string的模拟实现
- msvc和g++下的string内存比较
- 成员变量
- 构造函数与析构函数
- 拷贝构造函数
- 赋值拷贝
- c_str、size和capacity函数以及重载[]、clear、expand_capacity
- 迭代器与遍历
- reserve
- push_back、append、+=
- insert
- 字符串比较运算符
- erase
- <<流提取 >>流插入
- resize
- find
- substr
- 源码地址
msvc和g++下的string内存比较
int main()
{
std::string str1("HelloWorld");
std::string str2("HelloWorldHelloWorld");
cout << sizeof(str1) << " " << sizeof(str2) << endl;
return 0;
}
在windows下使用vs2022 x64运行输出都为40字节：调试上面程序，在自动窗口中观察str1和str2的原始视图：
发现buf是包含16个字符的数组，里面存放了字符串，ptr为空，size和res分别为字符个数和容量。
而查看str2的字符串视图：发现buf数组为随机值，ptr指向实际的字符串，size和res分别为字符个数和容量。
MSVC编译器使用的string类内存模型可大概表示如下：
class string
{
private:
char _buf[16];
char* _ptr;
size_t _size;
size_t _capacity;
};
在64位下是40字节。
在g++11.4.0版本下运行结果为32字节。可以验证g++和vs2022的msvc都没有使用写拷贝：
int main() {
string str1("hello world");
string str2(str1);
cout << (void *) str1.c_str() << endl; // 0xe8ce3ff7e0
cout << (void *) str2.c_str() << endl; // 0xe8ce3ff7c0
return 0;
}
现在我们来探索一下g++下的string内存结构：
int main() {
std::string s1 = "HelloWorld";
std::string s2 = "HelloWorldHelloWorld";
std::string s3 = "HelloWorldHelloWorldHelloWorld";
cout << (void *) &s1 << " " << (void *) s1.c_str() << endl;
cout << (void *) &s2 << " " << (void *) s2.c_str() << endl;
cout << (void *) &s3 << " " << (void *) s3.c_str() << endl;
return 0;
}
# 观察到s1的内存
0x00000000005ffe10 20 fe 5f 00 00 00 00 00 │ ·_····· │
0x00000000005ffe18 0a 00 00 00 00 00 00 00 │ ········ │
0x00000000005ffe20 48 65 6c 6c 6f 57 6f 72 │ HelloWor │
0x00000000005ffe28 6c 64 00 00 01 00 00 00 │ ld······ │
---------------------------------------------------------------------
# 观察到s2的内存
0x00000000005ffdf0 40 47 70 00 00 00 00 00 │ @Gp····· │
0x00000000005ffdf8 14 00 00 00 00 00 00 00 │ ········ │
0x00000000005ffe00 14 00 00 00 00 00 00 00 │ ········ │
0x00000000005ffe08 45 15 8c be f7 7f 00 00 │ E······· │
# s2指向的字符串
0x0000000000704740 48 65 6c 6c 6f 57 6f 72 │ HelloWor │
0x0000000000704748 6c 64 48 65 6c 6c 6f 57 │ ldHelloW │
0x0000000000704750 6f 72 6c 64 00 ab ab ab │ orld···· │
---------------------------------------------------------------------
# 观察到s3的内存
0x00000000005ffdd0 90 47 70 00 00 00 00 00 │ ·Gp····· │ # 指向字符数组
0x00000000005ffdd8 1e 00 00 00 00 00 00 00 │ ········ │ # size
0x00000000005ffde0 1e 00 00 00 00 00 00 00 │ ········ │ # capacity
0x00000000005ffdd8 00 00 00 00 00 00 00 00 │ ········ │
# s3指向的字符串
0x0000000000704790 48 65 6c 6c 6f 57 6f 72 │ HelloWor │
0x0000000000704798 6c 64 48 65 6c 6c 6f 57 │ ldHelloW │
0x00000000007047a0 6f 72 6c 64 48 65 6c 6c │ orldHell │
0x00000000007047a8 6f 57 6f 72 6c 64 00 ab │ oWorld·· │
观察上面3个string对象我们知道，第一个成员是char*的_str，指向保存的字符串地址。第二个成员是size。如果是小于等于15个字符，则后面16个字节保存短字符串。如果是大于15个字符的字符串，则第三个成员保存capacity容量，第四个成员则没有意义。
// 短字符串
class string {
private:
char* _str;
size_t _size;
char _buf[16];
};
// 长字符串
class string {
private:
char* _str;
size_t _size;
size_t _capacity;
size_t _reserve;
};
验证猜想：
int main() {
std::string s1 = "HelloWorldHello";
std::string s2 = "HelloWorldHelloWorld";
