How to initialize a Class-instance and print it's public variable?

Question

I've got the following code:

/*  1 */    class Market {
/*  2 */    public:
/*  3 */    
/*  4 */        double foo;
/*  5 */    
/*  6 */        /* 
/*  7 */         * Constructor:
/*  8 */         */
/*  9 */        Market() {
/* 10 */            foo = 2;
/* 11 */        }
/* 12 */    };
/* 13 */    
/* 14 */    Market market;              // GLOBALLY VISIBLE VAR
/* 15 */    
/* 16 */    void OnInit() {             // OnInit event-process
/* 17 */      market = new Market();    // <------------------- 17,10
/* 18 */      Print(market.foo);
/* 19 */    }
/* 20 */    
/* 21 */    void OtherFunc() {
/* 22 */      Print(market.foo);
/* 23 */    }

However when I'm trying to compile, I've got the following error:

test.mq4(17,10) : error 280: '=' - object required

Basically I'm planing to create an object instance and receive a pointer to it, so I can use it globally across the code.

I've checked the MQL4 docs page about classes, but it didn't help me.

I've also tried to assign the class as:

Market *market = new Market();

and then I declared that variable in the first line as:

Market market;

but then I've got this warning:

test.mq4(17,11) : warning 62: declaration of 'market' hides global declaration at line 14

and the file compiles, but I don't want to hide the global declaration (since it won't be available in the other functions), I want to assign an object instance into the global declaration, so I can print the object's variable across the whole code.

How do I do that?

Answer 1

So, let's start the "inventory of don't-s" in Advanced-MQL4

First advice, never rely on web-sources, always check the auto-updated IDE-Help. Even this does not contain a 100% correct text / code-samples, but it is by far closer to the actual compiler-grammar, than the obsoleting web texts.

Next advice, always remember MQL4 is not a C language - strange differences may kill otherwise smart idea.

---

Syntax FIRST:

Object Create Operator new

The new operator automatically creates an object of a corresponding size, calls the object constructor and returns a descriptor of created object. In case of failure, the operator returns a null descriptor that can be compared with the NULL constant.

The new operator can be applied only to class objects. It can't be applied to structures.

---

Object Pointers

In MQL4, there is a possibility to dynamically create objects of complex type. This is done by the new operator, which returns a descriptor of the created object. Descriptor is 8 bytes large. Syntactically, object descriptors in MQL4 are similar to pointers in C++.

Examples:

MyObject* hobject= new MyObject();

In contrast to C++, the hobject variable from example above is not a pointer to memory, but rather an object descriptor. Furthermore, in MQL5 all objects in function parameters must be passed by reference.

---

Retaining a value visible on global scope NEXT:

Using the above, hobject ought be visible on a global-scope, supposing it's declaration was in due place, before any system-event-handler / user defined code started ( and sure, on condition there is no name-masking taking place inside any lower-level scope-range of the code ).

Epilogue:

After an exposure to a bit more than a few hundreds man*years of MQL4 hands-on experience, all advanced solutions started to get designed as a distributed framework, rather than have to remain as victims of language-irregularities and syntax-creeps. One may consider this as a bit safer alternative than to invest some remarkable amount of software engineering into ecosystem, that changes faster, than an economy / performance of a repetitive code-base re-engineering might manage to keep the pace of the moving sands.

Anyway : Welcome to the Wild Worlds of MQL4