C++ STL丨string 基本使用（一）

C++ STL丨string 基本使用（一）

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1 string 字符串

  string是C++标准库的一个重要的部分，主要用于字符串处理。可以使用输入输出流方式直接进行操作，也可以通过文件等手段进行操作。同时C++的算法库对string也有着很好的支持，而且string还和c语言的字符串之间有着良好的接口。虽然也有一些弊端，但是瑕不掩瑜。
  以下是 C/C++ 中定义的字符串的内存表示：

  其实，您不需要把 null 字符放在字符串常量的末尾。C++ 编译器会在初始化数组时，自动把 ‘\0’ 放在字符串的末尾。

2 使用速查表(点击函数名查看使用代码)

函数	解释
string s	创建一个空的string
s.c_str()	返回指向以null结尾的字符数组的指针，其数据与存储在字符串中的数据相同
s.data()	返回指向字符串第一个字符的指针
s.begin(); s.cbegin()	返回字符串第一个字符的迭代器
s.end(); s.cend()	返回字符串最后一个字符的迭代器
s.empty()	true如果字符串为空, 否则返回false
s.size(); s.length()	字符串中CharT元素的数量
s.capacity()	返回字符串当前为其分配空间的字符数
s.shrink_to_fit()	请求删除未使用的容量
s.insert()	迭代器，如果没有插入字符，则引用第一个插入字符或pos的副本
s.clear()	从字符串中删除所有字符
s.earse(n, m)	删除[n, m)之间的数据
s.push_back(str); s.pop_back()	在s尾插入str字符串；删除尾字符串
s.append()	在字符串中附加其他字符
s.compare(str)	比较两个字符串是否相等
s.replaec	用新字符串替换[pos，pos + count]或[first，last）中的字符串部分
s.substr()	返回子串[pos，pos + count)。如果请求的子字符串超出字符串的结尾，或者如果count == npos，则返回的子字符串为[pos，size())
s.swap(str)	将字符串的内容与其他内容交换。所有迭代器和引用都可能无效
s.find(str)	查找与给定字符序列相等的第一个子字符串。搜索从pos开始，即找到的子字符串不得在pos之前的位置开始
string::npos	相当于s.end()

3 详细使用

3.1 string 初始化

string 初始化的多种方式：

#include 
#include 
#include 
#include 
#include 
 
int main()
{
  {
    // string::string()
    std::string s;
    assert(s.empty() && (s.length() == 0) && (s.size() == 0));
  }
 
  {
    // string::string(size_type count, charT ch)
    std::string s(4, '=');
    std::cout << s << '\n'; // "===="
  }
 
  {
    std::string const other("Exemplary");
    // string::string(string const& other, size_type pos, size_type count)
    std::string s(other, 0, other.length()-1);
    std::cout << s << '\n'; // "Exemplar"
  }
 
  {
    // string::string(charT const* s, size_type count)
    std::string s("C-style string", 7);
    std::cout << s << '\n'; // "C-style"
  }
 
  {
    // string::string(charT const* s)
    std::string s("C-style\0string");
    std::cout << s << '\n'; // "C-style"
  }
 
  {
    char mutable_c_str[] = "another C-style string";
    // string::string(InputIt first, InputIt last)
    std::string s(std::begin(mutable_c_str)+8, std::end(mutable_c_str)-1);
    std::cout << s << '\n'; // "C-style string"
  }
 
  {
    std::string const other("Exemplar");
    std::string s(other);
    std::cout << s << '\n'; // "Exemplar"
  }
 
  {
    // string::string(string&& str)
    std::string s(std::string("C++ by ") + std::string("example"));
    std::cout << s << '\n'; // "C++ by example"
  }
 
  {
    // string(std::initializer_list ilist)
    std::string s({ 'C', '-', 's', 't', 'y', 'l', 'e' });
    std::cout << s << '\n'; // "C-style"
  }
 
  {
    // overload resolution selects string(InputIt first, InputIt last) [with InputIt = int]
    // which behaves as if string(size_type count, charT ch) is called
    std::string s(3, std::toupper('a'));
    std::cout << s << '\n'; // "AAA"
  }
}

====
Exemplar
C-style
C-style
C-style string
Exemplar
C++ by example
C-style
AAA

3.2 c_str()

c_str 使用方法：

  std::string const s("Emplary");
  assert(s.size() == std::strlen(s.c_str()));
  assert(std::equal(s.begin(), s.end(), s.c_str()));
  assert(std::equal(s.c_str(), s.c_str() + s.size(), s.begin()));
  assert(0 == *(s.c_str() + s.size()));