std::string s3 = "HelloWorldHelloWorldHelloWorld";
cout << (void *) &s1 << " " << (void *) s1.c_str() << " " << s1.size() << " " << s1.capacity() << endl;
cout << (void *) &s2 << " " << (void *) s2.c_str() << " " << s2.size() << " " << s2.capacity() << endl;
cout << (void *) &s3 << " " << (void *) s3.c_str() << " " << s3.size() << " " << s3.capacity() << endl;
s1.push_back('*'); // 插入数据扩容
cout << (void *) &s1 << " " << (void *) s1.c_str() << " " << s1.size() << " " << s1.capacity() << endl;
s3.push_back('*'); // 插入数据扩容
cout << (void *) &s3 << " " << (void *) s3.c_str() << " " << s3.size() << " " << s3.capacity() << endl;
return 0;
}
/*
0xdd031ffde0 0xdd031ffdf0 15 15 # 扩容前s1
0xdd031ffdc0 0x2a486af25b0 20 20 # s2
0xdd031ffda0 0x2a486af6820 30 30 # s3
0xdd031ffde0 0x2a486af7860 16 30 # 扩容后s1
0xdd031ffda0 0x2a486af7890 31 60 # 扩容后s3
*/
成员变量
为了简单模拟字符串，我们不像g++和msvc一样保留对短字符串的处理，我们定义如下几个变量：
namespace my_std
{
class string
{
private:
char* _str; // 字符串数组
size_t _size;
size_t _capacity;
static const size_t npos;
};
const size_t string::npos = -1;
}
// size：当前有效字符的个数
// capacity：最大能容纳的有效字符的个数，不包括\0
在自己的命名空间 my_std 中实现，防止与std冲突。
下面我们将模拟g++下的string，可以对照g++的string输出作为参考。
构造函数与析构函数
std标准库中string构造函数的使用：
int main() {
std::string s1 = "HelloWorldHello";
std::string s2 = "HelloWorldHelloWorld";
std::string s3 = "HelloWorldHelloWorldHelloWorld";
cout << (void *) &s1 << " " << (void *) s1.c_str() << " " << s1.size() << " " << s1.capacity() << endl;
cout << (void *) &s2 << " " << (void *) s2.c_str() << " " << s2.size() << " " << s2.capacity() << endl;
cout << (void *) &s3 << " " << (void *) s3.c_str() << " " << s3.size() << " " << s3.capacity() << endl;
return 0;
}
/*
0x84b4fff660 0x84b4fff670 15 15
0x84b4fff640 0x23a3caa25b0 20 20
0x84b4fff620 0x23a3caa6820 30 30
*/
我们模拟实现如下：这里的默认参数旨在一个string空对象的正确打印。
string(const char* str = "")
: _size(strlen(str))
{
_capacity = (_size <= 15) ? 15 : _size;
_str = new char[_capacity + 1];
strcpy(_str, str);
}
如果是短字符串，我们将容量设置为15。如果是长字符串，容量设置为实际长度。
这里需要注意strcpy会拷贝 '\0' 。
下面是析构函数实现：因为构造函数中我们保证了容量至少是15，并且下面的实现resize中即使缩容也不会影响capacity实际容量，因此 _str不会为空。可以不加 _str 为空的判断。
~string()
{
delete[] _str;
_str = nullptr;
_size = _capacity = 0;
}
拷贝构造函数
先看标准库中string的使用：
void test() {
std::string str = "helloworldhelloworld";
cout << str.size() << " " << str.capacity() << " " << (void *) str.c_str() << endl; // 20 20 0x18ff84f25b0
str.push_back('#');
cout << str.size() << " " << str.capacity() << " " << (void *) str.c_str() << endl; // 21 40 0x18ff84f7830
std::string str1 = str;
cout << str1.size() << " " << str1.capacity() << " " << (void *) str1.c_str() << endl; // 21 21 0x18ff84f25b0
}
int main() {
test();
return 0;
}
可以观察到拷贝构造的str1的size和capacity都等于str的size，因此可以实现如下：
string(const string& s)
{
// apply the new space
_size = s._size;
_capacity = s._size;
_str = new char[_capacity + 1];
// copy str
strcpy(_str, s.c_str());
}
赋值拷贝
先看g++下的赋值拷贝：
void test() {
string str = "helloworldhelloworld";
