Passing a String/String Array reference to C++ or how to write a result handle for mysql

[Christian Stern]

Hey all,

since a few days i am fighting with MT to pass a string reference and a string array reference to c++ in order to handle mysql results. I started here a discussion but i thought to start fresh again.

My goal is to have like the same as Russel wrote here: https://www.mql5.com/en/code/8623

I don't know what i do wrong, i guess i understand C++ and MT, but even the examples won't work on my x64 windows maschine. 

Here is what is working:

- I can pass a string ( wchar_t*, char* ) to a C++ Function in a DLL

- I can return a string ( wchar_t*, char* ) from a C++ Function in a DLL

- I can overwrite the passed pointer in C++ and display the result in MT5

But i can't pass a array or a string reference. Since i know that i have to use the "C" prefix for exporting a C++ function, MT5 can find the function, but it's return value is in a unknown character set. Within C++ it works, no error during compiling, and the checks are okay. After a month of mysql driver writing and fighting with MT5 strings i want to start writing my trading agent.

I like MT very much and i am a programmer, but i getting p***** when i stuc at a simple problem. Normaly i would fight and solve the problem by my own, but i want to trade, and not doing troubleshooting :D If you like to write me the module and solve me the problem, i will pay for it: https://www.mql5.com/en/job/71666

C++:

#ifdef MYSQLWRAPPER_EXPORTS
#define MYSQLWRAPPER_API __declspec(dllexport)
#else
#define MYSQLWRAPPER_API __declspec(dllimport)
#endif

#ifdef _UNICODE
typedef wchar_t TCHAR;
#else
typedef char TCHAR;
#endif // _UNICODE

namespace std {
#ifdef _UNICODE
    typedef wstring tstring;
#else
    typedef string tstring;
#endif // _UNICODE
};

struct MqlStr {
    int               len;
    char             *string;
};

extern "C" MYSQLWRAPPER_API wchar_t*  __stdcall    returnString    ();
extern "C" MYSQLWRAPPER_API void __stdcall        changeString1    (wchar_t* &stringTest);
extern "C" MYSQLWRAPPER_API void __stdcall        changeString2    (char* &stringTest);
extern "C" MYSQLWRAPPER_API void __stdcall        changeString3    (LPWSTR &stringTest);
extern "C" MYSQLWRAPPER_API void __stdcall        changeString4    (LPWSTR stringTest);
extern "C" MYSQLWRAPPER_API void __stdcall        changeString5    (LPCSTR &stringTest);
extern "C" MYSQLWRAPPER_API void __stdcall        changeString6    (LPCSTR stringTest);
extern "C" MYSQLWRAPPER_API void __stdcall        fnReplaceString    (wchar_t* text, wchar_t *from, wchar_t *to);
extern "C" MYSQLWRAPPER_API void __stdcall        changeArrayString1(MqlStr *row);


wchar_t* __stdcall returnString() {
    return(L"this is a wchar_t string");
}

void  __stdcall changeString1(wchar_t* &stringTest) {
    stringTest = L"Hello you ....";
}

void  __stdcall changeString2(char* &stringTest) {
    stringTest = "Hello you ....";
}

void  __stdcall changeString3(LPWSTR &stringTest) {
    stringTest = L"Hello you ....";
}

void  __stdcall changeString4(LPWSTR stringTest) {
    stringTest = L"Hello you ....";
}

void  __stdcall changeString5(LPCSTR &stringTest) {
    stringTest = TEXT("Hello you ....");
}

void  __stdcall changeString6(LPCSTR stringTest) {
    stringTest = TEXT("Hello you ....");
}

void __stdcall fnReplaceString(wchar_t* text, wchar_t *from, wchar_t *to) {
    wchar_t *cp;
    //--- parameters check
    if (text == NULL || from == NULL || to == NULL) return;
    if (wcslen(from) != wcslen(to))             return;
    //--- search for substring
    if ((cp = wcsstr(text, from)) == NULL)         return;

    //--- replace it
    memcpy(cp, to, wcslen(to) * sizeof(wchar_t));
    text = cp;
}

void __stdcall changeArrayString1( MqlStr *row) {

    int pos = 1;

    for (int i = 0; i < pos; i++) {
        row[i].string = "Hello you ....";;
        row[i].len = static_cast<int>(strlen(row[i].string));
    }
}

MT5:

#import "Quantum\MySQL\mysql_wrapper.dll"
   string   returnString      ();
   void     changeString1      (string &value);
   void     changeString2     (string &value);
   void     changeString3     (string &value);
   void     changeString4     (string value);
   void     changeString5     (string &value);
   void     changeString6     (string value);
   void     fnReplaceString   (string &text,string from,string to);
   void     changeArrayString1(string& row[]);
#import


   string return_value1 = returnString();
   string return_value2 = "default...";
   string return_value3 = "default...";
   string return_value4 = "default...";
   string return_value5 = "default...";
   string return_value6 = "default...";
  
