MetaEditor/MQL4 ExpertAdviser: Local Variable Declaration More Than Once?

Question

I am looking at some old MetaEditor4 / MQL4 code, where a local variable was declared twice:

......
1 int start()                                     
2   {
3     if (1==2)
4       {
5          double myVar = 1;
6       } else
7       {
8          double myVar = 2;
9       }
10    return;                                      
11  }
.......

The compilation process in MetaEditor, version 5.00, build 1601, fails with:

'myVar' - variable already defined in line 8.

If I remove the line 8, the compilation goes well.

My questions are:

1. Is there any option in MetaEditor that tolerates the multiple declaration of a local variable?

2. In previous versions of MetaTrader Terminal 4 / MetaEditor and .MQ4 code: was it possible to declare a local variable more than once in such a situation?

3. The MetaEditor has the version 5.00, build 1601, but the extension of the code is .mq4 and it was installed together with the MetaTrader Terminal software MetaTrader4 ( from FXCM ). Therefore I assume I can still use .MQ4 code with it. Is there any chance to get a pure MQL4 installation from somewhere? Whenever I install mt4 ( from e.g.: mt4 download), it ends up with the mt5 installer.

user3666197 user3666197 · Accepted Answer · 2018-02-21T14:36:34

Prologue:

The worlds of MQL4 evolve. One may try to circumvent this fact, but finally, at one's own disappointment, attempts to avoid evolution will sooner or later go in vain.

Having been thrown into a need to re-engineer code-base spanning a few man*decades in size, I can tell you many stories about what worked and what did not.

An "Old code" v/s a New-`MQL4.56789`

If just one thing ought be taken from this, never try to "circumvent" New-MQL4, but rather review the code and refactor the "Old code" - this is a way safer way to survive ( way longer ).

Yes, there are chances ( zero warrants, just a few chances left temporarily on the table ) the new compiler version will remain able to generate an executable version of the code, but given a new set of rules have already come in the city, the game will not last long.

Ad 1 + 2 )
Compiler still tolerates multiple declarations, but not in one scope

If new version of a compiler defined that any variable is declared only relative to it's scope of validity, the serious programmer ought take this as a general principle. The code above actually has other problem, right nailed to the scope-of-validity:

2 ...
3 if (  1 == 2 ) {
4       ...
5       double myVar = 1; // myVar declared & known  |since HERE >
6       ...              //        masking any other,|known HERE :
7       ...             //                           |known HERE : 
8 } else               //                            |till  HERE . Undef further
9 {
10      ...
11      double myVar = 2; // myVar declared & known  |since HERE >
12                       //        masking any other,|known HERE :
13       ...            //                           |known HERE :
14 }                   //                            |till  HERE . Undef further

so, if there were any _global_ scope'd variable with the same name myVar, it will not be "visible" during an existence of locally declared variable, wearing the same name.

Finally, having the code-execution escaped from any of the lines 8 or 14 further, the locally there declared variable double myVar simply ceased to exist anymore and this behaviour is principally correct ( and the "older" compiler releases were tolerating a sort of dangerous habit of side-effects, during years of tolerating scope-of-validity spillover(s), so it was the high time to clean the rules, so as to meet a fair level of C/S standards.

Ad 3) language receives a lot from MQL5, even if not used in MQL4

Yes, MetaEditor will correctly compile a MQL4 code into .mq4 code-execution format, no problem here. Even an auto-update process started to go independently from MT4 Terminal platform (auto-)updates ( so you will quite often see new Help file coming and enforced re-compilation of all your localhost visible .MQ4 assets into updated .EX4 format, so "Do not panic."

Better never install a Broker-agnostic MT4, always go to your Broker's Support and get installation package & help from your Broker. This is business relation you have signed in a contract, so keep these strings as you are going to trade your money on a table they operate under the set Terms & Conditions. Some Brokers have means of platform customisations, so rather benefit from their custom settings that will match their Server-side automation.

It is more a question of economy of R&D efforts. ( May read a lot about language components injected from the MQL5 domain in the IDE Editor MQL4 Help ). This is a natural will of the product design strategy, not to double efforts on a dual-line. Without doubts, there are many details the Help file could be improved and better maintained, the common sense here is to live with the facts and re-learn what newly introduced features remain neutral for the MQL4 code base and what new things may actually help one a lot in aspects, where older compilers were short in powers.

If one objects that some compiler / platform re-design steps were bad, I would agree on a single-thread, platform-critical, potentially blocking, concentration of executing all the CustomIndicator-s in just one SPoF-thread.

But C'est La Vie, until system architects will not review this SPoF, the platform will remain susceptible to crashes from this feature, but the ball is on the other side of the court and a change will have to be implemented there.