Strange behavior using CString in swscanf directly - c++

Question

I have one problem with CString and STL's set.
It looks a bit strange to use CString and STL together, but I tried to be curious.
My code is below:
#include "stdafx.h"
#include <iostream>
#include <set>
#include <atlstr.h>
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
wchar_t line[1024] = {0};
FILE * pFile = _wfopen(L"F:\\test.txt", L"rt");
set<CString> cstr_set;
while (fgetws(line, 1024, pFile))
{
CString cstr;
swscanf(line, L"%s\n", cstr);
cstr_set.insert(cstr);
}
fclose(pFile);
cout << "count" << cstr_set.size();
return 0;
}
The contents of the test.txt is:
13245
123
2344
45
After the loop ends, cstr_set contains only one value.
It works as if cstr is static or const variable.
What is the problem?

Answer 1

A CString is a Microsoft implementation wrapping a character array into a C++ object to allow simpler processing.
But, swscanf is a good old C function that knows nothing about what a CString is: it just expects its arguments to be large enough to accept the decoded values. It should never be directly passed a CString.
The correct way would be:
...
#include <cstring>
...
while (fgetws(line, 1024, pFile))
{
line[wcscspn(line, L"\n")] = 0; // remove an optional end of line
CString cstr(line);
cstr_set.insert(cstr);
}
...