C++ no match for 'operator=' in my custom object array

Question

I have a class called KernelLock, and I am creating an array of KernelLocks called myLockArray. I declare it like this: KernelLock myLockArray[150];

When I try to add a new KernelLock to myLockArray, I get the aforementioned error. Here is the exact line I get the error on:

myLockArray[initializedLocksCounter] = new KernelLock(myAddrSpace, newLock);

and here is the exact error:

error: no match for 'operator=' in 'myLockArray[initializedLocksCounter] = (((KernelLock*)operator new(8u)), (<anonymous>->KernelLock::KernelLock(myAddrSpace, newLock), <anonymous>))

In case it helps, I am compiling with gcc through Nachos.

_"When I try to add a new KernelLock to myLockArray"_ Wait, what? You have an array of 150 KernelLocks. How can you add another one? Sounds like you're confusing Java and C++..... this is not an array of pointers...... — Lightness Races in Orbit, Oct 16 '15 at 18:17
Or you might just store the pointer. try myLockArray[initializedLocksCounter] = *(new KernelLock(myAddrSpace, newLock)) — 2785528, Oct 16 '15 at 18:18

NathanOliver · Answer 1 · 2015-10-16T18:19:09.347

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KernelLock myLockArray[150];

Creates an array of 150 KernelLocks. You then then try to assign a KernelLock * to it. If you want pointers then you need to change your array to:

KernelLock* myLockArray[150];

If you don't want pointers then you just need to change your assignment to

myLockArray[initializedLocksCounter] = KernelLock(myAddrSpace, newLock);

edited Oct 16 '15 at 18:19

answered Oct 16 '15 at 18:17

NathanOliver

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eerorika · Accepted Answer · 2015-10-16T18:24:46.537

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KernelLock myLockArray[150];

This is an array that contains 150 kernel locks.

When I try to add a new KernelLock to myLockArray

You cannot add any objects to an array. The size of an array is always constant. It always has 150 locks.

myLockArray[initializedLocksCounter] = new KernelLock(myAddrSpace, newLock);

This is wrong. new returns the address of a dynamically allocated object. You're trying to assign that address to the existing KernelLock object at the index initializedLocksCounter. You cannot assign an address to a non-pointer object (unless the object has corresponding (non-explicit) constructor).

If you want a growing array, use std::vector<KernelLock>

edited Oct 16 '15 at 18:24

answered Oct 16 '15 at 18:19

eerorika

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How would I do this with vectors? – Jonathan Allen Grant Oct 16 '15 at 19:28
@JonathanAllenGrant I recommend checking out one of the suggested books [here](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) for the basics. – eerorika Oct 16 '15 at 20:31

score 2 · Answer 3 · edited May 23 '17 at 12:29

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You are fundamentally misunderstanding what an object is in C++. It's not like in Java.

Your 150 KernelLock objects are 150 KernelLock objects. Not pointers, and not references. And there are 150 of them. You cannot dynamically "expand" the array whenever you please, and doing so with new is dynamic allocation which requires managing pointers (KernelLock*).

From the sounds of it, you actually want a std::vector<KernelLock>.

Here are some resources for learning the basics of C++.

edited May 23 '17 at 12:29

Community

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answered Oct 16 '15 at 18:19

Lightness Races in Orbit

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should I use vector or vector<*KernelLock>? – Jonathan Allen Grant Oct 16 '15 at 19:25
@JonathanAllenGrant: `vector<*KernelLock>` is not even valid syntax. You should use what I wrote in my answer. Which C++ book are you using? Guessing isn't a good way to program :( – Lightness Races in Orbit Oct 16 '15 at 23:54

Ilio Catallo · Answer 4 · 2015-10-16T18:27:32.133

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you cannot use a value of type KernelLock* as element of an array of KernelLock's. You simply do:

myLockArray[i] = KernelLock(myAddrSpace, newLock);

where i is a valid index.

However, notice that by doing so you will pay the price of default-initializing 150 KernelLock's no matter what (and this might be expensive). It would be better to use the standard library:

std::vector<KernelLock> kernel_locks;
kernel_locks.reserve(some_size); // better to reserve if you have some sensible upper bound
kernel_locks.emplace_back(myAddrSpace, newLock);

edited Oct 16 '15 at 18:27

answered Oct 16 '15 at 18:17

Ilio Catallo

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You are proposing writing an array overflow. – Lightness Races in Orbit Oct 16 '15 at 18:20
Because `myLockArray[initializedLocksCounter]` does not exist? – Lightness Races in Orbit Oct 16 '15 at 18:22
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As per the question, I assumed `KernelLock myLockArray[150];` to precede the assignment. If we moreover assume that `intiializedLocksCounter` is a valid index, the only problem I see is the fact that we are default-initializing 150 `KernelLock`. – Ilio Catallo Oct 16 '15 at 18:24
I read it as `initializedLocksCounter==150`, but I don't now :) – Lightness Races in Orbit Oct 16 '15 at 18:25
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I edited the question so as to use a generic, yet valid, `i` index :) – Ilio Catallo Oct 16 '15 at 18:26

C++ no match for 'operator=' in my custom object array

4 Answers4