cout << "str：" << str.size() << " " << str.capacity() << " " << (void *) str.c_str() << endl;
str.push_back('#');
cout << "str插入#：" << str.size() << " " << str.capacity() << " " << (void *) str.c_str() << endl;
string str1;
cout << "str1扩容前：" << str1.size() << " " << str1.capacity() << " " << (void *) str1.c_str() << endl;
string str2 = "Hello";
cout << str2.size() << " " << str2.capacity() << " " << (void *) str2.c_str() << endl;
// 短字符串 原地复制
str1 = str2;
cout << "str1短字符串原地复制：" << str1.size() << " " << str1.capacity() << " " << (void *) str1.c_str() << endl;
// 长字符串 两倍扩容
str1 = str;
cout << "str1长字符串两倍扩容：" << str1.size() << " " << str1.capacity() << " " << (void *) str1.c_str() << endl;
str += "helloworldhelloworld";
cout << str.size() << " " << str.capacity() << " " << (void *) str.c_str() << endl;
// 长字符串 两倍扩容不够 扩容到s.size()
string str3;
cout << "str3扩容前：" << str3.size() << " " << str3.capacity() << " " << (void *) str3.c_str() << endl;
str3 = str;
cout << "str3扩容后：" << str3.size() << " " << str3.capacity() << " " << (void *) str3.c_str() << endl;
}
int main() {
test();
return 0;
}
/*
str：20 20 0x17f061725b0
str插入#：21 40 0x17f06177830
str1扩容前：0 15 0x4dc55ffbc0
5 15 0x4dc55ffba0
str1短字符串原地复制：5 15 0x4dc55ffbc0
str1长字符串两倍扩容：21 30 0x17f061725b0
41 80 0x17f06177870
str3扩容前：0 15 0x4dc55ffb80
str3扩容后：41 41 0x17f06177830
*/
在等号赋值的过程中，如果赋值的字符串较短，则在原来的内存空间中直接复制。否则发生扩容，如果两倍扩容不够，则扩容到s._size。
string& operator=(const string& s)
{
if (&s != this)
{
if (s._size > _capacity)
{
// 两倍扩容不够，则扩容到s._size
_capacity = (s._size > 2 * _capacity) ? s._size : 2 * _capacity;
delete[] _str;
_str = new char[_capacity + 1];
}
strcpy(_str, s._str);
_size = s._size;
}
return *this;
}
下面写了reserve函数之后可以修改为：代码上更简洁，但是多了一次拷贝。
string& operator=(const string& s)
{
if (&s != this)
{
reserve(s._size);
strcpy(_str, s._str);
_size = s._size;
}
return *this;
}
c_str、size和capacity函数以及重载[]、clear、expand_capacity
const char* c_str() const
{
return _str;
}
size_t size() const
{
return _size;
}
size_t capacity() const
{
return _capacity;
}
char& operator[](size_t pos)
{
assert(pos < _size);
return _str[pos];
}
const char& operator[](size_t pos) const
{
assert(pos < _size);
return _str[pos];
}
这里还专门针对const string对象写了重载[]的const成员函数。
void expand_capacity(std::string &str) {
size_t old_capacity = str.capacity();
cout << "init capacity: " << old_capacity << endl;
for (int i = 0; i < 100; ++i) {
str.append("h");
//str.push_back('h');
if (old_capacity != str.capacity()) {
old_capacity = str.capacity();
cout << "expand capacity: " << old_capacity << endl;
}
}
}
int main() {
string str;
expand_capacity(str);
cout << str.size() << " " << str.capacity() << endl;
str.clear();
cout << str.size() << " " << str.capacity() << endl;
return 0;
}
/*
init capacity: 15
expand capacity: 30
expand capacity: 60
expand capacity: 120
100 120
0 120
*/
clear函数将size置0，容量保留：
void clear()
{
_str[0] = '\0';
_size = 0;
}
迭代器与遍历
class string
{
public:
typedef char* iterator;
iterator begin()
{
return _str;
}
iterator end()
{
return _str + _size;
