C++ STL string 类详解：接口、迭代器与常用操作

3.2 迭代器

1. 提供统一的方式遍历容器；
2. 算法可以泛型化，算法借助迭代器处理容器的数据。

3.2.1 迭代器的特点

• 提供统一的方式遍历容器。
• 算法可以泛型化。

3.2.2 const 迭代器

const 迭代器用于只读访问，不能修改通过迭代器指向的元素。

3.2.3 rbegin() && rend()

rbegin() 返回反向迭代器，rend() 返回反向结束迭代器。

3.3 string 类对象的容量操作

1. size() 与 length() 方法底层实现原理完全相同，一般用 size()。
2. clear() 只是将 string 中有效字符清空，不改变底层空间大小。
3. resize(n) 与 resize(n, char c) 都是将字符串中有效字符个数改变到 n 个，不同的是当字符个数增多时，resize(n) 用 '\0' 来填充，resize(n, c) 用字符 c 来填充。
4. reserve(n)：为 string 预留空间，不改变有效元素个数。

3.4 string 类对象的访问及遍历操作

3.5 string 类对象的修改操作（Modifiers）

1. 在 string 尾部追加字符时，s.push_back(c)、s.append(1,c)、s+='c' 三种的实现方式差不多，一般情况下 string 类的 += 操作用的比较多。
2. 对 string 操作时，如果能够大概预估到放多少字符，可以先通过 reserve 把空间预留好。

3.5.1 push_back && append

3.5.2 insert && erase

3.5.3 replace

replace 接口设计较复杂，效率不高，少用。如果是少量数据用没什么大问题，要挪动数据需谨慎。

示例：空格替换

// 所有空格替换为 %%%
string s4("hello world hello friend");
cout << s4 << endl;
size_t pos = s4.find(' ');
while (pos != string::npos) {
    s4.replace(pos, 1, "%%%" );
    pos = s4.find(' ', pos + 2);
}
cout << s4 << endl;

// 使用 reserve、范围 for 实现
string s5("hello world hello friend");
cout << s5 << endl;
string s6;
s6.reserve(s5.size());
for (auto ch : s5) {
    if (ch != ' ') {
        s6 += ch;
    } else {
        s6 += "%%%%";
    }
}
cout << s6 << endl;

3.6 string 类非成员函数

4. 本文完整代码

Test.cpp

#define _CRT_SECURE_NO_WARNINGS 1
#include <iostream>
#include <string>
#include <algorithm>
#include <list>
using namespace std;

void Print(const string& s) {
    string::const_iterator it1 = s.begin();
    while (it1 != s.end()) {
        cout << *it1 << " ";
        ++it1;
    }
    cout << endl;

    string::const_reverse_iterator it2 = s.rbegin();
    while (it2 != s.rend()) {
        cout << *it2 << " ";
        ++it2;
    }
    cout << endl;
}

void Test_string2() {
    string s1("hello world");
    cout << s1 << endl;
    s1[0] = 'x';
    cout << s1 << endl;
    cout << s1.size() << endl;

    for (size_t i = 0; i < s1.size(); i++) {
        s1[i]++;
    }
    cout << s1 << endl;

    string::iterator it1 = s1.begin();
    while (it1 != s1.end()) {
        (*it1)--;
        cout << *it1 << " ";
        ++it1;
    }
    cout << endl;

    list<int> lt;
    lt.push_back(1);
    lt.push_back(2);
    lt.push_back(3);
    list<int>::iterator lit = lt.begin();
    while (lit != lt.end()) {
        cout << *lit << " ";
        ++lit;
    }
    cout << endl;

