How do you convert CString and std::string std::wstring to each other?

CString is quite handy, while std::string is more compatible with STL container. I am using hash_map. However, hash_map does not support CStrings as keys, so I want to convert the CString into a std::string.

Writing a CString hash function seems to take a lot of time.

CString -----> std::string

How can I do this?

std::string -----> CString:


inline CString toCString(std::string const& str)
{
return CString(str.c_str());
}

Am I right?

EDIT:

Here are more questions:

How can I convert from wstring to CString and vice versa?

// wstring -> CString
std::wstring src;
CString result(src.c_str());


// CString -> wstring
CString src;
std::wstring des(src.GetString());

Is there any problem with this?

Additionally, how can I convert from std::wstring to std::string and vice versa?

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最佳答案

According to CodeGuru:

CString to std::string:

CString cs("Hello");
std::string s((LPCTSTR)cs);

BUT: std::string cannot always construct from a LPCTSTR. i.e. the code will fail for UNICODE builds.

As std::string can construct only from LPSTR / LPCSTR, a programmer who uses VC++ 7.x or better can utilize conversion classes such as CT2CA as an intermediary.

CString cs ("Hello");
// Convert a TCHAR string to a LPCSTR
CT2CA pszConvertedAnsiString (cs);
// construct a std::string using the LPCSTR input
std::string strStd (pszConvertedAnsiString);

std::string to CString: (From Visual Studio's CString FAQs...)

std::string s("Hello");
CString cs(s.c_str());

CStringT can construct from both character or wide-character strings. i.e. It can convert from char* (i.e. LPSTR) or from wchar_t* (LPWSTR).

In other words, char-specialization (of CStringT) i.e. CStringA, wchar_t-specilization CStringW, and TCHAR-specialization CString can be constructed from either char or wide-character, null terminated (null-termination is very important here) string sources.
Althoug IInspectable amends the "null-termination" part in the comments:

NUL-termination is not required.
CStringT has conversion constructors that take an explicit length argument. This also means that you can construct CStringT objects from std::string objects with embedded NUL characters.

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Solve that by using std::basic_string<TCHAR> instead of std::string and it should work fine regardless of your character setting.

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If you're looking to convert easily between other strings types, perhaps the _bstr_t class would be more appropriate? It supports converstion between char, wchar_t and BSTR.

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If you want something more C++-like, this is what I use. Although it depends on Boost, that's just for exceptions. You can easily remove those leaving it to depend only on the STL and the WideCharToMultiByte() Win32 API call.

#include <string>
#include <vector>
#include <cassert>
#include <exception>


#include <boost/system/system_error.hpp>
#include <boost/integer_traits.hpp>


/**
* Convert a Windows wide string to a UTF-8 (multi-byte) string.
*/
std::string WideStringToUtf8String(const std::wstring& wide)
{
if (wide.size() > boost::integer_traits<int>::const_max)
throw std::length_error(
"Wide string cannot be more than INT_MAX characters long.");
if (wide.size() == 0)
return "";


// Calculate necessary buffer size
int len = ::WideCharToMultiByte(
CP_UTF8, 0, wide.c_str(), static_cast<int>(wide.size()),
NULL, 0, NULL, NULL);


// Perform actual conversion
if (len > 0)
{
std::vector<char> buffer(len);
len = ::WideCharToMultiByte(
CP_UTF8, 0, wide.c_str(), static_cast<int>(wide.size()),
&buffer[0], static_cast<int>(buffer.size()), NULL, NULL);
if (len > 0)
{
assert(len == static_cast<int>(buffer.size()));
return std::string(&buffer[0], buffer.size());
}
}


throw boost::system::system_error(
::GetLastError(), boost::system::system_category);
}
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It is more efficient to convert CString to std::string using the conversion where the length is specified.

CString someStr("Hello how are you");
std::string std(someStr, someStr.GetLength());

In a tight loop, this makes a significant performance improvement.

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Works for me:

std::wstring CStringToWString(const CString& s)
{
std::string s2;
s2 = std::string((LPCTSTR)s);
return std::wstring(s2.begin(),s2.end());
}


CString WStringToCString(std::wstring s)
{
std::string s2;
s2 = std::string(s.begin(),s.end());
return s2.c_str();
}
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This is a follow up to Sal's answer, where he/she provided the solution:

CString someStr("Hello how are you");
std::string std(somStr, someStr.GetLength());

This is useful also when converting a non-typical C-String to a std::string

A use case for me was having a pre-allocated char array (like C-String), but it's not NUL terminated. (i.e. SHA digest). The above syntax allows me to specify the length of the SHA digest of the char array so that std::string doesn't have to look for the terminating NUL char, which may or may not be there.

Such as:

unsigned char hashResult[SHA_DIGEST_LENGTH];
auto value = std::string(reinterpret_cast<char*>hashResult, SHA_DIGEST_LENGTH);
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This works fine:

//Convert CString to std::string
inline std::string to_string(const CString& cst)
{
return CT2A(cst.GetString());
}
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All other answers didn't quite address what I was looking for which was to convert CString on the fly as opposed to store the result in a variable.

The solution is similar to above but we need one more step to instantiate a nameless object. I am illustrating with an example. Here is my function which needs std::string but I have CString.

void CStringsPlayDlg::writeLog(const std::string &text)
{
std::string filename = "c:\\test\\test.txt";


std::ofstream log_file(filename.c_str(), std::ios_base::out | std::ios_base::app);


log_file << text << std::endl;
}

How to call it when you have a CString?

std::string firstName = "First";
CString lastName = _T("Last");


writeLog( firstName + ", " + std::string( CT2A( lastName ) ) );

Note that the last line is not a direct typecast but we are creating a nameless std::string object and supply the CString via its constructor.

