假设要想获得一个STL序列容器
并且想将一个成员函数应用到这个容器包含的所有对象中
因为一个vector可以包含任意类型的对象
这就需要一个可以应用到任意类型的vector对象的函数
//: C05:ApplySequence.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Apply a function to an STL sequence container.
// const, 0 arguments, any type of return value:
template<class Seq, class T, class R>
void apply(Seq& sq, R (T::*f)() const) {
typename Seq::iterator it = sq.begin();
while(it != sq.end())
((*it++)->*f)();
}
// const, 1 argument, any type of return value:
template<class Seq, class T, class R, class A>
void apply(Seq& sq, R(T::*f)(A) const, A a) {
typename Seq::iterator it = sq.begin();
while(it != sq.end())
((*it++)->*f)(a);
}
// const, 2 arguments, any type of return value:
template<class Seq, class T, class R,
class A1, class A2>
void apply(Seq& sq, R(T::*f)(A1, A2) const,
A1 a1, A2 a2) {
typename Seq::iterator it = sq.begin();
while(it != sq.end())
((*it++)->*f)(a1, a2);
}
// Non-const, 0 arguments, any type of return value:
template<class Seq, class T, class R>
void apply(Seq& sq, R (T::*f)()) {
typename Seq::iterator it = sq.begin();
while(it != sq.end())
((*it++)->*f)();
}
// Non-const, 1 argument, any type of return value:
template<class Seq, class T, class R, class A>
void apply(Seq& sq, R(T::*f)(A), A a) {
typename Seq::iterator it = sq.begin();
while(it != sq.end())
((*it++)->*f)(a);
}
// Non-const, 2 arguments, any type of return value:
template<class Seq, class T, class R,
class A1, class A2>
void apply(Seq& sq, R(T::*f)(A1, A2),
A1 a1, A2 a2) {
typename Seq::iterator it = sq.begin();
while(it != sq.end())
((*it++)->*f)(a1, a2);
}
// Etc., to handle maximum likely arguments ///:~上面的apply()函数模板有一个对容器类进行引用的引用参数
还有一个指针参数
它指向容器类中包含的对象的一个成员函数
测试一下apply()几个重载版本
创建一个类Gromit
包含几个带有不同个数参数的函数
//: C05:Gromit.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// The techno-dog. Has member functions
// with various numbers of arguments.
#include <iostream>
class Gromit {
int arf;
int totalBarks;
public:
Gromit(int arf = 1) : arf(arf + 1), totalBarks(0) {}
void speak(int) {
for(int i = 0; i < arf; i++) {
std::cout << "arf! ";
++totalBarks;
}
std::cout << std::endl;
}
char eat(float) const {
std::cout << "chomp!" << std::endl;
return 'z';
}
int sleep(char, double) const {
std::cout << "zzz..." << std::endl;
return 0;
}
void sit() const {
std::cout << "Sitting..." << std::endl;
}
}; ///:~可以用apply()模板函数来调用vector<Gromit*>对象中包含的Gromit的成员函数
//: C05:ApplyGromit.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Test ApplySequence.h.
#include <cstddef>
#include <iostream>
#include <vector>
#include "ApplySequence.h"
#include "Gromit.h"
#include "../purge.h"
using namespace std;
int main() {
vector<Gromit*> dogs;
for(size_t i = 0; i < 5; i++)
dogs.push_back(new Gromit(i));
apply(dogs, &Gromit::speak, 1);
apply(dogs, &Gromit::eat, 2.0f);
apply(dogs, &Gromit::sleep, 'z', 3.0);
apply(dogs, &Gromit::sit);
purge(dogs);
getchar();
} ///:~
输出
arf!
arf! arf!
arf! arf! arf!
arf! arf! arf! arf!
arf! arf! arf! arf! arf!
chomp!
chomp!
chomp!
chomp!
chomp!
zzz...
zzz...
zzz...
zzz...
zzz...
Sitting...
Sitting...
Sitting...
Sitting...
Sitting...
purge()函数是一个小型的实用程序
函数调用delete来清除序列上的所有元素贴剩下的代码
//: :purge.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Delete pointers in an STL sequence container.
#ifndef PURGE_H
#define PURGE_H
#include <algorithm>
template<class Seq> void purge(Seq& c) {
typename Seq::iterator i;
for(i = c.begin(); i != c.end(); ++i) {
delete *i;
*i = 0;
}
}
// Iterator version:
template<class InpIt> void purge(InpIt begin, InpIt end) {
while(begin != end) {
delete *begin;
*begin = 0;
++begin;
}
}
#endif // PURGE_H ///:~