Using the value _T("") and CString variables

Question

This might sounds simple but I want to know if one way is actually more efficient.

Example one:

• CString strName = _T("");
• CString strName = CString();

Example two:

• if (strFilterText == _T(""))
• if (strFilterText.IsEmpty())

Is there any negative impact is using _T("") in this way? And if so, why?

Answer 1

CString has different constructors and operator overloads, it is prepared for different scenarios. It's a complicated class but in general, such classes work like this:

Use default constructor CString() (initializes for _T("")):

CString s1 = CString();     
CString s2;                 //same as above, shortcut
CString s3{};               //same as above
CString s4 = {};            //same as above

Use copy constructor CString(const CString& src):

const CString src = _T("");

CString s1 = CString(src);  
CString s2 = src;           //same as above
CString s3 = { src};        //same as above
CString s4(src);            //same as above

Use different constructor CString(const TCHAR* src):

const TCHAR* src = _T("");

CString s1 = CString(src);  
CString s2 = src;           //same as above
CString s3 = { src };       //same as above
CString s4(src);            //same as above

CString also supports = assignment operator:

CString s;
s = _T("");                 //uses `=` assignment operator

s = CString();              //uses default constructor for right side, 
                            //calls `=` assignment operator for left side.

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== operator overload may use CString::Compare to compare the two sides. So

if (s == _T("")) is same as if (s.Compare(_T("")) == 0), it might be 1 nanosecond slower than s.IsEmpty(), otherwise is valid.