（自用）template泛型编程，传function,lambda,仿函数,函数指针-例子

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#include<bits/stdc++.h>

template<class S,class OP=S(*)(S,S)>
class SegmentTree
{
private:
    OP op;
public:
    SegmentTree(OP _op=OP()) : op(_op) {}
    S f(S a,S b)
    {
        return op(a,b);
    }
};

int mul(int a,int b) 
{
    return a*b;
}
struct Max
{
    int operator()(int a, int b)
    {
        return std::max(a,b);
    }
};

void mytest()
{
    std::function<int(int,int)> add = [](int a,int b){return a+b;};
    SegmentTree<int,decltype(add)>t1(add);
    std::cout<<t1.f(3,5)<<"\n";//8
    
    SegmentTree<int>t2(mul);
    std::cout<<t2.f(3,5)<<"\n";//15
    
    SegmentTree<int,Max>t3;
    std::cout<<t3.f(3,5)<<"\n";//5
}
int main()
{
    mytest();
    return 0;
}