6.5.1

6.40

 (a) int ff(int a, int b = 0, int c = 0);   //正常的 (b) char *init(int ht = 24, int wd, char backgrnd);//ht有默认值，所以后面的形参都必须有默认值

6.41

char *init(int ht, int wd = 80, char backgrnd = ' ');(a)  init();(b)  init(24,10);(c)  init(14,'*');

（a）不合法，至少应该传入一个实参
（b）合法
（c）合法，但是匹配有问题，char会被转化成int，是否是你想要的呢

6.42

include<iostream>  #include<string>  using namespace std;  //如果ctr不为1则返回word的复数版本  string make_plural(size_t ctr, const string &word,                                 const string &ending = "s")  {      return (ctr = 1) ? word : word + ending;  }  int main()  {      cout << "Singular version: " << make_plural(1, "success", "es")          << "\t\t plural version: "          << make_plural(0, "success", "es") << endl          << "Singular version: " << make_plural(1, "failure")          << "\t\t plural version: "          << make_plural(0, "failure") << endl;      return 0;  }