C++ string简介
本节内容主要介绍C++中的string
1. 简介
string是C++中的字符串类（字符串就是字符的序列），string其实是一个类型别名
using string = std::basic_string<char>;
头文件
#include 2. 初始化方法
示例
#include 输出
str1 is: C++
str2 is: C++
str3 is: C++
str4 is: C++
str5 is: aaaaa
str6 is: thon
3. 使用
3.1 长度计算
| 方法 | 说明 |
|---|---|
| size() | 长度计算 |
| length() | 长度计算 |
| max_size() | 字符串可以达到的最大长度 |
int main(int argc, char *argv[])
{
std::string str1{"nothing is difficult to the man who will try."};
std::cout<<"str1.size() size is: \t"<<str1.size()<<std::endl;
std::cout<<"str1.length() size is: \t"<<str1.length()<<std::endl;
std::cout<<"str1.max_size() size is: "<<str1.max_size()<<std::endl;
return 0;
}
//输出
str1.size() size is: 45
str1.length() size is: 45
str1.max_size() size is: 9223372036854775807
3.2 元素访问和遍历
| 方法 | 说明 |
|---|---|
| [] 或 at() | 元素访问的方法，at()越界会抛出异常 |
| auto | 自动推导类型 |
| 迭代器 | iterator 或 reverse_iterator 或 const_iterator (begin,cbegin,end,cend等) |
int main(int argc, char *argv[])
{
std::string str1{"nothing is difficult to the man who will try."};
//1.下标，at()
std::cout<<str1[4]<<std::endl; //下标
std::cout<<str1.at(3)<<std::endl; //at
std::cout<<"at is: \t\t";
for(size_t i = 0;i<str1.size(); i++)
{
//std::cout<
std::cout<<str1.at(i)<<" ";
}
std::cout<<std::endl;
//2. auto
std::cout<<"auto is: \t";
for(auto &au : str1)
{
std::cout<<au<<" ";
}
std::cout<<std::endl;
//3. 迭代器
std::string::iterator it = str1.begin();
//std::string::const_iterator it = str1.cbegin(); //返回const
std::cout<<"iterator is: \t";
while(it != str1.end())
{
std::cout<<*it<<" ";
it++;
}
std::cout<<std::endl;
//反序输出
std::string::reverse_iterator rit = str1.rbegin();
std::cout<<"reverse_iterator is: \t";
while(rit != str1.rend())
{
std::cout<<*rit<<" ";
rit++;
}
std::cout<<std::endl;
return 0;
}
输出
i
h
at is: n o t h i n g i s d i f f i c u l t t o t h e m a n w h o w i l l t r y .
auto is: n o t h i n g i s d i f f i c u l t t o t h e m a n w h o w i l l t r y .
iterator is: n o t h i n g i s d i f f i c u l t t o t h e m a n w h o w i l l t r y .
reverse_iterator is: . y r t l l i w o h w n a m e h t o t t l u c i f f i d s i g n i h t o n
元素类型的判断
string中的每个元素时char类型
int main(int argc, char *argv[])
{
std::string str1{"nothing is difficult to the man who will try."};
char ch = 'a';
std::cout<<typeid(std::string).name()<<std::endl;
std::cout<<typeid(str1).name()<<std::endl;
std::cout<<typeid(char).name()<<std::endl;
std::cout<<typeid(str1.at(0)).name()<<std::endl;
std::cout<<typeid(ch).name()<<std::endl;
return 0;
}
输出
NSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE
NSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE
c
c
c
3.3 改变长度
| 方法 | 说明 |
|---|---|
| empty() | 判断字符串是否为空 |
| capacity() | 空间大小（所占内存） |
| clear() | 情况字符，size()为0，capacity()不变 |
| reserve(n) | 调整capacity()的大小，size()不变；n |
| resize(n) | 调整size()的大小，只有n>size时，capacity()才会随之改变 |
int main(int argc, char *argv[])
{
std::string str1{"nothing is difficult to the man who will try."};
//std::string str1{"n"};
//输出所占空间的大小
std::cout<<"size \t"<<str1.size()<<std::endl;
std::cout<<"capacity \t"<<str1.capacity()<<std::endl<<std::endl;
//判断是否为空串,以下三种都会判空
//std::string str2="";
//std::string str2;
std::string str2{};
//std::string str2(); //这个一个函数声明，函数声明，函数声明
if(str2.empty())
{
std::cout<<"empty"<<std::endl;
}
else
{
std::cout<<"not empty"<<std::endl<<std::endl;
}
//情况字符串
str1.clear();
std::cout<<"size after clear is: \t"<<str1.size()<<std::endl;
std::cout<<"capacity after clear is: \t"<<str1.capacity()<<std::endl<<std::endl;
//改变容量的大小（调整内存的大小），size不会改变
str1 = "nothing is difficult to the man who will try.";
str1.reserve(100);
std::cout<<"(reserve)size after clear is: \t"<<str1.size()<<std::endl;
