string - Strncpy Causing Segmentation Fault c++ -

i have c++ program reads text file , converts text file string. then, converts string character array using strncpy. have seen stackoverflow question on strncpy , taken necessary precautions in order avoid issues causes when creating array. please explain why still causes stack error.

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

this:

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

is variable-length array, , not standard c++.

also, don't need convert std::string character array. kind of is:

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

if want separate character buffer, you'll have allocate dynamically:

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