C++ template
Templates are the basis of generic programming, which is writing code in a way that is independent of any specific type.
A template is a blueprint or formula for creating a generic class or function. Library containers, such as iterators and algorithms, are examples of generic programming that use the concept of templates.
Each container has a single definition, such as vector, and we can define many different types of vectors, such as vector <int> or vector <string>.
You can use templates to define functions and classes, let's take a look at how to use them.
Function template
The general form of a template function definition is as follows:
template <class type> ret-type func-name(parameter list) { // 函数的主体 }
Here, type is the placeholder name of the data type used by the function. This name can be used in function definitions.
The following is an example of a function template that returns the maximum value of two numbers:
#include <iostream> #include <string> using namespace std; template <typename T> inline T const& Max (T const& a, T const& b) { return a < b ? b:a; } int main () { int i = 39; int j = 20; cout << "Max(i, j): " << Max(i, j) << endl; double f1 = 13.5; double f2 = 20.7; cout << "Max(f1, f2): " << Max(f1, f2) << endl; string s1 = "Hello"; string s2 = "World"; cout << "Max(s1, s2): " << Max(s1, s2) << endl; return 0; }
When the above code is compiled and executed, it will produce the following results:
Max(i, j): 39 Max(f1, f2): 20.7 Max(s1, s2): World
Class Template
Just as we define function templates, we can also define class templates. The general form of a generic class declaration is as follows:
template <class type> class class-name { . . . }
Here, type is a placeholder type name that can be specified when the class is instantiated. You can define multiple generic data types using a comma-separated list.
The following example defines the class Stack<> and implements generic methods to push and pop elements onto the stack:
#include <iostream> #include <vector> #include <cstdlib> #include <string> #include <stdexcept> using namespace std; template <class T> class Stack { private: vector<T> elems; // 元素 public: void push(T const&); // 入栈 void pop(); // 出栈 T top() const; // 返回栈顶元素 bool empty() const{ // 如果为空则返回真。 return elems.empty(); } }; template <class T> void Stack<T>::push (T const& elem) { // 追加传入元素的副本 elems.push_back(elem); } template <class T> void Stack<T>::pop () { if (elems.empty()) { throw out_of_range("Stack<>::pop(): empty stack"); } // 删除最后一个元素 elems.pop_back(); } template <class T> T Stack<T>::top () const { if (elems.empty()) { throw out_of_range("Stack<>::top(): empty stack"); } // 返回最后一个元素的副本 return elems.back(); } int main() { try { Stack<int> intStack; // int 类型的栈 Stack<string> stringStack; // string 类型的栈 // 操作 int 类型的栈 intStack.push(7); cout << intStack.top() <<endl; // 操作 string 类型的栈 stringStack.push("hello"); cout << stringStack.top() << std::endl; stringStack.pop(); stringStack.pop(); } catch (exception const& ex) { cerr << "Exception: " << ex.what() <<endl; return -1; } }
When the above code is compiled and executed, It will produce the following results:
7 hello Exception: Stack<>::pop(): empty stack