问题
I'm using GetOpenFileName
function from Winapi
, and I'm applying filter to the select file dialog.
THIS works perfectly:
LPSTR mfilter = "Filter\0*.PDF\0";
ofn.lpstrFilter = mfilter;
if(GetOpenFileName(&ofn)){
...
THIS fails (dialog opens but no filters apply):
string mfilter = "Filter\0*.PDF\0";
ofn.lpstrFilter = mfilter.c_str();
if(GetOpenFileName(&ofn)){
...
I need to use std:string
because I'm getting the file extension via parameters and this type facilitates the concatenation but I'm getting incompatibility issues...
This would be my code if it worked as expected (IT FAILS the same as previous example):
const char * ext = &(4:); //Ampersand parameter (from CA Plex) It contains "PDF"
string mfilter = "Filter\0*." + ext + "\0"; //Final string: Filter\0*.PDF\0;
ofn.lpstrFilter = mfilter.c_str();
When I use this method, I'm getting runtime exception:
string mf;
mf.append("Filter")
.append('\0')
.append("*.pdf")
.append('\0');
ofn.lpstrFilter = mf.c_str();
回答1:
The GetOpenFileName
function uses TCHARs, and TCHARs become WCHARs in case of UNICODE character set is used.
Here's an example:
std::wstring getOpenFileName(HWND hWnd, const std::wstring& sFilter)
{
wchar_t buffer[MAX_PATH] = L"";
OPENFILENAMEW ofn = {0};
ofn.lStructSize = sizeof(ofn);
ofn.hwndOwner = hWnd;
ofn.lpstrFilter = sFilter.c_str();
ofn.nFilterIndex = 1;
ofn.lpstrFile = buffer;
ofn.nMaxFile = MAX_PATH;
ofn.Flags = OFN_HIDEREADONLY | OFN_FILEMUSTEXIST;
if( !::GetOpenFileNameW( &ofn ) )
return L"";
return buffer;
}
If you want to parametrize lpstrFilter
based on std::wstring
you can just use wstring::c_str()
to get LPCTSTR
which is const wchar*
in case of UNICODE.
IMPORTANT: The problem is that the std::wstring
constructor that takes a const wchar*
assumes the input is a C string. C strings are '\0' terminated and thus parsing stops when it reaches the '\0' character. To compensate for this you need to use the constructor that takes two parameters a pointer to the char array and a length.
You can also use string::push_back()
method to append NULLs.
std::wstring sFilter = L"PDF Files";
sFilter.push_back('\0');
sFilter.append(L"*.pdf");
sFilter.push_back('\0');
回答2:
With
string mfilter = "Filter\0*.PDF\0";
you are calling an std::string
contructor, which terminates the string at the first \0
.
The following code:
string mfilter = "Filter\0*.PDF\0";
cout << "string:" << mfilter << " len: " << mfilter.length() << endl;
prints
string: Filter len: 6
The string is only constructed until the first \0
terminator. Do the string is only composed of the word "Filter".
回答3:
string mfilter = "Filter\0*.PDF\0";
This calls a std::basic_string
constructor that uses a null-terminated string. It will stop parsing the string literal at "Filter"
.
Try this one instead:
string mfilter( "Filter\0*.PDF", 13 ); // need double null at end
This calls a std::basic_string
constructor that uses "the first count characters of character string pointed to by s. s can contain null characters."
You have to either count the characters yourself, or write wrapper code if you encounter this problem more often.
Related: std::basic_string constructors.
As for your runtime error:
string mf;
mf.append("Filter")
.append('\0')
.append("*.pdf")
.append('\0');
append()
does not have an overload for a single character type. You are probably hitting the const CharT* s
overload, with a null pointer.
Use either append( 1, '\0' )
or append( "", 1 )
, either of which should append a null byte.
来源:https://stackoverflow.com/questions/34201213/c-lpstr-and-string-trouble-with-zero-terminated-strings