How to avoid explicitly specifying size for static array in class

Question

I have two classes A and B in C++11. There's a static array in class A, which is used in class B. Here is the simplified problem:

// A.h
class A {
public:
    static int const a[4];
};

// A.cpp
int const A::a[4] = {1,2,3,4};

// B.h
class B {
public:
    void method();  
};

// B.cpp
void B::method() {
    for(auto const& it:a) {
        /* do something */
    }
}

It works, but it requires the explicit definition of the array size. I would like to change the content of the array initializer from time to time, at compile time, without the manual redefinition of its size.

What would be an equivalent, but more efficient way to do this?

@Someprogrammerdude there is a great difference between the two. — SergeyA
– SergeyA, Commented Aug 1, 2017 at 14:20
What do you mean? Change size of the array at compilation time or at runtime? — SergeyA
– SergeyA, Commented Aug 1, 2017 at 14:20
@underscore_d, since std::array::size is marked as constexpr, I don't see any issues with that. — Volodymyr Lashko
– Volodymyr Lashko, Commented Aug 1, 2017 at 14:30
@VolodymyrLashko - You don't follow, the OP doesn't want to be forced into modifying the size parameter of the type in the class definition whenever they update the initializer for the array. With std::array they are forced to do so. Only with a C-style array can the size be omitted. — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Aug 1, 2017 at 14:34

StoryTeller - Unslander Monica · Accepted Answer · 2017-08-01 15:27:47Z

10

If it's const you can't do it. If it's constexpr however, you can:

class A {
public:
    static constexpr int a[] = {1, 2, 3, 4};
};

Just move the initializer into the class definition. Then you can omit the array size completely.

A caveat to note, as pointed out by @SergeBallesta, is that you cannot skip on defining the storage for the array if you odr-use it. Which your example obviously does in the range for loop (it needs pointers to the beginning and past the end). So your cpp file still needs a const int A::a[]; declaration.

In C++17 you can at least make the definition header only thanks to inline variables being added to the language.

edited Aug 1, 2017 at 15:27

answered Aug 1, 2017 at 14:30

StoryTeller - Unslander Monica

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3 Comments

curious_amateur Over a year ago

It works only if the type of 'a' is a literal like 'int'. What if its type is 'std::string'?

StoryTeller - Unslander Monica Over a year ago

@blzs - It works for any type which has a constexpr constructor. std::string doesn't, but it doesn't mean you can't define types that are.

Serge Ballesta Over a year ago

Much better now :-)

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