    Print(s1);

    auto ret1 = find(s1.begin(), s1.end(), 'x');
    if (ret1 != s1.end()) {
        cout << "找到了 x" << endl;
    }

    int i = 0;
    auto j = i;
    auto k = 10;
    auto p1 = &i;
    auto p2 = &i;
    cout << p1 << endl;
    cout << p2 << endl;

    int& r1 = i;
    auto r2 = r1;
    auto& r3 = r1;
    cout << &r2 << endl;
    cout << &r1 << endl;
    cout << &i << endl;
    cout << &r3 << endl;

    for (char ch : s1) {
        cout << ch << ' ';
    }
    cout << endl;

    for (auto e : lt) {
        cout << e << ' ';
    }
    cout << endl;

    for (auto& ch : s1) {
        ch -= 1;
    }
    cout << endl;

    for (const auto& ch : s1) {
        cout << ch << ' ';
    }
    cout << endl;

    int a[10] = { 1,2,3 };
    for (auto e : a) {
        cout << e << " ";
    }
    cout << endl;
}

void TestCapacity() {
    string s1;
    s1.reserve(200);
    size_t old = s1.capacity();
    for (size_t i = 0; i < 200; i++) {
        s1.push_back('x');
        if (s1.capacity() != old) {
            cout << s1.capacity() << endl;
            old = s1.capacity();
        }
    }
    cout << endl << endl;
}

void test_string3() {
    string s1("hello world");
    cout << s1.max_size() << endl;
    cout << s1.size() << endl;
    cout << s1.capacity() << endl;

    s1.clear();
    cout << s1.size() << endl;
    cout << s1.capacity() << endl << endl;

    TestCapacity();

    string s2("hello world");
    cout << s2.size() << endl;
    cout << s2.capacity() << endl;
    s2.reserve(5);
    s2.shrink_to_fit();
    cout << s2.size() << endl;
    cout << s2.capacity() << endl;

    string s3("hello world");
    cout << s3 << endl;
    s3.resize(5);
    cout << s3 << endl;
    s3.resize(10, 'x');
    cout << s3 << endl;
    s3.resize(20, 'y');
    cout << s3 << endl;
}

void Test_string4() {
    string s1("hello world");
    s1.push_back('%');
    s1.append("hello friend");
    cout << s1 << endl;
    s1.append(10, '#');
    cout << s1 << endl;

    string s2("pear hello!");
    s1.append(++s2.begin(), --s2.end());
    cout << s1 << endl;

    string s3("hello world");
    s3 += ' ';
    s3 += "hello world";
    cout << s3 << endl;
    cout << s3 + "xxxxx" << endl;
    cout << "xxxxx" + s3 << endl;

    s3.assign("yyyy");
    cout << s3 << endl;
}

void Test_string5() {
    string s1("hello world");
    s1.insert(0, "xxxxx");
    cout << s1 << endl;
    s1.insert(0, 1, '#');
    cout << s1 << endl;
    s1.insert(5, 1, '#');
    cout << s1 << endl;
    s1.insert(s1.begin(), '$');
    cout << s1 << endl;

    string s2("hello world");
    s2.erase(s2.begin());
    cout << s2 << endl;
    s2.erase(0, 1);
    cout << s2 << endl;
    s2.erase(5, 2);
    cout << s2 << endl;
    s2.erase(5);
    cout << s2 << endl << endl;

    string s3("hello world");
    s3.replace(5, 1, "%%%%" );
    cout << s3 << endl;
    s3.replace(5, 3, "*");
    cout << s3 << endl;

    string s4("hello world hello friend");
    cout << s4 << endl;
    size_t pos = s4.find(' ');
    while (pos != string::npos) {
        s4.replace(pos, 1, "%%%" );
        pos = s4.find(' ', pos + 2);
    }
    cout << s4 << endl;

    string s5("hello world hello friend");
    string s6;
    s6.reserve(s5.size());
    for (auto ch : s5) {
        if (ch != ' ') {
            s6 += ch;
        } else {
            s6 += "%%%%";
        }
    }
    cout << s6 << endl;
}

int main() {
    string str;
    cin >> str;
    getline(cin, str);
    getline(cin, str, '#');
    size_t pos = str.rfind(' ');
    if (pos != str.size()) {
        cout << str.size() - (pos + 1) << endl;
    } else {
        cout << str.size() << endl;
    }
    return 0;
}