How to pre-initialize an object array in c++ like an actual array?

Question

DISCLAIMER Good day, I am a novice programmer so please don't judge me if I have any mistakes that may be obvious or not.

I have a code that looks like this.

class Book{
public:
    string author;
    string title;
    int yearPub;
}

int main(){
    Book books[999];
}

However, I don't know how to predefine a certain amount of titles and authors like how they are defined in other variable types like this:

string author[99] = {"asdf","asf","asdff"};
int x[] = {9,10,19};

I tried initializing a constructor and did this:

class Book{
public:
...
    Book(string AUTH, string TTL, int YROP){
        author = AUTH;
        title = TTL;
        yearPub = YROP
    }
...
}
int main(){

Book books[999] = Book({"author1","author2"},{"title1","title2},{2022,2022});
return 0;
}

But it doesn't work. Do you know any way to do this?

what is the meaning of "doesnt work"? Please include the compiler error in the question — 463035818_is_not_an_ai, May 03 '22 at 14:34
no instance of constructor "Book::Book" matches the argument list — nyawng, May 03 '22 at 14:36
@nyawng The other 997 books need a constructor as well. `Book() = default;` — WhozCraig, May 03 '22 at 14:39
I think you might need to study your text-books a little more about arrays, object and initialization. Or invest in [some good C++ books](https://stackoverflow.com/a/388282/440558). — Some programmer dude, May 03 '22 at 14:39

HolyBlackCat · Answer 1 · 2022-05-03T14:46:29.477

1

Assuming you don't have a custom constructor, you can do this:

Book books[999] = {{"author1","author2"},{"title1","title2"}};

If you do want a custom constructor, you'll also need to add the second constructor: a default one (thanks @WhozCraig) that will be used for array elements that don't have their own initializers: Book() {}.

Some alternative syntax variants:

Book books[999] = {Book{"author1","author2"},Book{"title1","title2"}};

Book books[999] = {Book("author1","author2"),Book("title1","title2")};

The latter required the two-argument constructor until C++20.

Also consider using std::vector to avoid wasting storage on non-existent books`.

edited May 03 '22 at 14:46

answered May 03 '22 at 14:36

HolyBlackCat

78,603
9
131
207

When i use the first one, this error message shows: "no instance of constructor "Book::Book" matches the argument list". Same applies to the second one. – nyawng May 03 '22 at 14:46
@nyawng Remove your custom constructor, or add a default one. – HolyBlackCat May 03 '22 at 14:47
Removed my custom constructor, followed the code you showed, and it worked. Thanks! Appreciate it. – nyawng May 03 '22 at 15:06

Vlad from Moscow · Answer 2 · 2022-05-03T15:04:22.143

In the both cases you can write

Book books[999] = { { "author1", "title1" }, { "author2", "title2"} };

Provided that in the second case (when the construct with two parameters is defined) there is also declared the default constructor as for example

class Book {
public:
    Book( std::string author, std::string title )
        : author( std::move( author ) ), title( std::move( title ) )
    {
    }
    Book() = default;        
    
    std::string author;
    std::string title;
};

In the first case when your class is an aggregate you can also write

Book books[999] = { "author1", "title1", "author2", "title2" };

or even

Book books[999] = { "author1", "title1", { "author2", "title2" } };

or

Book books[999] = { { "author1", "title1" }, "author2", "title2" };

If the compiler supports C++ 20 you can also write

Book books[999] =  { { .author = "author1", .title = "title1" }, { .author = "author2", .title = "title2" }};

Removed my custom constructor like what was told above me (same as what you commented), followed the code they showed, and it worked. Thanks! Appreciate it. — nyawng, May 03 '22 at 15:06

How to pre-initialize an object array in c++ like an actual array?

2 Answers2