How to init a constant std::array? [duplicate]

Question

#include <array>
struct A
{
    A(char value, char another);
    const std::array<char, 42> something;  // how to init?
}

Considered approaches:

a const_cast is always dangerous and often UB
the docs list the only ctor as taking an aggregate parameter - what I want to do is accept some other parameter and initialize the constant
removing the constness is ... undesirable. The whole semantics are "create this; it will never need to change"
using std::string worries me as inefficient and this is an embedded device
std::experimental::make_array looks promising but not standard yet
an init list in the ctor seems promising but how?

Can it be done and how?

Not sure what is wrong with the usual way, using an initialiser list. A(char value) : a{{value}} {} — john
– john, Commented Apr 5, 2023 at 5:29
The object isn't const during construction, or the constructor wouldn't be able to initialize the object — Richard
– Richard, Commented Apr 5, 2023 at 5:29
@john this solves it. Sorry for bothering everyone: my brain is non-existent. — Vorac
– Vorac, Commented Apr 5, 2023 at 5:33

Remy Lebeau · Accepted Answer · 2023-04-05 05:45:07Z

1

Use the member initialization list of A's constructor to initialize the array, eg:

A::A(char value) : arr{value} {}

UPDATE: you edited your question after I posted this answer.

In your original code, the constructor took a single char, and the array held 1 char, so it was possible to fully initialize the array with that char.

Now, in your new code, the constructor takes 2 chars, but the array holds 42 chars, so the only way to fully initialize the array with any values other than zeros requires using a helper function that returns a filled array, eg:

std::array<char, 42> makeArr(char value) {
    std::array<char, 42> temp;
    temp.fill(value);
    return temp;
}

A::A(char value, char another) : arr{makeArr(value)} {}

edited Apr 5, 2023 at 5:45

answered Apr 5, 2023 at 5:30

Remy Lebeau

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2 Comments

Richard Over a year ago

Here is the dupe covering this: stackoverflow.com/a/14495926/12002570

Pepijn Kramer Over a year ago

makeArr can be constexpr (without change in C++20, replace fill with for loop pre C++20)

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