四、C++语言进阶：Boost入门

2023-11-18

4 Boost入门

4.1 简介

Boost库是一个可移植、提供源代码的C++库，作为标准库的后备，是C++标准化进程的开发引擎之一，是为C++语言标准库提供扩展的一些C++程序库的总称。

4.2 使用

4.2.1 lamdba表达式

lambda库通过创建一个匿名的lambda表达式来代替实名的函数对象

HelloWorld.cpp

#include <iostream>  
#include <boost/lambda/lambda.hpp> 
using namespace boost::lambda; 
using namespace std; 
int main() {
    (cout << _1 << " " << _2)("Hello","World");
}

结果：

[root@localhost 4]# ./a.out 
Hello World[root@localhost 4]#

HelloWorld2.cpp

#include <iostream>  
#include <boost/lambda/lambda.hpp> 
using namespace boost::lambda; 
using namespace std;

int main(){  
    auto func = std::cout << _1 << " " << _2;
    func("Hello","World");
}

for_each.cpp

#include <iostream>  
#include <iterator>  
#include <algorithm>  
#include <vector>  
#include <boost/lambda/lambda.hpp>  
int main()  
{  
    using namespace boost::lambda;  
    using namespace std;  
    int arr[] = {1,2,3,4,5,6};
    for_each(arr,arr+6, cout << _1 << " " );
    cout << endl;

    vector<int> vec(arr,arr+6);
    for_each(vec.begin(),vec.end(), cout << _1 << " " );
    cout << endl;
}

结果：

[root@localhost 4]# ./a.out 
1 2 3 4 5 6 
1 2 3 4 5 6

注：如果需要使用_1、_2的成员变量。需要使用绑定bind()。

#include <iostream>  
#include <string>  
#include <map>  
#include <algorithm>  
#include "boost/lambda/lambda.hpp"  
#include "boost/lambda/bind.hpp"  

using namespace boost::lambda; 
using namespace std; 
int main() {  
    map<int,string> kv;
    kv[110] = "匪警";
    kv[119] = "火警";
    kv[120] = "急救中心";
    for_each(kv.begin(),kv.end(), cout << bind(&map<int,string>::value_type::first,_1) << '\t' <<  bind(&map<int,string>::value_type::second,_1) << '\n'); 
}

结果：

[root@localhost 4]# ./a.out 
110	匪警
119	火警
120	急救中心

4.2.2 容器中存放任意类型值

#include <iostream>
#include <string>
#include <vector>
#include <iterator>
#include <boost/any.hpp>

int main(){
    using namespace std;
    using namespace boost;

    any a1 = 10;
    any a2 = 1.1f;
    any a3 = string("abc");
    any a4 = "abcefg";

    cout << any_cast<int>(a1) << endl;
    cout << any_cast<float>(a2) << endl;
    cout << any_cast<string&>(a3) << endl;
    cout << any_cast<const char*>(a4) << endl;

    vector<any> vec2 = {"abcd",123,3.14};
    cout << any_cast<const char*>(vec2[0]) << endl;
    cout << any_cast<int>(vec2[1]) << endl;
    cout << any_cast<double>(vec2[2]) << endl;

    vector<any> vec;
    vec.push_back(10);
    vec.push_back(12.4);
    vec.push_back(string("string"));
    char const* c_str = "const char*";
    vec.push_back(c_str);

    cout << any_cast<int>(vec[0]) << endl
            << any_cast<double>(vec[1]) << endl
            << any_cast<string&>(vec[2]) << endl
            << any_cast<char const*>(vec[3]) << endl;

    // copy(vec.begin(),vec.end(),ostream_iterator<any>(cout,","));
}

结果：

10
1.1
abc
abcefg
abcd
123
3.14
10
12.4
string
const char*

注：
any_cast<>中的类型不是原来的类型会抛出异常。
any已经成为C++17的标准库组件。

4.2.3 数据转化

字符串转数字

#include <iostream>
#include <boost/lexical_cast.hpp>
int main(){
    int i = boost::lexical_cast<int>("100");
    float f = boost::lexical_cast<float>("2.01");
    char chars[] = "1234";
    int n = boost::lexical_cast<int>(chars,strlen(chars));
    std::cout << i << " " << f << " " << n << std::endl;
    
    // f = boost::lexical_cast<int>("2.01"); // 类型不匹配抛异常
}

结果：

[root@localhost 4]# ./a.out 
100 2.01 1234

注：C++11可以使用stoi()、stof()等

数字转字符串

#include <string>
#include <iostream>
#include <boost/lexical_cast.hpp>
int main(){
    std::string str1 = boost::lexical_cast<std::string>(100);
    std::string str2 = boost::lexical_cast<std::string>(2.01);
    std::cout << str1 << " " << str2 << std::endl;
}

