How to fix serialization problems MQL4? - c++

Question

Today I get problems with serialization in MQL4.
I have a method, which I imported from a DLL:
In MQL4:
void insertQuery( int id,
string tableName,
double &values[4],
long &times[3],
int volume
);
In DLL:
__declspec(dllexport) void __stdcall insertQuery( int id,
wchar_t *tableName,
double *values,
long *times,
int volume
);
I tested it with this function calls in MQL4:
string a = "bla";
double arr[4] = { 1.1, 1.3, 0.2, 0.9 };
long A[3] = { 19991208, 19991308, 19992208 };
int volume = 1;
insertQuery( idDB, a, arr, A, volume );
Inside of this method I collect this values to files.
C++ :
stringstream stream;
stream << " '";
for (int i = 0; i < 2; ++i) {
stream << times[i] << "' , '";
}
stream << times[2] << ", ";
for (int i = 0; i < 4; ++i) {
stream << values[i] << ", ";
}
stream << volume;
wstring table(tableName);
query.append("INSERT INTO ");
query.append(table.begin(), table.end());
query.append(" VALUES (");
query.append(stream.str());
query.append(" )");
std::ofstream out("C:\\Users\\alex\\Desktop\\text.txt");
out << query;
out.close();
But in output file I receive this record:
INSERT INTO bla VALUES ( '19991208' , '0' , '19991308, 1.1, 1.3, 0.2, 0.9, 1 )
So my question is : why I lose one long value in array when I receive my record in DLL?
I tried a lot of ways to solve this problem ( I transfered two and three long values, etc ) and always I get a result that I lose second long value at serialization. Why?

Answer 1

The problem is cause because in MQL4, a long is an 8 bytes, while a long in C++ is a 4 bytes.
What you want is a long long in your C++ constructor.
Or you could also pass them as strings, then convert them into the appropriate type within your C++ code.

Answer 2

Well, be carefull, New-MQL4.56789 is not a c-compatible language
The first thing to test is to avoid passing MQL4 string into DLL calling interface, where really a c-lang string is expected.
Since old-MQL4 has been silently re-defined into a still-WIP-creeping syntax New-MQL4,the MQL4 string is not a string, but a struct.
Root-cause [ isolation ]:
Having swallowed the shock about string/struct trouble, if you can, first try to test the MQL4/DLL interactions without passing any string to proof, that all other parameters, passed by value and addressed by-ref, do get their way to the hands of a DLL-function as you expect.
If this works as you wish, proceed to the next step:
How to pass the very data to expected string representation, then?
Let me share a dirty hack I used for passing data where DLL expects string-s
#import "mql4TOOL.dll"
...
int mql4TOOL_msg_init_data ( int &msg[],
uchar &data[],
int size
);
...
#import
...
int tool_msg_init_data ( int &msg[], string data, int size ) { uchar dataChar[]; StringToCharArray( data, dataChar );
return ( mql4TOOL_msg_init_data ( msg, dataChar, size ) );
}
Yes, dirty, but works for years and saved us many tens-of-man*years of re-engineering upon a maintained code-base with heavy dependence on the MQL4/DLL interfacing in massively distributed heterogeneous computing systems.
The last resort:
If all efforts went in vain, go low level, passing a uchar[] as needed, where you assemble some serialised representation in MQL4 and parse that on the opposite end, before processing the intended functionality.
Ugly?
Yes, might look like that,butkeeps you focused on core-functionality and isolates you from any next shift of paradigm if not only strings cease to be strings et al.