}
}
测试：
int main()
{
// 遍历测试
string str1("hello world");
cout << str1.c_str() << endl;
for (size_t i = 0; i < str1.size(); ++i)
{
cout << str1[i] << " ";
}
cout << endl;
string::iterator it = str1.begin();
while (it != str1.end())
{
cout << *it << " ";
++it;
}
cout << endl;
for (auto ch : str1)
{
cout << ch << " ";
}
cout << endl;
return 0;
}
reserve
void test() {
string s = "hello";
cout << s.size() << " " << s.capacity() << " " << s << endl;
s.reserve(5);
cout << "(05)" << s.size() << " " << s.capacity() << " " << s << endl;
s.reserve(18);
cout << "(18)" << s.size() << " " << s.capacity() << " " << s << endl;
s.reserve(61);
cout << "(61)" << s.size() << " " << s.capacity() << " " << s << endl;
}
int main() {
test();
return 0;
}
/*
5 15 hello
(05)5 15 hello
(18)5 30 hello
(61)5 61 hello
*/
标准库中的reserve函数的使用，reserve参数如果比capacity小，则没有变化。reserve参数比capacity大，发生扩容，如果两倍扩容不够，则扩容到输入的参数。
void reserve(size_t n)
{
if (n > _capacity)
{
// 两倍扩容不够，则扩容到n
_capacity = (n > 2 * _capacity) ? n : 2 * _capacity;
char* tmp = new char[_capacity + 1];
strcpy(tmp, _str);
delete[] _str;
_str = tmp;
}
}
push_back、append、+=
void push_back(char ch)
{
/*if (_size == _capacity)
{
reserve(_capacity * 2);
}*/
reserve(_size + 1);
_str[_size++] = ch;
_str[_size] = '\0';
}
void append(const char* str)
{
size_t len = strlen(str);
reserve(_size + len);
strcpy(_str + _size, str);
_size += len;
}
string& operator+=(const char* str)
{
append(str);
return *this;
}
string& operator+=(char ch)
{
push_back(ch);
return *this;
}
值得说的是，有了上面的reserve，我们在扩容时，代码上就可以更简洁，不用进行 _size == _capacity 的判断：
void push_back(char ch)
{
/*if (_size == _capacity)
{
reserve(_capacity * 2);
}*/
reserve(_size + 1);
_str[_size++] = ch;
_str[_size] = '\0';
}
insert
要从 '\0' 开始向后移动，使用时注意while循环对size_t类型的判断，避免导致死循环：
void insert(size_t pos, char ch)
{
assert(pos <= _size);
reserve(_size + 1);
/*
size_t end = _size; // from '\0' start
while (end >= pos) // when pos=0, while loop will not exit
{
_str[end + 1] = _str[end];
--end;
}
*/
size_t end = _size + 1; // from '\0' next pos start
while (end > pos)
{
_str[end] = _str[end - 1];
--end;
}
_str[pos] = ch;
_size++;
}
下面是插入字符串str：
void insert(size_t pos, const char* str)
{
assert(pos <= _size);
size_t len = strlen(str);
reserve(_size + len);
// 向后挪动数据
size_t end = _size + 1; // from '\0' next pos start
while (end > pos)
{
_str[end - 1 + len] = _str[end - 1];
--end;
}
// 拷贝数据
/*for (int i = 0; i < len; ++i)
{
_str[i + pos] = str[i];
}*/
strncpy(_str + pos, str, len);
_size += len;
}
这里使用 strncpy 指定拷贝的字节数。
字符串比较运算符
bool operator<(const string& s) const
{
return strcmp(_str, s._str) < 0;
}
bool operator==(const string& s) const
{
return strcmp(_str, s._str) == 0;
}
bool operator<=(const string& s) const
{
return *this < s || *this == s;
}
bool operator>(const string& s) const
{
return !(*this <= s);
}
bool operator>=(const string& s) const
{
return !(*this < s);
}
bool operator!=(const string& s) const
{
return !(*this == s);
}
erase
