CODEWITHSUNDEEP
c++vectorstruct
A Friedrich
A Friedrich

Reputation: 603

Vector allocation when declaring struct

You can create a C++ structure with an array within:

struct foo{
    int bar[42];
};

What I would like to do is: 

struct foo{
    std::vector<int> bar(42);
};

Of course this doesn't compile, but you get the intent. I'm aware of .reserve() and the like, but I would like to have the space already allocated when declaring a foo.

The reason is that I'm supposed to alter a rather complicated Perl script which is generating C code with arrays within structs. These arrays should be replaced by std::vectors. The script subsequently initializes the arrays depending on an XML file and I would rather not mess around with push_back in the script since the structures are deeply nested (structs of arrays and arrays of structs). The sizes of the arrays do of course vary.

Thanks for your suggestions.

Upvotes: 1

Views: 4816

Answers (5)

Igor Bulgakov
Igor Bulgakov

Reputation: 183

Use constructor, as suggested in other answers. But I must fix you:

  1. Space will be "already allocated" after definition (in case of using constructor);
  2. You should never use .reserve(), only .resize() (before you understand difference);
  3. You can call .resize() after declaration safely and almost without overhead (instead of using constructor). But anyway, learn and use constructor.

Upvotes: 0

anon
anon

Reputation:

The existing answers have explained how to do what you want to do, but I think they've missed the important part of explaining why. The thing that you need to grasp is that struct in C++ is simply a near-synonym for class with the only difference being that a struct defaults to having public members and a class defaults to having private. You could replace every instance of class in your code with

struct X
{
    private:
    ...
}

and it would work exactly the same and vice versa for replacing structs with class. This means that just as you would use a constructor to do what you want with a class, you use a constructor with your struct:

struct foo
{
    std::vector<int> bar;
    foo() : bar(42) {}
}

This constructor for std::vector, btw, also take a couple more arguments with the second being the value to fill your vector with so you could do foo() : bar(42, -1) {} to fill your vector with values of -1.

Upvotes: 3

hmjd
hmjd

Reputation: 122011

Add a constructor and use the initializer list:

struct foo {
    foo() : bar(42) {}
    std::vector<int> bar;
};

If your compiler supports C++11 features use std::array<> instead.

struct foo {
    std::array<int, 42> bar;
};

Or boost::array<> if C++11 is not available and boost is permitted.

Upvotes: 3

Luchian Grigore
Luchian Grigore

Reputation: 258678

Use a constructor:

struct foo{
    std::vector<int> bar;
    foo() : bar(32) {}
};

Memory for the vector will automatically be allocated when creating an instance. The initialization list initializes members before the constructor body executes.

Upvotes: 6

Ivaylo Strandjev
Ivaylo Strandjev

Reputation: 71009

You should set the size of the vector(i.e. call std::vector's constructor) in the constructor of the structure(preferably in its initialization list).

Like so:

struct foo{
    foo() : bar(42) {}
    std::vector<int> bar;
};

Upvotes: 0

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