How to initialize an array of structs with another stuct?

Question

Hello I am writing an algorithm for back propagation in c++, it consists of 1 input layer 1 hidden layer and 1 output layer. I am curious how to initialize my inputlayer, hiddenlayer, and outputlayer.

This is what i got:

struct Neuron {
double activation;
double bias;
double *incomingWeights;
};

struct Network {
double *targetLayer;
Neuron *inputLayer;
Neuron *hiddenLayer;
Neuron *outputLayer;
};

ann.inputLayer = malloc(din * sizeof * Neuron); //ERROR
for (int i = 0; i < din; i++) {
    ann.inputLayer[i].activation = 0;
    ann.inputLayer[i].bias = 0;
    ann.inputLayer[i].incomingWeights = NULL;
}

ann.outputLayer = malloc(dout * sizeof * Neuron); //ERROR
for (int i = 0; i < dout; i++) {
    ann.outputLayer[i].activation = 0;
    ann.outputLayer[i].bias = 0;
    ann.outputLayer[i].incomingWeights = new double[dhid];
}

din represents the number of neurons in my input layer. dhid represents the number of neurons in my hidden layer. dout repreesnts the number of neurons in my output layer. i didnt bother showing the initialization of my hidden layer cause its not important thank you

i wrote //ERROR on the lines where errors are shown according to my IDE(netbeans)

What do you think is wrong with the code you have written? It does have some issues but it helps if you explain yourself rather than we guess. — john, Apr 14 '13 at 19:05
If you use `new` instead of `malloc`, the constructor method would be called after memory allocation. The constructor is where you should initialized the objects. — Thomas Matthews, Apr 14 '13 at 19:05
main.cpp:138:50: error: expected primary-expression before ‘)’ token that is the error that i am getting it refers to the way i am using malloc, which makes me think i am not using malloc correctly — jamalsabs, Apr 14 '13 at 19:06
Another method is to use `std::vector` and copy the items during the construction of the `vector`. — Thomas Matthews, Apr 14 '13 at 19:06
The code posted is legal C. It shouldn't compile with a C++ compiler, however. Which leads to the question: should it be C or C++. If the intent is to write code in C, then the label should be C (and if the above code doesn't compile, it's probably because you're using a C++ compiler). If the intent is to write code in C++, then there should be no pointers and no dynamic allocation in the code, at least not for the parts we've been shown. — James Kanze, Apr 14 '13 at 19:13
In general, we need the error message as well as the location; in this case, the errors are trivial. — Alex Chamberlain, Apr 14 '13 at 19:15
Very similar to [this question](http://stackoverflow.com/q/15961835/1084416). — Peter Wood, Apr 14 '13 at 19:43

score 3 · Answer 1 · answered Apr 14 '13 at 19:10

First, you never want to do default initializations in calling code, you can create a constructor for your struct and do it there:

struct Neuron {
   Neuron() : activation(0.0), bias(0.0), incomingWeights(0) {}
   double activation;
   double bias;
   double *incomingWeights;
};

Then (since this is C++)

for (int i = 0; i < din; i++) {
   ann.inputLayer[i] = new Neuron[din];

You should use new over malloc in C++, though if you ever did need malloc your statement would need to be fixed, to:

ann.inputLayer = (Neuron*)malloc(din * sizeof(Neuron));

@jamalsabs My pleasure, esp. since it's neuroscience. :) – Matt Phillips Apr 14 '13 at 19:20 — Matt Phillips, Apr 14 '13 at 19:20

score 1 · Answer 2 · edited May 23 '17 at 12:18

1

Don't use malloc in C++.

As a minimal change to what you have, change this:

ann.inputLayer = malloc(din * sizeof * Neuron);

To this:

ann.inputLayer = new Neuron[din];

new here is more clear for readers of your code, performs construction when needed, and returns the correct pointer type.

(Same with the dout line)

Edit:

You would also likely find std::vector much easier to manage over pointers.

edited May 23 '17 at 12:18

Community

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answered Apr 14 '13 at 19:08

Drew Dormann

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No. Don't use `new[]` either. `inputLayer` should be an `std::vector`. (In fact, I don't think there should be any pointers in this code.) – James Kanze Apr 14 '13 at 19:10
@JamesKanze I completely agree with your reasoning, although your suggestion would require changes in unposted code. That would be a good lesson for another day. :) – Drew Dormann Apr 14 '13 at 19:13

score 0 · Answer 3 · answered Apr 14 '13 at 19:17

Just for the record: the reason you're getting errors is because you are compiling C code with a C++ compiler. malloc returns a void*. In C, this converts implicitly to any other pointer type; in C++, you need an explicit conversion.

Except that, as others are saying as well, you should never use malloc in C++ (unless for reasons of compatibility with C—i.e. you are passing the pointer to a function written in C which will call free on it). In fact, in the little you've shown, there should be no pointers and no dynamic allocation whatsoever: define your types, add constructors, and when you need more than one, use std::vector.

How to initialize an array of structs with another stuct?

3 Answers3