高亮部分怎么了？

[msg198734]

#include <string>
#include <fstream>
#include <iostream>

using namespace std;

void main(void){

string line,sentence, sentence_append;
ifstream infilestream("data.txt"); //File object.
char end_of_sentence[3]={'.','?','!'}; //End Mark.
int position_end=0, position_start=0; // The position which could copy to the sentence.
int sentence_size=0, append_mark; //If it needs append, append=1;
bool appendmark=0;

//cout<<"Please input a file name"<<endl;
//cin>>filename;




while (!infilestream.eof()){
getline(infilestream,line);
position_end=0;
position_start=0;
string sentence_append;

do{
position_end=line.find_first_of(end_of_sentence, position_start);

//Count the size of sentence
if(position_start==0 && position_end!=string::npos)
sentence_size=position_end;

else if (position_start!=0 && position_end!=string::npos)
sentence_size=position_end-position_start;

else if(position_start!=0 && position_end==string::npos)
sentence_size=line.length()-position_start;

else
sentence_size=line.length();


//Create the sentence
if(position_end!=string::npos && appendmark==false){

line.copy(sentence,sentence_size, position_start);
}

else if(position_end==string::npos && appendmark==false){
line.copy(sentence,sentence_size, position_start);
appendmark=true;

}

else if(position_end!=string::npos && appendmark==true){
line.copy(sentence_append, sentence_size,position_start);
sentence.append(sentence_append);
appendmark=false;
}

else if(position_end==string::npos && appendmark==true){
line.copy(sentence_append, sentence_size,position_start);
sentence.append(sentence_append);
}

position_start=position_end;

}while(position_end!=string::npos);
cout<<sentence<<endl;

}
infilestream.close();

}

[msg198734]

为什么运行之后就报错 ？
应该怎么改？
望各位不吝赐教～

[a_rockboy]

拷贝之前
sentence.resize(sentence_size)
sentence_append.resize(sentence_size)
初始化一下

[msg198734]

[Quote=引用 2 楼 a_rockboy 的回复:]
拷贝之前
sentence.resize(sentence_size)
sentence_append.resize(sentence_size)
初始化一下
[/Quote]

什么意思啊？

[a_rockboy]

line.copy(sentence,sentence_size, position_start);
line.copy(sentence_append, sentence_size,position_start);

sentence和sentence_append没申请可用内存呢，向里面写东西肯定失败，而且在VS2008下根本编译不过去。
使用
sentence.resize(sentence_size)
line.copy(sentence,sentence_size, position_start);

sentence_append.resize(sentence_size)
line.copy(sentence_append, sentence_size,position_start);