ranges::starts_with 与 C 风格的字符串参数有什么关系？

Question

以下代码打印

1010

#include <iostream>
#include <range/v3/algorithm/starts_with.hpp>

int main () {
    using namespace std::literals;
    std::cout << ranges::starts_with("hello world"s, "hello"s)
              << ranges::starts_with("hello world"s, "hello")
              << ranges::starts_with("hello world",  "hello"s)
              << ranges::starts_with("hello world",  "hello") << std::endl; // prints 1010

}

为什么会这样？

我还注意到，如果两个刺痛相同，那么最后一个案例也会返回

1

：

    std::cout << ranges::starts_with("hello world"s, "hello world"s)
              << ranges::starts_with("hello world"s, "hello world")
              << ranges::starts_with("hello world",  "hello world"s)
              << ranges::starts_with("hello world",  "hello world") << std::endl; // prints 1011

我知道

std::string

，例如

"bla"s

，是一个范围，而

char[N]

也是一个范围，给定一个常量

N

。所以我不明白为什么

Answer 1

像

"hello"

这样的字符串文字是一个

const char[N]

数组，其中

N

包含空终止符的空间，例如

char[6] = {'h','e','l','l','o','\0'}

.

像

"hello"s

这样的字符串文字是一个

std::string

对象，它的

size()

中不包含空终止符。

所以，你最终得到：

starts_with("hello world"s, "hello"s) = 1
// {'h','e','l','l','o',' ','w','o','r','l','d'}
// DOES begin with
// {'h','e','l','l','o'}

starts_with("hello world"s, "hello") = 0
// {'h','e','l','l','o',' ','w','o','r','l','d'}
// DOES NOT begin with
// {'h','e','l','l','o','\0'}

starts_with("hello world",  "hello"s) = 1
// {'h','e','l','l','o',' ','w','o','r','l','d','\0'}
// DOES begin with
// {'h','e','l','l','o'}

starts_with("hello world",  "hello") = 0
// {'h','e','l','l','o',' ','w','o','r','l','d','\0'}
// DOES NOT begin with
// {'h','e','l','l','o','\0'}

starts_with("hello world"s, "hello world"s) = 1
// {'h','e','l','l','o',' ','w','o','r','l','d'}
// DOES begin with
// {'h','e','l','l','o',' ','w','o','r','l','d'}

starts_with("hello world"s, "hello world") = 0
// {'h','e','l','l','o',' ','w','o','r','l','d'}
// DOES NOT begin with
// {'h','e','l','l','o',' ','w','o','r','l','d','\0'}

starts_with("hello world",  "hello world"s) = 1
// {'h','e','l','l','o',' ','w','o','r','l','d','\0'}
// DOES begin with
// {'h','e','l','l','o',' ','w','o','r','l','d'}

starts_with("hello world",  "hello world") = 1
// {'h','e','l','l','o',' ','w','o','r','l','d','\0'}
// DOES begin with
// {'h','e','l','l','o',' ','w','o','r','l','d','\0'}