C++ 中 find 主要分为两种

一种是 string 里封装的，只能针对 string 搜索，返回类型是 int 下标，没搜到时返回 string::npos

一种是 <algorithm> 里封装的，能对各种东西搜索，返回类型是 iterator，没搜到时返回 end() 迭代器

1. string 中的 find

现有一个 string s 搜索目标为 "abc"

find(string& str, pos = 0)

用法：s.find("abc")

Finds the first substring equal to the given character sequence. Search begins at pos, i.e. the found substring must not begin in a position preceding pos.

找到搜索目标第一次出现的位置，即第一个 "abc" 出现的位置

如果搜索目标为空，空字符串第一次出现的位置为字符串开头，即 s.find(t) 会返回 0

find_first_of(string& str, pos = 0)

用法：s.find_first_of("abc")

Finds the first character equal to one of the characters in the given character sequence. The search considers only the interval [pos, size()). If the character is not present in the interval, npos will be returned.

找到搜索目标中任一字符第一次出现的位置，即第一个 "a" 或 "b" 或 "c" 出现的位置

如果搜索目标为空，即搜索目标中不包含任何字符，所以什么都找不到， s.find_first_of(t) 会返回 npos

2. find

#include <iostream>
#include <algorithm>
#include <vector>
#include <iterator>
 
int main()
{
    std::vector<int> v{1, 2, 3, 4};
    int n1 = 3;
    int n2 = 5;
    auto is_even = [](int i){ return i%2 == 0; };
 
    auto result1 = std::find(begin(v), end(v), n1);
    auto result2 = std::find(begin(v), end(v), n2);
    auto result3 = std::find_if(begin(v), end(v), is_even);
 
    (result1 != std::end(v))
        ? std::cout << "v contains " << n1 << '\n'
        : std::cout << "v does not contain " << n1 << '\n';
 
    (result2 != std::end(v))
        ? std::cout << "v contains " << n2 << '\n'
        : std::cout << "v does not contain " << n2 << '\n';
 
    (result3 != std::end(v))
        ? std::cout << "v contains an even number: " << *result3 << '\n'
        : std::cout << "v does not contain even numbers\n";
}

Output:

v contains 3
v does not contain 5
v contains an even number: 2