參考

分析

如果一個class帶有指標型別的資料成員，那麼請思考預設的copy ctor和copy assign是否合適。如果你需要深拷貝那麼預設的行為就不合適了（預設為淺拷貝）。並且如果你的編譯器支援c++11，可以考慮是否可以利用右值引用來提高效率，這時需要實現move ctor和move assign（注意nonexcept的使用）。最後，當然別忘了dtor。

原始碼

#include <stdint.h>
#include <stdio.h>
#include <utility>
#include <string.h>
 

#include <malloc.h>
#include <iostream>
#include <vector>

class String {
public:

    // ctor
    String() : data_(new char[1]) 
    { 
        std::cout << "default ctor this:" << std::hex << this << std::endl;
        data_[0] = '\0'; 
    }

    String(const
 
 char* str) : data_(new char[strlen(str) + 1]) 
    {
        std::cout << "ctor witch const char* this:" << std::hex << this << std::endl;
        data_[0] = '\0'; 
        strcpy(data_, str);
    }

    // copy ctor
    String(const String& rhs) noexcept : data_(new char[strlen
 
(rhs.data_) + 1])
    {
        std::cout << "copy ctor this:" << std::hex << this 
                  << " rhs: " << std::hex << &rhs
                  << std::endl;
        strcpy(data_, rhs.data_);
    }

    // copy assign


    String& operator= (const String& rhs) {
        String tmp(rhs);
        swap(tmp);
        return *this;
    }

/*
    String& operator= (String rhs) 
    {
        std::cout << "copy assign this:" << std::hex << this 
                  << " rhs: " << std::hex << &rhs
                  << std::endl;
        swap(rhs);
        return *this;
    }
    //注意：
    //String s("hello");
    //s = "world"; 會提示和move copy assign ambiguous
*/

    // move ctor
    String(String&& rhs) noexcept : data_(rhs.data_) 
    {
        std::cout << "move ctor this:" << std::hex << this 
                  << " rhs: " << std::hex << &rhs
                  << std::endl;
        rhs.data_ = nullptr;
    }

    // move assign
    String& operator= (String&& rhs)
    {
        std::cout << "move assign this:" << std::hex << this 
                  << " rhs: " << std::hex << &rhs
                  << std::endl;
        swap(rhs);
        return *this;
    }

    // dtor
    ~String() 
    { 
        std::cout << "dtor " << std::hex << this << std::endl;
        delete [] data_; 
    }

    // swap
    void swap(String& rhs) 
    {
        std::cout << "member swap this:" << std::hex << this 
                  << " rhs: " << std::hex << &rhs
                  << std::endl;
        using std::swap;
        swap(data_, rhs.data_);
    }

    const char* c_str() const { return data_; }

private:
    char* data_;
};

void swap(String& lhs, String& rhs) {
    std::cout << "non-member swap" << std::endl;
    lhs.swap(rhs);
}

namespace std {
template<>
void swap<String>(String& lhs, String& rhs) {
    std::cout << "std::swap total template specialization" << std::endl;
    lhs.swap(rhs);
}
}

std::ostream& operator<< (std::ostream& os, const String& s)
{
    os << s.c_str();
    return os;
}

注意上面的class根據Effective C++ rule 25，實現了member-swap， non-member-swap以及std::swap的全特化版本。

在vector中使用

  String s0;
  String s1("hello");
  std::vector<String> svec;
  //svec.reserve(4); // 開啟此行分析有什麼變化
  //svec.resize(4);  
  std::cout << "size: "<< svec.size()  << " cap:"<< svec.capacity() << std::endl;
  svec.push_back(s0);
  std::cout << "size: "<< svec.size()  << " cap:"<< svec.capacity() << std::endl;
  svec.push_back(s1);
  std::cout << "size: "<< svec.size()  << " cap:"<< svec.capacity() << std::endl;
  svec.push_back(baz());
  std::cout << "size: "<< svec.size()  << " cap:"<< svec.capacity() << std::endl;
  svec.push_back("good job");
  std::cout << "size: "<< svec.size()  << " cap:"<< svec.capacity() << std::endl;
  return 0;

右值引用 和 nonexcept

move ctor在標明nonexcept時，才會被vector使用（在需要reallocation時），否則會使用copy ctor