How C++ call parameterized ctor to create object arrays?

Question

I know in c++11 I can use initializer to create array/vector elements one by one like:

int ar[10]={1,2,3,4,5,6};
vector<int> vi = {1,2,3,4};

This is done manually, but what if I wish to initialize an array/vector with 10000 elements and specify their value, can it be done in one statement, RAII pattern?

E.g. I've defined a class:

class Somebody{
    int age;
    string name;
public:
    Somebody():age(20),name("dummy"){} // default ctor
    Somebody(int a, const char* n):age(a),name(n){} // but I wish to call this ctor
    Somebody& operator = (const Somebody& s) {...}
};

Then in main function:

int main(int argc, char const *argv[])
{
    Somebody obj(2, "John"); // parameterized ctor
    Somebody ar[300];        // initialized, but called default ctor
    for(int i=0;i<300;++i){  // initialize again in a loop
        ar[i] = ... ;        // call operator = 
...

Question: could Somebody ar[300] be constructed with parameterized ctor?

If not, then I have to create this object array ar first(Compiler generated code will loop and call default parameter), then apply value changes for each object. This seems quite a bit redundant of cpu cycles.

Is there a way to do what I expected with just one call? (Loop and update each array element with parameterized ctor)

Thanks.

Use std::vector's with the appropriate constructor. Raw arrays take from C and the only thing they do is default constricting objects. — 3CEZVQ
– 3CEZVQ, Commented Jun 20, 2022 at 1:30
Note: having 2 constructors doing the same thing is error prone and totally needless here. You can use Somebody() : Somebody(20, "dummy") { } to forward the constructor or Somebody(int a = 20, const char * n = "dummy") { } to provide default values for the arguments. That why when you later add int money; you can't accidentally add money in one of the constructors but not the other leading to UB. — Goswin von Brederlow
– Goswin von Brederlow, Commented Jun 20, 2022 at 3:58

Goswin von Brederlow · Accepted Answer · 2022-06-20 04:11:48Z

You can not do this with C-style arrays. One of the many reasons you should not be using them in the first place.

You should be using std::vector, which provides a constructor to populate the vector using a single value:

std::vector<Somebody> ar{300, Somebody("42", "filler")};

But often it is better to not create such dummy entries in the first place. A std::vector can grow dynamically so you can just add Somebody objects to it as you get their age and name.

std::vector people; people.reserve(2); // optional but makes adding lots of objects more efficient Somebody adam(42, "Adam"); people.push_back(adam); people.emplace_back(23, "Eve");

This shows you 2 ways to add objects to the vector, the push_back adds objects to the vector (copies/moves it) while emplace_back will construct the object directly in the vector, which is often more efficient.

If you want a fixed size array then you can use

std::array<Somebody, 300> ar;
ar.fill(Somebody(23, "filler"));

But this has the same problem as the C-style array of using the default construtor. std::arrays::fill replaces all the objects with a new value. Unfortunately there is no array constructor to do this in one go which is rather unfortunate.

Sam Varshavchik · Accepted Answer · 2022-06-20 01:49:52Z

Well, with simple arrays there's always the direct approach:

Somebody ar[300]={ {1, "Larry"}, {2, "Moe"}, {3, "Curly"} ... };

You'd have to write out whatever you want to write out 297 more times. This gets real old, real quickly.

One could potentially concoct, in a pinch, something that involves a variadic template to generate the whole bunch of them. Something like that is the classical approach to generating a fast 256-value array lookup for a textbook-style base64 decoder.

But for the simplistic case where you only want the same value, just #n copies of it, the simplest solution is to use a std::vector instead of a plain array and use one of its contructors that takes a single object and the vector size, and then constructs the vector by copying the object the requisite number of times. That involves slightly more overhead but is generally good enough, for this simple use case.

Nguyen Duc · Accepted Answer · 2022-06-20 03:32:39Z

1

You can use std::fill_n:

Somebody obj(2, "John");
Somebody ar[300];
fill_n(ar, 300, obj);

answered Jun 20, 2022 at 3:32

Nguyen Duc

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