C++ Primer 第5版--练习12.2

练习 12.2：编写你自己的StrBlob类，包含const版本的front和back。

#include <iostream>#include <vector>#include <memory>#include <stdexcept>using namespace std;class StrBlob{public:    typedef vector<string>::size_type size_type;    StrBlob();    StrBlob(initializer_list<string> i1);    size_type size() const { return data->size(); }    bool empty() const { return data->empty(); }    // 添加和删除元素    void push_back(const string &t) { data->push_back(t);}    void pop_back();    // 元素访问    string& front() const;    string& back() const;private:    shared_ptr<vector<string>> data;    void check(size_type i, const string &msg) const;};StrBlob::StrBlob() : data(make_shared<vector<string>>()) {}StrBlob::StrBlob(initializer_list<string> i1) :    data(make_shared<vector<string>>(i1)) {}void StrBlob::check(size_type i, const string &msg) const{    if (i >= data->size())        throw out_of_range(msg);}string& StrBlob::front() const{    check(0, "front on empty StrBlob");    return data->front();}string& StrBlob::back() const{    check(0, "back on empty StrBlob");    return data->back();}void StrBlob::pop_back(){    check(0, "pop_back on empty StrBlob");    data->pop_back();}int main(){    StrBlob b1;    StrBlob b2 = {"a", "an", "the"};    b1 = b2;    b2.push_back("about");    cout << "Elements number of b1: " << b1.size() << endl         << "Elements number of b2: " << b2.size();    return 0;}