我的程序出现这个错误，我不明白为什么。代码本质上必须检查存储在声明为全局变量的集合中的标记。如果它是有效标签，则将其存储在堆栈中，否则返回错误消息。然后它检查（如果它是一个有效的标签）结束标签是否有序。这就是 is_well_formed 方法的全部内容。对于 print_well_formed_file 方法，它本质上检查给定文件是否格式正确，如果它会显示该文件。：
terminate called after throwing an instance of 'std::out_of_range'
what(): basic_string::substr
我能做些什么来解决这个错误？这是代码的一部分：
bool is_well_formed(ifstream& ifs, string& error_msg) {
// your code goes here
string fname, line;
Token tok;
Lexer lexer;
tags.insert("blue");
tags.insert("red");
tags.insert("cyan");
tags.insert("white");
tags.insert("yellow");
tags.insert("magenta");
tags.insert("dim");
tags.insert("underline");
tags.insert("bold");
while (getline(cin, fname)) {
// tries to open the file whose name is in string fname
string name = fname.substr(1, fname.length() - 2);
cout << "Name" + name;
ifs.open(name.c_str());
if (ifs.fail()) {
cerr << "ERROR: Failed to open file " << fname << endl;
ifs.clear();
} else {
while (getline(ifs, line)) {
lexer.set_input(line);
while (lexer.has_more_token()) {
tok = lexer.next_token();
string tmpTok = tok.value;
switch (tok.type) {
case TAG:
// If it has /, remove / from tmpTok
if (tok.value[0] == '/') {
tmpTok = tmpTok.substr(1, tmpTok.length() - 1);
}
if (tags.find(tmpTok) == tags.end()) {
// Check whether the encountered tag is valid
error_return("Tag " + tmpTok + " is invalid!");
} else {
// Valid Tag encountered
stack < string > tagstack;
tagstack.push(tmpTok);
// Check if the tags are formed properly
if (tok.value[0] == '/') {
// Remove / from tmpTok
string closingTag = tmpTok;
string openingTag = tagstack.top();
tagstack.pop();
if (closingTag.compare(openingTag) != 0) {
error_return(
closingTag + "doesn't match"
+ openingTag);
} //else
// return true; // if the file is well formed
}
}
break;
case IDENT:
cout << "IDENT: " << tok.value << endl;
break;
case ERRTOK:
error_return("Syntax error on this line\n");
//cout << "Syntax error on this line\n";
break;
case ENDTOK:
break;
}
}
}
}
}
return true; // if the file is well-formed
}
void print_well_formed_file(ifstream& ifs) {
//Check if file is well formed.
string line;
Lexer command;
if (is_well_formed(ifs, line)) { //if well formed display
command.set_input(line);
display(command);
}
}
void display(Lexer cmd_lexer) {
string file_name;
if (!parse_input(cmd_lexer, file_name)) {
error_return("Syntax error: display <filename>");
return;
}
ifstream ifs(file_name.c_str());
string error_msg;
if (ifs) {
if (!is_well_formed(ifs, error_msg)) {
error_return(error_msg);
} else {
ifs.clear(); // clear EOF flag
ifs.seekg(0, ios::beg); // go back to the very beginning
print_well_formed_file(ifs);
}
} else {
error_return("Can't open " + file_name + " for reading");
}
ifs.close();
}
用户输入示例：
validate <file name>
display <file name>
exit