1 + 2 + 3 +... + N

1 + 2 + 3 + n

1. Constructor

Code:

#include <iostream>#include <vector>#include <assert.h>using namespace std;class temp{private :static unsigned int N,sum;public :temp(){++N;sum += N;}    staticvoid reset(){N=0;sum=0;}static int getSum(){return sum;}};unsigned int temp::N = 0;unsigned int temp::sum = 0;int main(){temp::reset();temp *a = new temp[3];delete [] a;a = NULL;cout<<temp::getSum()<<" ";    return 0;}

2. Use template Meta

#include <iostream>using namespace std;template<int N> struct sum{<span style="white-space:pre"></span>enum value {n = N + sum<N-1>::n};};template <> struct sum<1>{<span style="white-space:pre"></span>enum value { n= 1};};void main(){<span style="white-space:pre"></span>int a[sum<4>::n];<span style="white-space:pre"></span>cout<<sizeof(a) / sizeof( int) <<endl ;}

Running result:

For more templates, see http://blog.csdn.net/buyingfei8888/article/details/38413031

3. Use function pointers

#include <iostream>using namespace std;typedef unsigned int (*fun) (unsigned int);unsigned int soluation_tem(unsigned int n){<span style="white-space:pre"></span>return 0;}unsigned int soluation(unsigned int n){<span style="white-space:pre"></span>static fun f[2] = {soluation_tem,soluation};<span style="white-space:pre"></span>return n + f[!!n](n-1);}void main(){<span style="white-space:pre"></span>cout << soluation(3);<span style="white-space:pre"></span>}

4. Use virtual functions:

Code:

#include <iostream>using namespace std;class A;A * Array[2];class A{public :virtual unsigned int sum(int N){return 0;}};class B :public A{public:virtual unsigned int sum(int N){return Array[!!N]->sum(N-1) + N;}};unsigned int soluation(unsigned int n){A a;B b;Array[0] = &a;Array[1] = &b;return Array[1]->sum(n);}void main(){cout << soluation(3);}