将特定数量的字符从 std::basic_istream 复制到 std::string

Question

从 std::basic_istream 中提取特定数量的字符并将其存储在 std::string 中的安全好方法是什么？

在下面的程序中，我使用 char[] 最终获得 result，但我想避免 POD 类型并确保更安全和更易于维护：

#include <sstream>
#include <string>
#include <iostream>
#include <exception>

int main()
{
    std::stringstream inss{std::string{R"(some/path/to/a/file/is/stored/in/50/chars         Other data starts here.)"}};
    char arr[50]{};
    if (!inss.read(arr,50))
        throw std::runtime_error("Could not read enough characters.\n");

    //std::string result{arr}; // Will probably copy past the end of arr
    std::string result{arr,arr+50};

    std::cout << "Path is: " << result << '\n';
    std::cout << "stringstream still has: " << inss.str() << '\n';

    return 0;
}

备选方案：

预先将整个流转换为字符串：std::string{inss.c_str()}
- 这似乎很浪费，因为它会复制整个流。
写一个模板函数接受char[]
- 这仍将使用中间 POD 阵列。
在循环中使用std::basic_istream::get与std::basic_string::push_back一起读取所需数量的字符
- 循环看起来有点笨拙，但它确实避开了数组。

Answer 1

直接读入result字符串即可。

#include <sstream>
#include <string>
#include <iostream>
#include <exception>

int main()
{
    std::stringstream inss{std::string{R"(some/path/to/a/file/is/stored/in/50/chars         Other data starts here.)"}};
    std::string result(50, '[=10=]');

    if (!inss.read(&result[0], result.size()))
        throw std::runtime_error("Could not read enough characters.\n");

    std::cout << "Path is: " << result << '\n';
    std::cout << "stringstream still has: " << inss.str() << '\n';

    return 0;
}

从C++11开始，下面保证了std::string(from cppreference)的内存布局。

The elements of a basic_string are stored contiguously, that is, for a basic_string s, &*(s.begin() + n) == &*s.begin() + n for any n in [0, s.size()), or, equivalently, a pointer to s[0] can be passed to functions that expect a pointer to the first element of a CharT[] array. (since C++11)

将特定数量的字符从 std::basic_istream 复制到 std::string

Copy specific number of characters from std::basic_istream to std::string

c++

string

stringstream