In C++ language, how do you create a new object which is a pointer to an array?

Question

I'm using a 3rd party library in which a method is declared this way:

void nexLoop(NexTouch *nex_listen_list[]);

In case of a single .cpp file, this is working:

NexButton b0 = NexButton(0, 1, "b0");
NexTouch *nex_listen_list[] = 
{
    &b0,
    NULL
};
...
nexLoop(nex_listen_list);

Full example code can be seen here: https://github.com/itead/ITEADLIB_Arduino_Nextion/blob/master/examples/CompText/CompText.ino

Now I'm incorporating this library in a class which has a separate header and c file, so that:

MyNextion.h

#include "Nextion.h"

class MyNextion {
   public:
       MyNextion();
       void loop();
   private: 
       NexButton *b0;
       NexTouch *nex_listen_list[];
}

MyNextion.cpp

#include "MyNextion.h"

MyNextion::MyNextion() {
    b0 = new NexButton(0, 1, "b0");
    nex_listen_list = new ???
}

MyNextion::loop() {
    nexLoop(nex_listen_list); // ???
}

I was messing around with pointers and address operators but I couldn't find the right combination to a successful compilation.

My question is: what would you put here: nex_listen_list = new ???;

Also, I'm not sure the title well defines my problem. Please correct it if you have a better idea for the title.

Thank you!

What language is this? Doesn't look like C. Tries to look like C++, but it's failing. — Ivaylo Valchev
– Ivaylo Valchev, Commented Nov 29, 2016 at 20:35
Okay you might be right. Actually this is Arduino code which is C/C++. — papaiatis
– papaiatis, Commented Nov 29, 2016 at 20:36
"A new object which is a pointer to an array"? Doesn't make any sense to me. — dandan78
– dandan78, Commented Nov 29, 2016 at 20:37
Please, feel free to edit the question to make more sense. I'm not a native english speaker. — papaiatis
– papaiatis, Commented Nov 29, 2016 at 20:38

rodrigo · Accepted Answer · 2016-11-29 21:26:11Z

3

You should not declare a member array without a size. Instead you may declare a pointer-to-pointer:

class MyNextion {
   ...
   private: 
       NexButton *b0;
       NexTouch **nex_listen_list;
};

And then the construction is easy:

nex_listen_list = new NexTouch*[10];
nex_listen_list[0] = b0;
....

That is, nex_listen_list is a dynamic array of pointers to NexTouch objects. And do not forget the delete[] when you are finished:

delete[] nex_listen_list;

That said, if the size of the array is constant, a normal array is just fine:

       NexButton *b0;
       NexTouch *nex_listen_list[3];

If not, I'd rather use a std::vector. Something like:

class MyNextion {
   ...
   private: 
       NexButton *b0;
       std::vector<NexTouch*> nex_listen_list;
};

And then no new or delete required:

nex_listen_list.push_back(b0);
nexLoop(nex_listen_list.data());

And going on, you could declare the b0 as a full object, not a pointer:

class MyNextion {
   ...
   private: 
       NexButton b0;
       std::vector<NexTouch*> nex_listen_list;
};
...
nex_listen_list.push_back(&b0); //use the pointer!

No new so no delete and no leaks.

edited Nov 29, 2016 at 21:26

answered Nov 29, 2016 at 20:39

rodrigo

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5 Comments

papaiatis Over a year ago

This is great help already! I managed to get rid of a lot of compiler errors, however I'm facing with others. Please see my updated question. Thank you for your help!

DevSolar Over a year ago

I'd suggest showcasing the use of smart pointers instead of "naked" new / delete, C++11 and all...

rodrigo Over a year ago

@papaiatis: You should really close this question and create a new one. Anyway, you cannot convert a pointer-to-member function into a pointer-to-function, they may look the same but are very different!

rodrigo Over a year ago

@DevSolar: Well, in this case, probably you don't even need the dynamic object at all. But I don't know the used library at all or even if the OP has C++11 available. It is an Arduino after all...

papaiatis Over a year ago

@rodrigo Sure, I'm opening a separate question for that. Thank you again!

Kuba hasn't forgotten Monica · Accepted Answer · 2016-11-29 21:13:46Z

If nex_listen_list has static size, you should keep it by value:

// Interface
class MyNextion {
public:
   MyNextion() {}
   void loop();
private:
   NexButton b0{0, 1, "b0"};
   NexTouch *nex_listen_list[2] = {&b0, nullptr};
};

// Implementation
void MyNextion::loop() {
   nexLoop(nex_listen_list);
}

If the size is variable, you have two choices. If you don't mind dynamic memory allocation, use std::vector:

// Interface
class MyNextion {
public:
   MyNextion() {}
   void loop();
   void foo();
private:
   NexButton b0{0, 1, "b0"};
   NexButton b1{0, 2, "b1"};
   std::vector<NexTouch *> nex_listen_list{&b0, nullptr};
   void append(NexTouch *t);
};

// Implementation
void MyNextion::append(NexTouch *t) {
   nex_listen_list.back() = t;
   nex_listen_list.push_back(nullptr);
}

void MyNextion::foo() {
   append(&b1);
}

void MyNextion::loop() {
   nexLoop(nex_listen_list.data());
}

Alternatively, you can pre-allocate a fixed-size list and avoid dynamic memory allocation:

lass MyNextion {
public:
   MyNextion() {}
   void loop();
   void foo();
private:
   void append(NexTouch *);
   NexButton b0{0, 1, "b0"};
   NexButton b1{0, 2, "b1"};
   std::array<NexTouch *, 10> nex_listen_list{{&b0, nullptr}};
   std::array<NexTouch *, 10>::iterator it_el =
         std::find(nex_listen_list.begin(), nex_listen_list.end(), nullptr);
};

// Implementation
void MyNextion::append(NexTouch *t) {
   *it_el++ = t;
   *it_el++ = nullptr;
}

void MyNextion::foo() {
   append(&b1);
}

void MyNextion::loop() {
   nexLoop(nex_listen_list.data());
}

You're given a rather powerful C++ compiler. You should leverage it to write code for you. In particular, you should let the compiler generate the code for the constructor.

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