《C++ primer》里面讲到函数指针定义并调用，做一下练习6.54-6.56，题目如下：

vector元素的类型是函数指针的类型是int (*)(int, int)，指向的元素是有两个int形参，返回类型也是int的函数。

#include <vector>
#include <iostream>
using namespace std;

int add(int num1, int num2)
{
  return num1 + num2;
}
int subtract(int num1, int num2)
{
  return num1 - num2;
}
int multiply(int num1, int num2)
{
  return num1 * num2;
}
int divide(int num1, int num2)
{
  return num1 / num2;
}
int main()
{
  vector<int (*)(int, int)> a;
  a.push_back(add);
  a.push_back(subtract);
  a.push_back(multiply);
  a.push_back(divide);

  int res_add = a[0](6, 3);
  int res_subtract = a[1](6, 3);
  int res_multiply = a[2](6, 3);
  int res_divide = a[3](6, 3);
  cout << res_add << " " << res_subtract << " " << res_multiply << " " << res_divide;
  return 0;
}

输出