   Print("1.  Return value is: " + return_value1 );
  
   changeString1(return_value1);
   changeString2(return_value2);
   changeString3(return_value3);
   changeString4(return_value4);
   changeString5(return_value5);
   changeString6(return_value6);
  
   Print("1.1 Return value is: " + return_value1 );
   Print("2.  Return value is: " + return_value2 );
   Print("3.  Return value is: " + return_value3 );
   Print("4.  Return value is: " + return_value4 );
   Print("5.  Return value is: " + return_value5 );
   Print("6.  Return value is: " + return_value6 );
  
   //--- modify the string
   string text="A quick brown fox jumps over the lazy dog";
  
   fnReplaceString(text,"fox","cat");
   Print("Replace: ",text);
  
  // string row[];
  // ArrayResize(row, 2);
  // changeArrayString1(row);

Terminal:

2017.11.20 08:18:32.548    Test (EURUSD,M1)    1.  Return value is: this is a wchar_t string
2017.11.20 08:18:32.548    Test (EURUSD,M1)    1.1 Return value is: ㎘߾
2017.11.20 08:18:32.548    Test (EURUSD,M1)    2.  Return value is: ㎸߾
2017.11.20 08:18:32.548    Test (EURUSD,M1)    3.  Return value is: ㎘߾
2017.11.20 08:18:32.548    Test (EURUSD,M1)    4.  Return value is: default...
2017.11.20 08:18:32.548    Test (EURUSD,M1)    5.  Return value is: ㎸߾
2017.11.20 08:18:32.548    Test (EURUSD,M1)    6.  Return value is: default...
2017.11.20 08:18:32.548    Test (EURUSD,M1)    Replace: A quick brown cat jumps over the lazy dog

Passing a array, like as in the mysql_wrapper cause a panic:

2017.11.20 07:56:38.438    Test (EURUSD,M1)    Access violation at 0x000007FEF6CA10EA write to 0x0000000000000008 in 'C:\Users\tesla\AppData\Roaming\MetaQuotes\Terminal\D0E8209F77C8CF37AD8BF550E51FF075\MQL5\Libraries\Test\MySQL\mysql_wrapper.dll'
2017.11.20 07:56:38.438    Test (EURUSD,M1)       crash -->  000007FEF6CA10EA 48895008          mov        [rax+0x8], rdx
2017.11.20 07:56:38.438    Test (EURUSD,M1)                  000007FEF6CA10EE 4883C8FF          or         rax, 0xff
2017.11.20 07:56:38.438    Test (EURUSD,M1)                  000007FEF6CA10F2 488B11            mov        rdx, [rcx]
2017.11.20 07:56:38.438    Test (EURUSD,M1)                  000007FEF6CA10F5 488B4A08          mov        rcx, [rdx+0x8]
2017.11.20 07:56:38.438    Test (EURUSD,M1)                  000007FEF6CA10F9 0F1F8000000000    nop        [rax+0x0]
2017.11.20 07:56:38.438    Test (EURUSD,M1)                  000007FEF6CA1100 48FFC0            inc        rax
2017.11.20 07:56:38.438    Test (EURUSD,M1)                  000007FEF6CA1103 803C0100          cmp        byte [rcx+rax], 0x0
2017.11.20 07:56:38.438    Test (EURUSD,M1)    
2017.11.20 07:56:38.438    Test (EURUSD,M1)    00: 0x000007FEF6CA10EA
2017.11.20 07:56:38.438    Test (EURUSD,M1)    01: 0x00000000042D212B
2017.11.20 07:56:38.438    Test (EURUSD,M1)    02: 0x000000000C405200
2017.11.20 07:56:38.438    Test (EURUSD,M1)    03: 0x000000000881113E
2017.11.20 07:56:38.438    Test (EURUSD,M1)    04: 0x0000000000000002

[Ex Ovo Omnia]

Passing of string arrays is blocked by MQ.

[William Roeder]

Christian Stern:

- I can pass a string ( wchar_t*, char* ) to a C++ Function in a DLL

- I can return a string ( wchar_t*, char* ) from a C++ Function in a DLL

- I can overwrite the passed pointer in C++ and display the result in MT5

But i can't pass a array or a string reference.

1. When you post code please use the SRC button! Please edit your post.
             General rules and best pratices of the Forum. - General - MQL5 programming forum

2. MT4/5 doesn't have pointers, (only handles to classes,) so you can't pass a string reference.
   
3. Of course you can pass an array (as a reference.) Remember the array will not be asSeries.
   
4. You can't change the pointer (array reference.) Change the passed array, don't exceed it's allocated size.
5. Since C++ can't do anything but manipulate variables, stop making things more complicated. Just write the code in MT4/5, KISS.