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std::fstream LoadFile;
LoadFile.open ( "C:\\3V.txt", std::ios::binary | std::ios::_Nocreate );
fstream fs;
fs.open("test.txt", ios_base::binary);
// fstream.open有一个默认参数放在最后,就是ios_base::_Nocreate
// 所以上面的代码与下面这行功能一样
// fs.open("test.txt", ios_base::binary, ios_base::_Nocreate);
if (fs)
{
cout << "yes" << endl; // 这个yes不会输出
}
template <typename CharT> FILE * _Xfiopen(const CharT *filename,
ios_base::openmode mode, int prot)
{
static const int valid[] =
{ // valid combinations of open flags
ios_base::in,
ios_base::out,
ios_base::out | ios_base::trunc,
ios_base::out | ios_base::app,
ios_base::in | ios_base::binary,
ios_base::out | ios_base::binary,
ios_base::out | ios_base::trunc | ios_base::binary,
ios_base::out | ios_base::app | ios_base::binary,
ios_base::in | ios_base::out,
ios_base::in | ios_base::out | ios_base::trunc,
ios_base::in | ios_base::out | ios_base::app,
ios_base::in | ios_base::out | ios_base::binary,
ios_base::in | ios_base::out | ios_base::trunc
| ios_base::binary,
ios_base::in | ios_base::out | ios_base::app
| ios_base::binary,
0};
FILE *fp = 0;
int n;
ios_base::openmode atendflag = mode & ios_base::ate;
ios_base::openmode norepflag = mode & ios_base::_Noreplace;
// ===========重点看这里===========
if (mode & ios_base::_Nocreate)
mode |= ios_base::in; // file must exist
if (mode & ios_base::app)
mode |= ios_base::out; // extension -- app implies out
mode &= ~(ios_base::ate | ios_base::_Nocreate | ios_base::_Noreplace);
for (n = 0; valid[n] != 0 && valid[n] != mode; ++n)
; // look for a valid mode
if (valid[n] == 0)
return (0); // no valid mode
else if (norepflag && (mode & (ios_base::out | ios_base::app))
&& (fp = _Xfsopen(filename, 0, prot)) != 0)
{ // file must not exist, close and fail
fclose(fp);
return (0);
}
else if (fp != 0 && fclose(fp) != 0)
return (0); // can't close after test open
else if ((fp = _Xfsopen(filename, n, prot)) == 0)
return (0); // open failed
if (!atendflag || fseek(fp, 0, SEEK_END) == 0)
return (fp); // no need to seek to end, or seek succeeded
fclose(fp); // can't position at end
return (0);
}
上面的函数被fstream的open函数间接调用。可以看到,当第二个参数包含ios_base::_Nocreate标志时,会被自动加上ios::in。楼主不必惊奇。