std::cout<<"(reserve)capacity after clear is: \t"<<str1.capacity()<<std::endl;
std::cout<<"str1 is: "<<str1<<std::endl<<std::endl;
//10小于str1 size，该函数不生效
str1.reserve(10);
std::cout<<"(reserve)size after clear is: \t"<<str1.size()<<std::endl;
std::cout<<"(reserve)capacity after clear is: \t"<<str1.capacity()<<std::endl;
std::cout<<"str1 is: "<<str1<<std::endl<<std::endl;
//size和容量都会扩充，size扩充的部分会被's'填充
str2 = "nothing is difficult to the man who will try.";
str2.resize(60, 's');
//str2.resize(100); //填充的部分无数据
std::cout<<"(resize)size after clear is: \t"<<str2.size()<<std::endl;
std::cout<<"(resize)capacity after clear is: \t"<<str2.capacity()<<std::endl;
std::cout<<str2<<std::endl<<std::endl;
//size变为10，容量大小不变；内容会被截取
str2.resize(10);
std::cout<<"(resize)size after clear is: \t"<<str2.size()<<std::endl;
std::cout<<"(resize)capacity after clear is: \t"<<str2.capacity()<<std::endl;
std::cout<<str2<<std::endl<<std::endl;
return 0;
}
输出
size 45
capacity 45
empty
size after clear is: 0
capacity after clear is: 45
(reserve)size after clear is: 45
(reserve)capacity after clear is: 100
str1 is: nothing is difficult to the man who will try.
(reserve)size after clear is: 45
(reserve)capacity after clear is: 45
str1 is: nothing is difficult to the man who will try.
(resize)size after clear is: 60
(resize)capacity after clear is: 90
nothing is difficult to the man who will try.sssssssssssssss
(resize)size after clear is: 10
(resize)capacity after clear is: 90
nothing is
3.3 原始字符串的输出
| 方法 | 说明 |
|---|---|
| R | 输出原始字符串 |
int main(int argc, char *argv[])
{
std::string str1{"\n\t\r"};
std::cout<<"str1 is: "<<str1<<std::endl<<std::endl;
std::string str2 = R"(\n\t\r)";
std::cout<<"str2 is: "<<str2<<std::endl;
std::string str3{R"(\n\t\r)"};
std::cout<<"str3 is: "<<str3<<std::endl;
return 0;
}
输出
str1 is:
str2 is: \n\t\r
str3 is: \n\t\r
3.4 修改&&连接
| 方法 | 说明 |
|---|---|
| append() | 在字符串后面追加字符串 |
| front() | 头元素 |
| compare() | 比较 |
| push_back() | 插入元素（char） |
| pop_back() | 删除元素 |
| swap() | 交换两个string的数值 |
| “+” | 连接两个string |
| c_str() | 将string转换为const char* |
| data() | 将string转换为const char*，c++11之后，其功能同c_str()；c++11之前，末尾不会加’\0’ |
| substr(n, m) | 截取string下标从n开始的m个字符 |
template<typename T, typename Y>
void print(T& t, Y& y)
{
std::cout<<t<< " is: \t" <<y<<std::endl;
}
int main(int argc, char *argv[])
{
std::string str1{"apple"};
str1.append(" orange");
print("str1 ",str1);
str1 = "apple";
str1.append(" orange",3); //追加3个字符
print("str1 ",str1);
str1 = "apple";
str1.append(" orange",2, 3); //从下标为2的位置开始，追加3个字符
print("str1 ",str1);
/* append的函数原型
string& append (const string& str);
string& append (const string& str, size_t subpos, size_t sublen);
string& append (const char* s);
string& append (const char* s, size_t n);
string& append (size_t n, char c);
template string& append (InputIterator first, InputIterator last);
string& append (initializer_list il);
*/
str1 = "apple";
str1.push_back('s'); //添加字符
print("str1 ",str1);
str1.pop_back(); //删除字符
print("str1 ",str1);
str1 = "nothing is difficult ";
std::string str2{"to the man who will try."};
str1.swap(str2); //交换两个字符串的内容
print("str1",str1);
print("str2",str2);
str1.swap(str2);
std::string str3 = str1 + str2;
print("str3",str3);
const char* pch = str3.c_str(); //const char*
//const char* pch = str3.data(); //const char*
print("pch",pch);
/* //需要包含头文件#include
char* pch1 = nullptr;
std::strcpy(str3,pch1);
print("pch1",pch1);
*/
std::string str4 = str1.substr(2,7); //从下标为2的位置，截取7个字符
print("str4",str4);
std::string str5 = str1.substr(2); //从下标为2的位置,到最后
print("str5",str5);
return 0;
}
输出
str1 is: apple orange
str1 is: apple or
str1 is: appleran
str1 is: apples
str1 is: apple
str1 is: to the man who will try.