3.3 data()

data 使用方法：

  std::string const s("Emplary");
  assert(s.size() == std::strlen(s.data()));
  assert(std::equal(s.begin(), s.end(), s.data()));
  assert(std::equal(s.data(), s.data() + s.size(), s.begin()));
  assert(0 == *(s.data() + s.size()));

3.4 empty()

empty 使用代码：

    std::string s;
    std::boolalpha(std::cout);
    std::cout << "s.empty():" << s.empty() << "\t s:'" << s << "'\n";
 
    s = "Exemplar";
    std::cout << "s.empty():" << s.empty() << "\t s:'" << s << "'\n";
 
    s = "";
    std::cout << "s.empty():" << s.empty() << "\t s:'" << s << "'\n";

s.empty():true s:’’
s.empty():false s:‘Exemplar’
s.empty():true s:’’

3.5 size() length()

size和length 使用代码：

	std::string s("Exemplar");
    assert(8 == s.size());
    assert(s.size() == s.length());
    assert(s.size() == static_cast(
        std::distance(s.begin(), s.end())));
 
    std::u32string a(U"ハロー・ワールド"); // 8 code points
    assert(8 == a.size()); // 8 code units in UTF-32
 
    std::u16string b(u"ハロー・ワールド"); // 8 code points
    assert(8 == b.size()); // 8 code units in UTF-16
 
    std::string c(u8"ハロー・ワールド"); // 8 code points
    assert(24 == c.size()); // 24 code units in UTF-8

3.6 shrink_to_fit()

shrink_to_fit 使用代码：

	std::string s;
    std::cout << "Default-constructed capacity is " << s.capacity() << '\n';
    s.resize(100);
    std::cout << "Capacity of a 100-element string is " << s.capacity() << '\n';
    s.clear();
    std::cout << "Capacity after clear() is " << s.capacity() << '\n';
    s.shrink_to_fit();
    std::cout << "Capacity after shrink_to_fit() is " << s.capacity() << '\n';

Default-constructed capacity is 0
Capacity of a 100-element string is 100
Capacity after clear() is 100
Capacity after shrink_to_fit() is 0

3.7 earse()

earse 删除元素的方式：

	std::string s = "This is an example";
    std::cout << s << '\n';
 
    s.erase(0, 5); // Erase "This "
    std::cout << s << '\n';
 
    s.erase(std::find(s.begin(), s.end(), ' ')); // Erase ' '
    std::cout << s << '\n';
 
    s.erase(s.find(' ')); // Trim from ' ' to the end of the string
    std::cout << s << '\n';

输出：

This is an example
is an example
isan example
isan

3.8 append ()

append 追加元素的几种方式：

	std::basic_string str = "string";
    const char* cptr = "C-string";
    const char carr[] = "Two and one";
 
    std::string output;
 
    // 1) Append a char 3 times. 
    // Notice, this is the only overload accepting chars.
    output.append(3, '*');
    std::cout << "1) " << output << "\n";
 
    //  2) Append a whole string
    output.append(str);
    std::cout << "2) " << output << "\n";
 
    // 3) Append part of a string (last 3 letters, in this case)
    output.append(str, 3, 3);
    std::cout << "3) " << output << "\n";
 
    // 4) Append part of a C-string
    // Notice, because `append` returns *this, we can chain calls together
    output.append(1, ' ').append(carr, 4);
    std::cout << "4) " << output << "\n";
 
    // 5) Append a whole C-string
    output.append(cptr);
    std::cout << "5) " << output << "\n";
 
    // 6) Append range
    output.append(&carr[3], std::end(carr));
    std::cout << "6) " << output << "\n";
 
    // 7) Append initializer list
    output.append({ ' ', 'l', 'i', 's', 't' });
    std::cout << "7) " << output << "\n";

输出：

1)***
2)***string
3)***stringing
4)***stringing Two
5)***stringing Two C-string
6)***stringing Two C-string and one
7)***stringing Two C-string and one list

3.9 compare()

compare 比较元素的几种方式：

#include 
#include 
#include 
int main() 
{
    // 1) Compare with other string
    {
        int compare_value{
            std::string{"Batman"}.compare(std::string{"Superman"})
        };
        std::cout << (
            compare_value < 0 ? "Batman comes before Superman\n" :
            compare_value > 0 ? "Superman comes before Batman\n" :
            "Superman and Batman are the same.\n"
        );
    }
 
    // 2) Compare substring with other string
    {
        int compare_value{
            std::string{"Batman"}.compare(3, 3, std::string{"Superman"})
        };
        std::cout << (
            compare_value < 0 ? "man comes before Superman\n" :
            compare_value > 0 ? "Superman comes before man\n" :
            "man and Superman are the same.\n"
        );
    }
 