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One interesting approach is to cast CString to CStringA inside a string constructor. Unlike std::string s((LPCTSTR)cs); this will work even if _UNICODE is defined. However, if that is the case, this will perform conversion from Unicode to ANSI, so it is unsafe for higher Unicode values beyond the ASCII character set. Such conversion is subject to the _CSTRING_DISABLE_NARROW_WIDE_CONVERSION preprocessor definition. https://msdn.microsoft.com/en-us/library/5bzxfsea.aspx

        CString s1("SomeString");
string s2((CStringA)s1);
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From this post (Thank you Mark Ransom )

Convert CString to string (VC6)

I have tested this and it works fine.

std::string Utils::CString2String(const CString& cString)
{
std::string strStd;


for (int i = 0;  i < cString.GetLength();  ++i)
{
if (cString[i] <= 0x7f)
strStd.append(1, static_cast<char>(cString[i]));
else
strStd.append(1, '?');
}


return strStd;
}
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(Since VS2012 ...and at least until VS2017 v15.8.1)

Since it's a MFC project & CString is a MFC class, MS provides a Technical Note TN059: Using MFC MBCS/Unicode Conversion Macros and Generic Conversion Macros:

A2CW      (LPCSTR)  -> (LPCWSTR)
A2W       (LPCSTR)  -> (LPWSTR)
W2CA      (LPCWSTR) -> (LPCSTR)
W2A       (LPCWSTR) -> (LPSTR)

Use:

void Example() // ** UNICODE case **
{
USES_CONVERSION; // (1)


// CString to std::string / std::wstring
CString strMfc{ "Test" }; // strMfc = L"Test"
std::string strStd = W2A(strMfc); // ** Conversion Macro: strStd = "Test" **
std::wstring wstrStd = strMfc.GetString(); // wsrStd = L"Test"


// std::string to CString / std::wstring
strStd = "Test 2";
strMfc = strStd.c_str(); // strMfc = L"Test 2"
wstrStd = A2W(strStd.c_str()); // ** Conversion Macro: wstrStd = L"Test 2" **


// std::wstring to CString / std::string
wstrStd = L"Test 3";
strMfc = wstrStd.c_str(); // strMfc = L"Test 3"
strStd = W2A(wstrStd.c_str()); // ** Conversion Macro: strStd = "Test 3" **
}

--

Footnotes:

(1) In order to for the conversion-macros to have space to store the temporary length, it is necessary to declare a local variable called _convert that does this in each function that uses the conversion macros. This is done by invoking the USES_CONVERSION macro. In VS2017 MFC code (atlconv.h) it looks like this:

#ifndef _DEBUG
#define USES_CONVERSION int _convert; (_convert); UINT _acp = ATL::_AtlGetConversionACP() /*CP_THREAD_ACP*/; (_acp); LPCWSTR _lpw; (_lpw); LPCSTR _lpa; (_lpa)
#else
#define USES_CONVERSION int _convert = 0; (_convert); UINT _acp = ATL::_AtlGetConversionACP() /*CP_THREAD_ACP*/; (_acp); LPCWSTR _lpw = NULL; (_lpw); LPCSTR _lpa = NULL; (_lpa)
#endif
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Is there any problem?

There are several issues:

  • CString is a template specialization of CStringT. Depending on the BaseType describing the character type, there are two concrete specializations: CStringA (using char) and CStringW (using wchar_t).
  • While wchar_t on Windows is ubiquitously used to store UTF-16 encoded code units, using char is ambiguous. The latter commonly stores ANSI encoded characters, but can also store ASCII, UTF-8, or even binary data.
  • We don't know the character encoding (or even character type) of CString (which is controlled through the _UNICODE preprocessor symbol), making the question ambiguous. We also don't know the desired character encoding of std::string.
  • Converting between Unicode and ANSI is inherently lossy: ANSI encoding can only represent a subset of the Unicode character set.

To address these issues, I'm going to assume that wchar_t will store UTF-16 encoded code units, and char will hold UTF-8 octet sequences. That's the only reasonable choice you can make to ensure that source and destination strings retain the same information, without limiting the solution to a subset of the source or destination domains.

The following implementations convert between CStringA/CStringW and std::wstring/std::string mapping from UTF-8 to UTF-16 and vice versa:

#include <string>
#include <atlconv.h>


std::string to_utf8(CStringW const& src_utf16)
{
return { CW2A(src_utf16.GetString(), CP_UTF8).m_psz };
}


std::wstring to_utf16(CStringA const& src_utf8)
{
return { CA2W(src_utf8.GetString(), CP_UTF8).m_psz };
}

The remaining two functions construct C++ string objects from MFC strings, leaving the encoding unchanged. Note that while the previous functions cannot cope with embedded NUL characters, these functions are immune to that.

#include <string>
#include <atlconv.h>


std::string to_std_string(CStringA const& src)
{
return { src.GetString(), src.GetString() + src.GetLength() };
}


std::wstring to_std_wstring(CStringW const& src)
{
return { src.GetString(), src.GetString() + src.GetLength() };
}
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You can use CT2CA 

CString datasetPath;
CT2CA st(datasetPath);
string dataset(st);
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to convert CString to std::string. You can use this format.

std::string sText(CW2A(CSText.GetString(), CP_UTF8 ));
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CString has method, GetString(), that returns an LPCWSTR type if you are using Unicode, or LPCSTR otherwise.

In the Unicode case, you must pass it through a wstring:

CString cs("Hello");
wstring ws = wstring(cs.GetString());
string s = string(ws.begin(), ws.end());

Else you can simply convert the string directly:

CString cs("Hello");
string s = string(cs.GetString());

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