbitrary Associative function and arbitrary data type.I did the following for the max_stack part:

```
template<typename T,class F=function<T(const T&,const T&)> >
class Stack{
F func;
stack<pair<T,T> > st;
public:
Stack(F f):func(f){};
bool empty(){
return st.empty();
}
int size(){
return st.size();
}
void push(T nw){
if(empty()){
st.emplace(nw,nw);
return;
}
T prev=st.top().second;
prev=func(prev,nw);
st.emplace(nw,prev);
}
T pop(){
return st.pop();
}
T getval(){
return st.top().second;
}
};
```

The program worked well and the test was sucessful but then initializing two max_stacks for a max_queue caused some problems I implemented it as follows:

```
class Queue{
F func;
Stack<T> right;
Stack<T> left;
public:
explicit Queue(F f): func(f){
right(f);
left(f);
}
void push(T nw){
right.push(nw);
}
void pop(){
if(left.empty()){
while(!right.empty()){
left.push(right.pop());
}
}
assert(!left.empty());
left.pop();
}
T getval(){
return func(right.getval(),left.getval());
}
};
```

my editor didn’t give me any errors until I tried to use it.I initialized the queue as follows:

```
Queue<int> qp([&](int a,int b){
return min(a,b);
});
```

Then my editor gives the error: "Constructor for ‘Queue<int, std::function<int (const int &, const int &)>>’ must explicitly initialize the member ‘left’ which does not have a default constructor"

And the similar for the left stack.It seems that The problmes is about the template types.How can I fix that?

Source: Windows Questions C++