MQL4 C++ Dll change string argument in function call

Question

Here is my code for a MetaTraderWrapper.dll :

#define MT4_EXPFUNC __declspec(dllexport)

MT4_EXPFUNC void __stdcall PopMessageString(wchar_t *message)
{
    auto result = L"Hello world !";
    int  n      = wcslen( result );
    wcscpy_s( message, n + 1, result );
}

On the MQL4-Caller side this Script is used:

#property strict
#import "MetaTraderWrapper.dll"
     int PopMessageString( string & );
#import
//
void OnStart(){
     string message;
     if (  StringInit( message, 64 ) ){
           PopMessageString(  message );
           int n = StringLen( message );
           MessageBox(        message );
     }
}

In this way it works, when a message have been properly initialized with a StringInit() function and enough memory was allocated.

What I need to do is, to allocate the message variable not in MQL4 script, but within the DLL.

In a c++ function, should be something like this:

MT4_EXPFUNC void __stdcall PopMessageString(wchar_t *message)
{
    auto result = L"Hello world !";
    int  n      = wcslen( result );

    // allocate here, but does not work
    message = new wchar_t[n + 1];        // <--------- DOES NOT WORK
    //
    wcscpy_s( message, n + 1, result );
}

What can I do ?

Answer 1

Get acquainted with Wild Worlds of MQL4:
Step 1: forget a string to be string _{( it is a struct ... since 2014 )}

Internal representation of the string type is a structure of 12 bytes long:

 #pragma pack(push,1) struct MqlString { int size; // 32-bit integer, contains size of the buffer, allocated for the string. LPWSTR buffer; // 32-bit address of the buffer, containing the string. int reserved; // 32-bit integer, reserved. }; #pragma pack(pop,1) 

So,
having headbanged into this one sunny Sunday afternoon, as the platform undergone a LiveUpdate and suddenly all DLL-call-interfaces using a string stopped work, it took a long way to absorb the costs of such "swift" engineering surprise.

You can re-use the found solution:

use an array of bytes - uchar[] and convert appropriately bytes of returned content on MQL4 side into string by service functions StringToCharArray() resp. CharArrayToString()

The DLL- .mqh -header file may also add these tricks and make these conversions "hidden" from MQL4-code:

#import <aDLL-file>                                       // "RAW"-DLL-call-interfaces
...
// Messages:
           int      DLL_message_init(       int &msg[] );
           int      DLL_message_init_size ( int &msg[], int    size );
           int      DLL_message_init_data ( int &msg[], uchar &data[], int size );
           ...
#import

// ------------------------------------------------------ // "SOFT"-wrappers
...
int                MQL4_message_init_data ( int &msg[], string data, int size ) {  uchar dataChar[]; StringToCharArray( data, dataChar );
   return (         DLL_message_init_data (      msg,          dataChar, size ) );
}

Always be pretty carefull with appropriate deallocations, not to cause memory leaks.

Always be pretty cutious when new LiveUpdate changes the code-base and introduces new compiler + new documentation. Re-read whole documentation, as many life-saving details come into the help-file only after some next update and many details are hidden or reflected indirectly in chapters, that do not promise such information on a first look -- so, become as ready as D'Artagnan or red-scarfed pioneer -- you never know, where the next hit comes from :)