    // 3) Compare substring with other substring
    {
        std::string a{"Batman"};
        std::string b{"Superman"};
 
        int compare_value{a.compare(3, 3, b, 5, 3)};
 
        std::cout << (
            compare_value < 0 ? "man comes before man\n" :
            compare_value > 0 ? "man comes before man\n" :
            "man and man are the same.\n"
        );
        // Compare substring with other substring
        // defaulting to end of other string
        assert(compare_value == a.compare(3, 3, b, 5));
    }
 
    // 4) Compare with char pointer
    {
        int compare_value{std::string{"Batman"}.compare("Superman")};
 
        std::cout << (
            compare_value < 0 ? "Batman comes before Superman\n" :
            compare_value > 0 ? "Superman comes before Batman\n" :
            "Superman and Batman are the same.\n"
        );
    }
 
    // 5) Compare substring with char pointer
    {
        int compare_value{std::string{"Batman"}.compare(3, 3, "Superman")};
 
        std::cout << (
            compare_value < 0 ? "man comes before Superman\n" :
            compare_value > 0 ? "Superman comes before man\n" :
            "man and Superman are the same.\n"
        );
    }
 
    // 6) Compare substring with char pointer substring
    {
        int compare_value{std::string{"Batman"}.compare(0, 3, "Superman", 5)};
 
        std::cout << (
            compare_value < 0 ? "Bat comes before Super\n" :
            compare_value > 0 ? "Super comes before Bat\n" :
            "Super and Bat are the same.\n"
        );
    }
}

Batman comes before Superman
Superman comes before man
man and man are the same.
Batman comes before Superman
Superman comes before man
Bat comes before Super

3.10 replace()

replace 的简单使用：

std::string str("The quick brown fox jumps over the lazy dog.");
str.replace(10, 5, "red"); // (5)
str.replace(str.begin(), str.begin() + 3, 1, 'A'); // (6)
std::cout << str << '\n';

A quick red fox jumps over the lazy dog.

3.11 substr()

substr 的简单使用：

	std::string a = "0123456789abcdefghij";
 
    // count is npos, returns [pos, size())
    std::string sub1 = a.substr(10);
    std::cout << sub1 << '\n';
 
    // both pos and pos+count are within bounds, returns [pos, pos+count)
    std::string sub2 = a.substr(5, 3);
    std::cout << sub2 << '\n';
 
    // pos is within bounds, pos+count is not, returns [pos, size()) 
    std::string sub4 = a.substr(a.size()-3, 50);
    std::cout << sub4 << '\n';
 
    try {
        // pos is out of bounds, throws
        std::string sub5 = a.substr(a.size()+3, 50);
        std::cout << sub5 << '\n';
    } catch(const std::out_of_range& e) {
        std::cout << "pos exceeds string size\n";
    }

abcdefghij
567
hij
pos exceeds string size

3.12 swap()

swap 的简单使用：

 	std::string a = "AAA";
    std::string b = "BBB";
 
    std::cout << "before swap" << '\n';
    std::cout << "a: " << a << '\n';
    std::cout << "b: " << b << '\n';
 
    a.swap(b);
 
    std::cout << "after swap" << '\n';
    std::cout << "a: " << a << '\n';
    std::cout << "b: " << b << '\n';

before swap
a: AAA
b: BBB
after swap
a: BBB
b: AAA

3.13 find()

swap 的简单使用：

#include 
#include 
 
void print(std::string::size_type n, std::string const &s)
{
    if (n == std::string::npos) {
        std::cout << "not found\n";
    } else {
        std::cout << "found: " << s.substr(n) << '\n';
    }
}
 
int main()
{
    std::string::size_type n;
    std::string const s = "This is a string";
 
    // search from beginning of string
    n = s.find("is");
    print(n, s);
 
    // search from position 5
    n = s.find("is", 5);
    print(n, s);
 
    // find a single character
    n = s.find('a');
    print(n, s);
 
    // find a single character
    n = s.find('q');
    print(n, s);
}

found: is is a string
found: is a string
found: a string
not found

3.14 string::npos()

npos 的使用：

	// string search functions return npos if nothing is found
    std::string s = "test";
    if(s.find('a') == std::string::npos)
        std::cout << "no 'a' in 'test'\n";
 
    // functions that take string subsets as arguments 
    // use npos as the "all the way to the end" indicator
    std::string s2(s, 2, std::string::npos);
    std::cout << s2 << '\n';
 
    std::bitset<5> b("aaabb", std::string::npos, 'a', 'b');
    std::cout << b << '\n';

no ‘a’ in ‘test’
st
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