导致分段错误 C++

Question

我有一个c++程序，它读取一个文本文件，然后将该文本文件转换为字符串。然后，它使用 strncpy 将字符串转换为字符数组。我已经在 strncpy 上看到了堆栈溢出问题，并采取了必要的预防措施以避免它在创建数组时导致的问题。有人可以解释为什么它仍然会导致堆栈错误。

#include <iostream>
#include <string.h>
#include <random>
#include <fstream>
#include <istream>
#include <sstream>
#include <stdio.h>
using namespace std;
int main()
{
    //open a stream reader
    ifstream fin;
    //opens the text file in the stream reader
    fin.open("songlyrics.txt");
    //will be used to aggregate all characters in text file
    string song;
    //used as a pointer when reading each character in text file
    char ch;
    //while the end of file is not reached
    while(!fin.eof())
    {
        //get the character from the file and add it to the song string
        fin.get(ch);
        song += ch;
    }
    //close the file
    fin.close();
    //make a character array called lyrics_ with a length of the length of song
    char lyrics_[song.length()];
    //use strncpy to convert song to a char array, lyrics_
    strncpy(lyrics_, song.c_str(), sizeof(lyrics_));
    //avoid the segmentation fault
    lyrics_[sizeof(lyrics_) - 1] = 0;
    cout<<lyrics_;
    return 0;
}

Answer 1

这个：

char lyrics_[song.length()];
//           ^^^^^^^^^^^^^
//           not a compile-time constant

是一个可变长度数组，不是标准C++。

此外，您无需将std::string转换为字符数组。它已经是：

char* lyrics = &song[0]; // assuming you don't append to song in the future

如果你真的想要一个单独的字符缓冲区，你必须动态分配它：

char* lyrics = new char[song.length() + 1];
memcpy(lyrics, song.c_str(), song.length() + 1); // this will copy the null terminator
delete [] lyrics; // don't forget this

Answer 2

C++不支持

C.1999的标准功能，也是C.2011中的可选功能。

如果要将字符串的内容复制到动态大小的char数组中，可以使用vector：

std::vector<char> lyrics_(song.begin(), song.end());
lyrics_.push_back('');
std::cout << &lyrics_[0];