void erase(size_t pos, size_t len = npos)
{
assert(pos < _size);
if (len == npos || pos + len >= _size)
{
// 从pos删除到结尾
_str[pos] = '\0';
_size = pos;
}
else
{
// 从pos删除len长度个
strcpy(_str + pos, _str + pos + len);
_size -= len;
}
}
验证：
int main()
{
string str1("helloworld");
cout << str1 << " " << str1.size() << " " << str1.capacity() << endl;
str1.erase(6);
cout << str1 << " " << str1.size() << " " << str1.capacity() << endl;
str1.erase(0);
cout << str1 << " " << str1.size() << " " << str1.capacity() << endl;
return 0;
}
/*
helloworld 10 15
hellow 6 15
0 15
*/
<<流提取 >>流插入
将<<和 >> 重载为全局函数，放在my_std命名空间中：
std::ostream& operator <<(std::ostream& out, const string& s)
{
out << s.c_str();
return out;
}
使用 istream::get() 方法，每次读取一个字符进行插入。
std::istream& operator>>(std::istream& in, string& s)
{
s.clear();
char ch;
// in >> ch;
ch = in.get(); // istream::get()
while (ch != ' ' && ch != '\n')
{
s += ch;
ch = in.get();
}
return in;
}
每次字符串执行+= s += ch; 效率太低，我们加入buf，提高插入的效率：
std::istream& operator>>(std::istream& in, string& s)
{
s.clear();
char buf[128] = { '\0' };
size_t i = 0;
char ch;
// in >> ch;
ch = in.get(); // istream::get()
while (ch != ' ' && ch != '\n')
{
//s += ch;
if (i == 127)
{
s += buf;
i = 0;
}
buf[i] = ch;
i++;
ch = in.get();
}
if (i >= 0)
{
buf[i] = '\0';
s += buf;
}
return in;
}
resize
cplusplus的resize
void resize (size_t n);
void resize (size_t n, char c);
int main()
{
//test_main();
string s1("hello world");
cout << s1 << " " << s1.size() << " " << s1.capacity() << endl;
s1.resize(5);
cout << s1 << " " << s1.size() << " " << s1.capacity() << endl;
s1.resize(50, 'x');
cout << s1 << " " << s1.size() << " " << s1.capacity() << endl;
return 0;
}
/*
hello world 11 15
hello 5 15
helloxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx 50 50
*/
假设有字符串 str = "hello world" 的size为11，capacity为10。
针对以下三种参数调整空间：
1.当n=5时，也就是n<=size，可以直接令 str[5] = '\0'，size变为5，容量不变。
2.当n=15时，也就是n>size，需要插入数据，直接调用reserve扩容为n个空间，然后将剩下的插入指定字符c。
我们自己实现的resize：
void resize(size_t n, char ch = '\0')
{
if (n <= _size)
{
_str[n] = '\0';
_size = n;
}
else
{
reserve(n);
while (_size < n)
{
_str[_size] = ch;
++_size;
}
_str[_size] = '\0';
}
}
find
find返回指定字符或者字符串从pos开始的下标：
size_t find(char ch, size_t pos = 0)
{
for (size_t i = pos; i < _size; i++)
{
if (_str[i] == ch)
{
return i;
}
}
return npos;
}
size_t find(const char* sub, size_t pos = 0)
{
const char* p = strstr(_str + pos, sub);
if (p)
{
return p - _str;
}
else
{
return npos;
}
}
substr
cplusplus.com/reference/string/string/substr/
string substr (size_t pos = 0, size_t len = npos) const;
实现如下：
string substr(size_t pos, size_t len = npos)
{
string s;
size_t end = pos + len;
if (len == npos || end >= _size) // 取到结尾
{
// len为字串的size
len = _size - pos; // end >=_size
end = _size;
}
s.reserve(len);
for (size_t i = pos; i < end; i++)
{
s += _str[i];
}
return s;
}
值得说的是从pos开始指定取的len如果太长超过结尾，则需要修正取到的len的大小。
源码地址
https://github.com/shlyyy/stl/blob/main/my_string.h
https://gitee.com/shlyyy/stl/blob/master/my_string.h
