Thread: C++ console application works; Excel crashes when using the same code.

Hi all,

Greetings! this is my first post to the forum.

My question is the following. I have a c++ code which, after being compiled as a console application (I'm using DEV-C++ 4.9.9.2), the code does what I expect it to do.

However, if I compile the same code as a DLL and after that I call it from within Excel (using Excel 2007), then it will crash.

I have no problem in using DLLs from within Excel; I've done that before and the work as a charm. The problem arises with this code and I cannot figure out what is wrong.

I want to bring up this: if compiled as a console application it works; if compiled as a DLL and called from Excel, Excel crashes.

Here's the code

Code:

#include <windows.h>
#include <string>

using namespace std;

extern "C" __declspec( dllexport ) __stdcall string str_separator(string in_string, string sep_char, int id);

string str_separator(string in_string, string sep_char, int id)
{
     int str_len=0,pos_sep_char=0,q=0,k=0;
     // temporal strings declared as 'string' types.
     string temp_str ("test_str");
     string out_str ("out_str");
     string temp_char ("0");
     
     temp_str = in_string;
     str_len=temp_str.length();
     
     for(k=0;k<=str_len-1;k++)
     {
           // here we get the kth char from the string to compare it after
           temp_char = temp_str.substr(k,1);
           
           if (temp_char==sep_char) // is the gotten char the one I'm expecting?
           {
                // I got sep_char within the string.
                q=q+1;
                
                if (id==q)
                {k=str_len;}
                else 
                {
                     temp_str = temp_str.substr(k+1,temp_str.length()-k);
                     str_len=temp_str.length();
                     k=-1;
                }
           }
           else if (temp_char != sep_char) // gotten char is not the one
           {
                if (k==0)
                {out_str = temp_char;}
                else
                {out_str += temp_char;}
           }
     }
     
     return out_str;
}

Any hint is highly appreciated.

Best regards,

Lucas

Originally Posted by Elysia

I don't believe Excel uses std::string, but rather some nuisance BSTR or something. You have to change the parameters you accept and return.

Hi Elysa, thanks for replying.

I'm no c++ expert, so I don't think I've completely understood what you've said.

Are you saying that Excel does not like the "string" type for input parameters? If so, would I have to use the "char" type for working with strings?

If that's true, oh well, I'll have to do a lot more work. I really liked the string library in order to work in a really easy way with strings ... :-(

Isn't there any other way of using "strings" as input parameters with Excel?

Cheers!

Lucas

Hi All,

Thanks everyone for the replies.

I've been reading some information on the web but I'm somehow confused (actually: a lot).

As far as I've read, the following occurs:

VBA, when calling the DLL, will sendout as input parameters string variables of 'string' type which has the following characteristics (please, correct me if I'm wrong in any of the following statements):

VBA string:

1) is UNICODE (meaning 2 bytes per char)
2) length of the string is located at the start of the variable
3) it ends with zero character

C++ string:

1) is ANSI (meaning 1 byte per char)
2) has no length information (length comes with the information)
3) ends with NULL char

On the other hand, the VBA string type is compatible with the C++ BSTR type. I understand by this that the BSTR type in C++ has the same structure as the VBA type.

So, my question now, if I'm expecting parameters coming from VBA, then the function must return a BSTR. Also, input parameters should be BSTR. So, it makes sense to declare the function as follows:

Code:

extern "C" __declspec( dllexport ) __stdcall BSTR str_separator(BSTR in_string, BSTR sep_char, int id);

The problem now is that, within the function, I must (and I prefer to) deal with c++ strings (I'm using #include <string>).

How do I convert then a BSTR variable (which comes in as an input parameter) into a c++ string (so I can work with strings within the function) and finally convert it back into BSTR so VBA will interpret it correctly?

I'll keep on reading but any hint is highly appreciated...

Many thanks,

Lucas

Hi!

So, some new and fresh information (at least for me).

Just for information, I've found this link very useful:

Passing Strings to a DLL Procedure

Now, going to the matter. I've managed myself to pass the DLL from VBA a string, and echo it (meaning, I got the original string back). I've done that by using both string types in C++: BSTR and LPSTR. Why is that working, not a clue, since, as per the link above, there is a little difference between them. However the two codes that follow have worked.

