Can make STL string::c_str() return NULL when it has no string?

Question

My project has legacy library which consider NULL pointer as empty string.

But when I get return data from std::wstring like this,

std::wstring strData; const wchar* pStr = strData.c_str(); ASSERT(NULL == pStr); // ASSERT!!

pStr is not NULL but pointer which wstring point.

Can I make std::string return NULL when it has no string data? Now I wrap every str member variable like this :

const wchar* GetName() { // I hate this kinds of wrapping function if (m_Name.empty()) { return NULL; } return m_Name.c_str(); }

My working environment is Visual Studio 2008 sp1 in Windows

Thanks in advance.

sharptooth · Accepted Answer · 2010-10-22 07:13:38Z

Since you only need that new behavior for interacting with that legacy library and not in all code (for example strlen() will break if you pass a null pointer into it) your best bet is to use an utility function for providing proper behavior.

Something like you suggested:

const wchar* GetStringBody( const std::string& str ) { if( str.empty() ) { return 0; } return str.c_str(); }

and call it where necessary.

CodeAngry · Accepted Answer · 2014-01-05 18:56:45Z

template <typename Char_t, typename Traits_t, typename Allocator_t> inline const Char_t* StrPtr(const std::basic_string<Char_t, Traits_t, Allocator_t>& aString) { return (aString.empty() || !aString.front()) ? nullptr : aString.c_str(); } template <typename Char_t> inline const Char_t* StrPtr(const Char_t* aString) { return (!aString || !*aString) ? nullptr : aString; }

Use this general functions to convert strings to pointers. All NULL or empty() / "" strings return nullptr while any 1+ length strings returns the proper pointer.

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