const char* 的 str::find() 函数

Question

const char *attribute[]  = 
{"abc","efg","hij","lmn","opq","rst","uvw","Xyz"};

想在上面的数组中找到“lmn”的布尔值和位置。

Answer 1

本示例将向您展示如何同时获取布尔值和索引。 在这里演示：https://onlinegdb.com/vHHJ9QG1M

#include <array>
#include <limits>
#include <algorithm>
#include <iostream>
#include <string_view>

// make a struct to be able to return two (readable) values from function
// I almost never use std::pair it results in hard to read code.
// where you have to check the semantics of first/second over and over again.
struct is_attribute_result_t
{
    // conversion to bool so result can be directly used in "if's" 
    constexpr operator bool() const 
    {
        return is_attribute;
    }

    bool is_attribute{false};
    std::size_t index{std::numeric_limits<std::size_t>::max()};
};


// make a std::array that is usable at compile time
// use string_view because it implements operator== for the whole string (const char* doesn't)
constexpr std::array<std::string_view,8> attributes = {"abc","efg","hij","lmn","opq","rst","uvw","xyz"};
 
// make a function that can be evaluated at compile time
// so no std::find, just use availability of (constexpr) operator== on string_view
constexpr is_attribute_result_t test_attribute(const std::string_view& attribute)
{
    is_attribute_result_t result;
    for(std::size_t n = 0; n < attributes.size(); ++n)
    {
       if(attribute == attributes[n])
       {
            result.is_attribute = true;
            result.index = n;
            return result;
       }
    }
    
    return result;
}


int main()
{
    // nice thing is you can now also check at compile time.
    static_assert(test_attribute("abc"));
    static_assert(test_attribute("abc").index == 0ul);
    static_assert(test_attribute("lmn"));
    static_assert(test_attribute("lmn").index == 3ul);
    static_assert(!test_attribute("123"));
    
    // and ofcourse still use the function at runtime too
    if ( auto result = test_attribute("lmn"))
    {
       std::cout << "lmn is an attribute, and is found at index = " << result.index << "\n";
    }

    return 0;
}