[2018年5月3号]C++ primer 课后练习 第十二章 动态内存

12.16

12.17

a.不合法 ix是一个整形,不能给unique_pt<int> 作为初始参数

b.不合法,IDE不会报错,但是pi不是一块动态内存,在退出p1作用域时候,会释放pi导致报错

c.正确

d不合法,同b

e.正确

f.不合法,但是IDE不错报错,p5和p2指向同一块内存,当退出p5作用域时候,p2的指针会称为空悬指针,再退出p2作用域,p2会再次删除指向的内存,导致程序无法继续执行

12.18

shared_ptr可能有多个指针同时指向同一块内存,假设存在release函数并执行,当前指针会放弃对该片内存的控制,但其他所有指向那块内存的指针会称为空悬指针

12.19

class StrBlobPtr {
public:
    StrBlobPtr():curr(0){}
    StrBlobPtr(StrBlob& a, size_t sz = 0):wptr(a.data),curr(sz){}
    string& deref() const;
    StrBlobPtr& incr();

private:
    std::shared_ptr<vector<string >> check(size_t,const string&) const;
    weak_ptr<vector<string>> wptr;
    size_t curr;
};
shared_ptr<vector<string >>
StrBlobPtr::check(size_t sz, const string& msg) const {
    shared_ptr<vector<string >> spv = wptr.lock();
    if(!spv){
        throw runtime_error(" unbound StrBlobPtr");
    }
    if(sz > spv->size()){
        throw range_error(msg);
    }
    return spv;
}
string& StrBlobPtr::deref()const{
    auto p = check(curr,"dereference past end");
    return (*p)[curr];
}
StrBlobPtr& StrBlobPtr::incr()  {
    check(curr,"increment past end of StrBlobPtr");
    ++curr;
    return *this;
}
StrBlobPtr StrBlob::begin() { return StrBlobPtr(*this); }
StrBlobPtr StrBlob::end() { return StrBlobPtr(*this, data->size()); } //begin 和 end要放在StrBlobPtr定以后再实现,不然编译不过

12.20

int main() {
    StrBlob b;
    ifstream ifs("test_2.txt");
    string str;
    int count = 0;
    while (ifs >> str)
    {   
        b.push_back(str);
        count++;
    }

    StrBlobPtr begin = b.begin();
    for(int i = 0 ; i < count ;i++, begin.incr()){
        cout << begin.deref() << ends;
    }
    for (;;);
    return 0;
}

begin和end没有定义!=比较符,不能直接比较后输出,需要再定义个计数器来进行计算当前个数操作

12.21

各有好处,练习里的写法对运算符分析熟练后,看起来比原版本的简洁

12.22

构造函数时候将传入的StrBlob改成const类型

    ConstStrBlobPtr(const StrBlob& a, size_t sz = 0) :wptr(a.data), curr(sz) {}

12.23

int main() {
    char a[] = "aaasd";
    char b[] = "asdasdas";
    size_t size = sizeof(a) + sizeof(b);
    char * charArray = new char[size];
    memset(charArray, 0, sizeof(charArray));
    strcat_s(charArray, size, a);
    strcat_s(charArray, size, b);
    cout << charArray << endl;

    string as = "qeqeqweqw";
    string bs = "asdasdasd";
    size_t size_s = as.size() + bs.size();
    char * stringArray = new char [size_s];
    int slot = 0;
    for(int i = 0 ; as[i] != '\0'; i++){
        *(stringArray+ slot) = as[i];
        slot++;
    }
    for (int i = 0; bs[i] != '\0'; i++) {
        *(stringArray + slot) = bs[i];
        slot++;
    }
    cout << stringArray<< endl;


    for (;;);
    return 0;
}

12.24

int main() {
    string tmp;
    cin >> tmp;

    char* a = new char [tmp.size()];

    for(size_t i = 0; i < tmp.size(); i ++){
        *(a+i) = tmp[i];
    }
    *(a + tmp.size())= '\0';
    cout << a << endl;
    for (;;);
    return 0;
}

用传入的对象来定义动态数组的长度

12.25

  delete [] pa;