Code:

BSTR string_ex(BSTR stringVar)
{
	BSTR buffer;
	buffer = stringVar;
	return buffer;
}

and ...

Code:

LPSTR string_ex(LPSTR stringVar)
{
	LPSTR buffer;
	buffer = stringVar;
	return buffer;
}

In both cases, I'm getting the input string back in Excel. My problem now is the following: I want to use the library <string> in order to easily manipulate strings withing C++.

How do I convert (BSTR or LPSTR) into a string-type object (in order to use the string class).

Any hint regarding this last question?

Many thanks,

Lucas

When calling your function, make the function accept a const char* if you will not modify the string, or char* otherwise. In VBA, declare that argument with ByVal (not ByRef).
Use std::string's constructor to make that C-style string into a C++-style string. For example:

Yes, I've seen this. If declare my C++ function with input parameter of type LPSTR then I can send BvVal a string from VBA and that will work (which is what I want).

Then, since I have a char* (because of LPSTR) I can assign it to a string object. In fact, look at my function now...

VBA:

Code:

Declare Function string_ex Lib "d:/datos/doc/tech/c++/excel/ipfnc/ipfnc.dll" _
    (ByVal stringVar As String) As String

C++:

Code:

LPSTR string_ex(LPSTR stringVar)
{
    string buffer(stringVar);
    int str_len;
    str_len=buffer.size();
	
    char *c_buffer = new char[str_len+1];
    strcpy(c_buffer,buffer.c_str());
    return c_buffer;
}

Now, the previous code is compiling, but excel crashes any way. My guess is that Excel does not like the string (or whatever) I'm sending back to it...

I tried to follow your advise, by building up a buffer of the size of the string and strcpy'ing the string into it, but I don't think that had worked ... :-(

On the other hand, I do not like your following statement ...

Finally, if you want to pass back a string, you have to do it the old fashioned way.
In VBA, declare a fixed-length string (that is, Dim MyVar as String * Size).
Make your function accept a char* and size (ie char* str, size_t size).

Why do I have to declare a fixed-length string in VBA? My function may return a variable length string ... So, this is the problem: which type of data must my function return? I think this is the question to answer.

I'll keep on looking for that solution. It must be possible somehow ... I know I lack of good c/c++ knowledge but I'll try to get this to work ... :-)

Many thanks anyway,

Lucas

Hi Codeplug,

Thanks for the reply and for your suggestion.

I've tried before what you suggest but I cannot compile your code. This is the compiler's output:

Code:

Compiler: Default compiler
Building Makefile: "d:\datos\doc\tech\c++\excel\ipfnc\Makefile.win"
Executing  make clean
rm -f ipfnc.o  ipfnc.dll
g++.exe -D__DEBUG__ -c ipfnc.cpp -o ipfnc.o  -DBUILDING_DLL=1   -g3
dllwrap.exe --output-def libipfnc.def --driver-name c++ --implib libipfnc.a ipfnc.o  --no-export-all-symbols --add-stdcall-alias  -g3  -o ipfnc.dll
ipfnc.o: In function `string_ex@4':
d:\datos\doc\tech\c++\excel\ipfnc/ipfnc.cpp:45: undefined reference to `SysAllocStringByteLen@8'
collect2: ld returned 1 exit status

c:\GNU\MinGW\bin\dllwrap.exe: no export definition file provided.
Creating one, but that may not be what you want
c:\GNU\MinGW\bin\dllwrap.exe: c++ exited with status 1

make.exe: *** [ipfnc.dll] Error 1
Execution terminated

Remeber I'm using Dev-C++ 4.9.9.2.

I've read that, in order to use the 'SysAllocStringByteLen' function I need to include the header 'oleauto.h' but I do not have it ... :-( ... may be if using VC++, but I'd rather go with Dev-C++

Rgds,

Lucas

@Codeplug:

Thanks! let me have a look a those ... any preference?

Cheers!

Lucas