c++简单string实现

string.h

#pragma once

class string
{
public:
    string(const char* str = nullptr);
    string(const string& str);
    ~string();

    string& operator=(const string& str);
    string& operator+=(const string& str);
    char& operator[](int n) const;
    char& operator[](int n);
    bool operator==(const string& str) const;

    int size() const;
    const char* c_str() const;
    bool empty() const;

    friend const string operator+(const string& lhs, const string& rhs);
private:
    char *data;
    int length;
};

string.cpp

#include "string.h"
#include <string.h>
#include <stdio.h>

const string operator+(const string& lhs, const string& rhs)
{
    string tmp(lhs);
    tmp += rhs;
    return tmp;
}

string::string(const char * str)
{
    if (!str)
    {
        length = ;
        data = new char[];
        *data = '\0';
    }
    else
    {
        length = strlen(str);
        data = new char[length + ];
        strcpy(data, str);
    }
    printf("string constructor\n");
}

string::string(const string & str)
{
    length = str.size();
    data = new char[length + ];
    strcpy(data, str.c_str());
    printf("string copy constructor\n");
}

string::~string()
{
    length = ;
    delete[] data;
    printf("string destructor\n");
}

string& string::operator=(const string & str)
{
    if (this == &str)
        return *this;
    delete[] data;

    length = str.size();
    data = new char[length + ];
    strcpy(data, str.c_str());

    return *this;
}

string & string::operator+=(const string & str)
{
    length += str.size();
    char *newdata = new char[length + ];
    strcpy(newdata, data);
    strcat(newdata, str.c_str());
    delete[] data;
    data = newdata;
    return *this;
}

char& string::operator[](int n) const
{
    return data[n];
}

char& string::operator[](int n)
{
    return data[n];
}

bool string::operator==(const string & str) const
{
    if (length != str.size())
        return false;
    return strcmp(data, str.c_str()) ? false : true;
}

int string::size() const
{
    return length;
}

const char * string::c_str() const
{
    return data;
}

bool string::empty() const
{
    return length == ;
}

main.cpp

#include"string.h"

int main()
{
    char a[] = "hello", b[] = "world";
    string s1(a), s2(b);
    string s3 = s1 + s2;
    return ;
}

string的+运算符重载进行了返回值优化，在Visual Studio Release模式下main函数中会调用两次构造函数、一次复制构造函数、一次析构函数，比起不做优化减少了一次构造函数和一次析构函数

string constructor
string constructor
string copy constructor
string destructor
string destructor
string destructor