C++模板与异常处理详解

# C++模板与异常处理详解 ## 1. 函数模板输出运算符 为`BinaryTree`类模板提供输出运算符时，若为非模板类，可按如下方式编写： ```cpp ostream& operator<<(ostream&, const int_BinaryTree&); ``` 对于类模板，若为每个生成的类定义提供显式实例，会十分繁琐且永无止境。更好的解决方案是将输出运算符定义为函数模板： ```cpp template <typename elemType> inline ostream& operator<<(ostream &os, const BinaryTree<elemType> &bt) { os << "Tree: " << endl; bt.print(os); return os; } ``` 当执行以下代码时： ```cpp BinaryTree< string > bts; cout << bts << endl; ``` 会生成支持`BinaryTree<string>`作为第二个参数的输出运算符实例。同理，对于`BinaryTree<int>`也会生成相应实例。 `print()`是`BinaryTree`类模板的私有成员函数，为使输出运算符能访问`print()`，需将其声明为`BinaryTree`的友元： ```cpp template <typename elemType> class BinaryTree { friend ostream& operator<<(ostream&, const BinaryTree<elemType>&); // ... }; ``` ## 2. 常量表达式与默认参数 模板参数不仅限于类型，还可以是常量表达式。例如，可将数值序列类层次结构重新定义为模板类，使对象包含的元素数量参数化： ```cpp template <int len> class num_sequence { public: num_sequence(int beg_pos=1); // ... }; template <int len> class Fibonacci : public NumericSeries<len> { public: Fibonacci(int beg_pos=1) : num_sequence<len>(beg_pos){} // ... }; ``` 创建`Fibonacci`对象时，如： ```cpp Fibonacci< 16 > fib1; Fibonacci< 16 > fib2(17); ``` 会生成`Fibonacci`派生类和`num_sequence`基类的实例，且`len`绑定为16。也可同时对长度和起始位置进行参数化： ```cpp template < int len, int beg_pos > class NumericSeries; ``` 由于多数类对象从位置1开始，可提供位置的默认值： ```cpp template <int len, int beg_pos=1> class Fibonacci : public num_sequence<len,beg_pos> { ... }; ``` 以下是这些类对象的定义示例： ```cpp num_sequence<32> *pns1to32 = new Fibonacci<32>; num_sequence<32,33> *pns33to64 = new Fibonacci<32,33>; ``` 默认参数值从右到左按位置解析。以下是重新定义的`num_sequence`和`Fibonacci`类： ```cpp template <int len, int beg_pos> class num_sequence { public: virtual ~num_sequence(){}; int elem(int pos) const; const char* what_am_i() const; static int max_elems(){ return _max_elems; } ostream& print(ostream &os = cout) const; protected: virtual void gen_elems(int pos) const = 0; bool check_integrity(int pos, int size) const; num_sequence(vector<int> *pe) : _pelems(pe){} static const int _max_elems = 1024; vector<int> *_pelems; }; template <int len, int beg_pos> ostream& operator<<(ostream &os, const num_sequence<len,beg_pos> &ns) { return ns.print(os); } template <int len, int beg_pos> int num_sequence<len,beg_pos>:: elem(int pos) const { if (! check_integrity(pos, _pelems->size())) return 0; return (*_pelems)[pos-1]; } template <int length, int beg_pos> const char* num_sequence<length, beg_pos>:: what_am_i() const { return typeid(*this).name(); } template <int length, int beg_pos> bool num_sequence<length, beg_pos>:: check_integrity(int pos, int size) const { if (pos <= 0 || pos > max_elems()){ cerr << "!! invalid position: " << pos << " Cannot honor request\n"; return false; } if (pos > size) gen_elems(pos); return true; } template <int length, int beg_pos> ostream& num_sequence<length, beg_pos>:: print(ostream &os) const { int elem_pos = beg_pos-1; int end_pos = elem_pos + length; if (! check_integrity(end_pos, _pelems->size())) return os; os << "(" << beg_pos << " , " << length << ") "; while (elem_pos < end_pos) os << (*_pelems)[elem_pos++] << ' '; return os; } template <int length, int beg_pos=1> class Fibonacci : public num_sequence<length, beg_pos> { public: Fibonacci() : num_sequence<length,beg_pos>(&_elems){} protected: virtual void gen_elems(int pos) const; static vector<int> _elems; }; template <int length, int beg_pos> vector<int> Fibonacci<length,beg_pos>::_elems; template <int length, int beg_pos> void Fibonacci<length,beg_pos>:: gen_elems(int pos) const { if (pos <= 0 || pos > max_elems()) return; if (_elems.empty()) { _elems.push_back(1); _elems.push_back(1); } if (_elems.size() < pos) { int ix = _elems.size(); int n_2 = _elems[ix-2], n_1 = _elems[ix-1]; int elem; for (; ix < pos; ++ix) { elem = n_2 + n_1; _elems.push_back(elem); n_2 = n_1; n_1 = elem; } } } ``` 以下是一个测试程序： ```cpp int main() { Fibonacci<8> fib1; Fibonacci<8,8> fib2; Fibonacci<12,8> fib3; cout << "fib1: " << fib1 << '\n' << "fib2: " << fib2 << '\n' << "fib3: " << fib3 << endl; ```