str2 is: nothing is difficult
str3 is: nothing is difficult to the man who will try.
pch is: nothing is difficult to the man who will try.
str4 is: thing i
str5 is: thing is difficult
3.5 查找
| 方法 | 说明 |
|---|---|
| find() | 正向查找 |
| rfind() | 反向查找 |
| find_first_of(str) | 正向查找str中任何一个字符出现的位置，不匹配返回std::string::npos |
| find_last_of(str) | 反向查找str中任何一个字符出现的位置 |
| find_first_not_of(str) | 正向遍历原始字符串中的元素，当该元素不在str中则返回当前位置 |
| find_last_not_of(str) | 反向遍历原始字符串中的元素，当该元素不在str中则返回当前位置 |
| std::string::npos | 常量静态成员，size_t，值为-1 |
template<typename T, typename Y>
void print(T& t, Y& y)
{
std::cout<<t<< " is: \t" <<y<<std::endl;
}
template<typename T>
void print(T& t)
{
std::cout<<t<<std::endl;
}
int main(int argc, char *argv[])
{
std::string str1{"nothing is difficult to the man who will try."};
std::cout<< "str size is: "<<str1.size()<<std::endl;;
//正向查找
//auto re = str1.find("difficult");
size_t re = str1.find('x');
re == std::string::npos ? print("re not find.") : print("re", re);
//反向查找
size_t re1 = str1.rfind("will");
re1 == std::string::npos ? print("re1 not find.") : print("re1", re1);
//find_first_of()
size_t re2 = str1.find_first_of("ig");
re2 == std::string::npos ? print("re2 not find.") : print("re2", re2);
//find_last_of()
size_t re3 = str1.find_last_of("ig");
re3== std::string::npos ? print("re3 not find.") : print("re3", re3);
//find_first_not_of
size_t re4 = str1.find_first_not_of("nothing");
re4 == std::string::npos ? print("re4 not find.") : print("re4", re4);
//find_last_not_of
size_t re5 = str1.find_last_not_of("will");
re5 == std::string::npos ? print("re5 not find.") : print("re5", re5);
/*
find函数，其它亦同
size_t find (const string& str, size_t pos = 0) const noexcept;
size_t find (const char* s, size_t pos = 0) const;
size_t find (const char* s, size_t pos, size_type n) const;
size_t find (char c, size_t pos = 0) const noexcept;
*/
return 0;
}
输出
str size is: 45
re not find.
re1 is: 36
re2 is: 4
re3 is: 37
re4 is: 7
re5 is: 44
3.5 类型转换
| 方法 | 说明 |
|---|---|
| atoi() | string类型转换为int类型 |
| atof() | string类型转换为double类型 |
| atol(str) | string类型转换为long类型 |
| atoll(str) | string类型转换为long long类型 |
int main(int argc, char *argv[])
{
std::string str1{"121"};
std::cout<<std::atoi(str1.c_str())<<std::endl<<std::endl;
std::string str2{"121.121"};
std::cout<<std::atoi(str2.c_str())<<std::endl;
std::cout<<std::atol(str2.c_str())<<std::endl;
std::cout<<std::atoll(str2.c_str())<<std::endl;
std::cout<<std::atof(str2.c_str())<<std::endl<<std::endl;
std::string str3{"121121212"};
std::cout<<std::atoi(str3.c_str())<<std::endl;
std::cout<<std::atol(str3.c_str())<<std::endl;
std::cout<<std::atoll(str3.c_str())<<std::endl;
return 0;
}
输出
121
121
121
121
121.121
121121212
121121212
121121212