为“find_first_of”引发错误“out_of_range”

Question

我不知道为什么第一个代码块正常工作而第二个代码块不工作： 对于这两个块，有：

struct Object
{
    vector <string> line;
};

Object myObject;
string a = "*Nset, nset=_PickedSet2, internal, generate";
myObject.line.push_back(a);
int iPos = 0; 
int iPos2 = 7;  
string sRefString = ",";     // So I want to find the ",".
iPos = myObject.line.at(0).find_first_of(sRefString, iPos2);    //Here is the issue.

所以这很好用：

cout <<"Output: "<< myObject.line.at(0).find_first_of(sRefString, iPos2);

它给出了：

Output: 23

但这不是：

iPos = myObject.line.at(0).find_first_of(sRefString, iPos2);

我在这里得到一个错误：

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
zweiterminate called after throwing an instance of 'std::out_of_range'
what():  basic_string::substr

感谢您的帮助！