结果：

[root@localhost 4]# ./a.out 
100 2.0099999999999998

多态对象转换

#include <iostream>
#include <boost/cast.hpp>
using namespace std;
class Base{
public:
    void Func(){
        cout << "Base" << endl;
    }
    virtual ~Base(){}
};
class Derive : public Base{
public:
    void Func(){
        cout << "Derive" << endl;
    }
};
int main(){
    Base* b = new Derive;
    boost::polymorphic_cast<Derive*>(b)->Func();
}

结果：

[root@localhost 4]# ./a.out 
Derive

4.2.4指针容器

特点：容器销毁自动释放指针

类型	说明
`ptr_vector`	指针向量
`ptr_set`	指针集合
`ptr_array`	指针数组
`ptr_multimap`	指针一对多映射

#include <iostream>
#include <boost/ptr_container/ptr_vector.hpp>
#include <boost/ptr_container/ptr_set.hpp>
using namespace std;
int main(){
    boost::ptr_vector<int> pvec;
    for(int i=0;i<10;++i){
        pvec.push_back(new int(i));
    }
    for(auto n:pvec){
        cout << n << " ";
    }
    cout << endl;

    boost::ptr_set<int> s;
    for(size_t i=0;i<10;i++){
        s.insert(new int(i));
    }
    for(auto n:s){
        cout << n << " ";
    }
    cout << endl;
}

结果：

[root@localhost 4]# ./a.out 
0 1 2 3 4 5 6 7 8 9 
0 1 2 3 4 5 6 7 8 9

4.2.5 退出处理

变量离开(正常/异常)作用域，触发处理代码。

#include <iostream>
#include <boost/scope_exit.hpp>
using namespace std;
int main(){
    {
        int *a = new int(10);
        BOOST_SCOPE_EXIT(a){
            delete a;
        }BOOST_SCOPE_EXIT_END

        cout << *a << endl;
    }
}

结果：

[root@localhost 4]# ./a.out 
10

注：不止用在new/delete，也用在fopen()/fclose()，dlopen()/dlclose()等。与析构函数相似的作用。

4.2.6 遍历`BOOST_FOREACH`

#include <iostream>
#include <boost/foreach.hpp>
using namespace std;
int main(){
    int arr[] = {1,2,3,4,5};
    BOOST_FOREACH(int a,arr) cout << a << endl;
}

结果：

[root@localhost 4]# ./a.out 
1
2
3
4
5

4.2.7 函数绑定

全局函数：boost::bind(函数名, 参数1，参数2，...)
成员函数：boost::bind(&类名::方法名，类实例指针，参数1，参数2）

#include <iostream>
#include <algorithm>
#include <boost/bind.hpp>
using namespace std;
bool comp(int a,int b){
    return a<b;
}
int main(){
    int arr[] = {1,2,3,4,5};
    cout << count_if(arr,arr+5,boost::bind(comp,_1,3)) << endl;
    cout << count_if(arr,arr+5,boost::bind(comp,2,_1)) << endl;
}

结果：

[root@localhost 4]# ./a.out 
2
3

注：bind支持最多九个参数。占位符被命名为 _1,_2, _3, _4, 直至 _9。

4.2.8 不可复制类

#include <boost/nocopyable.hpp>

class Test:private boost::nocopyable{
    // ...  
};

C++11禁用拷贝构造函数的两种方法：

（1）设置成delete

#include <iostream>
using namespace std;
class Test {
public:
    Test(){}
    Test(const Test&) = delete;
    Test& operator=(const Test&) = delete;
};
int main() {
    Test t;
    Test t2 = t;
    t2 = t;
}

报错：

[root@localhost 4]# g++ Test.cpp
Test.cpp: In function ‘int main()’:
Test.cpp:11:15: error: use of deleted function ‘Test::Test(const Test&)’
     Test t2 = t;
               ^
Test.cpp:6:5: note: declared here
     Test(const Test&) = delete;
     ^~~~
Test.cpp:12:10: error: use of deleted function ‘Test& Test::operator=(const Test&)’
     t2 = t;
          ^
Test.cpp:7:11: note: declared here
     Test& operator=(const Test&) = delete;
           ^~~~~~~~

（2）设置成private:

#include <iostream>
using namespace std;
class Test {
public:
    Test(){}
private:
    Test(const Test&) {}
    Test& operator=(const Test&) {
        return *this;
    };
};
int main() {
    Test t;
    Test t2 = t;
    t2 = t;
}

报错：

[root@localhost 4]# g++ Test.cpp
Test.cpp: In function ‘int main()’:
Test.cpp:14:15: error: ‘Test::Test(const Test&)’ is private within this context
     Test t2 = t;
               ^
Test.cpp:7:5: note: declared private here
     Test(const Test&) {}
     ^~~~
Test.cpp:15:10: error: ‘Test& Test::operator=(const Test&)’ is private within this context
     t2 = t;
          ^
Test.cpp:8:11: note: declared private here
     Test& operator=(const Test&) {
           ^~~~~~~~

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