13.6.1

最新推荐文章于 2024-11-10 12:52:56 发布

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最新推荐文章于 2024-11-10 12:52:56 发布

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分类专栏： C++ Primer第五版练习

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本文深入探讨了C++中字符串类String的设计与实现细节，包括构造函数、析构函数、赋值运算符重载等关键成员函数。通过具体代码示例展示了左值引用与右值引用的区别及应用场景。

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13.45
左值引用一般绑定在左值上
右值引用引用对象将被销毁，对象没有其他用户

13.46

int f();  
vector<int> vi(100);
int&& r1 = f();
int& r2 = vi[0];
int& r3 = r1;
int&& r4 = vi[0] * f();

13.47

#include <memory>

class String
{
public:
    String() : String("") { }
    String(const char *);
    String(const String&);
    String& operator=(const String&);
    ~String();

    const char *c_str() const { return elements; }
    size_t size() const { return end - elements; }
    size_t length() const { return end - elements - 1; }

private:
    std::pair<char*, char*> alloc_n_copy(const char*, const char*);
    void range_initializer(const char*, const char*);
    void free();

private:
    char *elements;
    char *end;
    std::allocator<char> alloc;
};

#include <algorithm>
#include <iostream>

std::pair<char*, char*>
String::alloc_n_copy(const char *b, const char *e)
{
    auto str = alloc.allocate(e - b);
    return{ str, std::uninitialized_copy(b, e, str) };
}

void String::range_initializer(const char *first, const char *last)
{
    auto newstr = alloc_n_copy(first, last);
    elements = newstr.first;
    end = newstr.second;
}

String::String(const char *s)
{
    char *sl = const_cast<char*>(s);
    while (*sl)
        ++sl;
    range_initializer(s, ++sl);
}

String::String(const String& rhs)
{
    range_initializer(rhs.elements, rhs.end);
    std::cout << "copy constructor" << std::endl;
}

void String::free()
{
    if (elements) {
        std::for_each(elements, end, [this](char &c){ alloc.destroy(&c); });
        alloc.deallocate(elements, end - elements);
    }
}

String::~String()
{
    free();
}

String& String::operator = (const String &rhs)
{
    auto newstr = alloc_n_copy(rhs.elements, rhs.end);
    free();
    elements = newstr.first;
    end = newstr.second;
    std::cout << "copy-assignment" << std::endl;
    return *this;
}

13.48

#include <vector>
#include <iostream>

// Test reference to http://coolshell.cn/articles/10478.html

void foo(String x)
{
    std::cout << x.c_str() << std::endl;
}

void bar(const String& x)
{
    std::cout << x.c_str() << std::endl;
}

String baz()
{
    String ret("world");
    return ret;
}

int main()
{
    char text[] = "world";

    String s0;
    String s1("hello");
    String s2(s0);
    String s3 = s1;
    String s4(text);
    s2 = s1;

    foo(s1);
    bar(s1);
    foo("temporary");
    bar("temporary");
    String s5 = baz();

    std::vector<String> svec;
    svec.reserve(8);
    svec.push_back(s0);
    svec.push_back(s1);
    svec.push_back(s2);
    svec.push_back(s3);
    svec.push_back(s4);
    svec.push_back(s5);
    svec.push_back(baz());
    svec.push_back("good job");

    for (const auto &s : svec) {
        std::cout << s.c_str() << std::endl;
    }
}