initialize std::array <struct, size>

Question

I am trying to defined and initialize an array of struct.

#include <iostream>
#include <array>

int main() {
    struct row{
        double a0;
        double a1;
    };

    //method 0: this way works
    row c[2] ={{1.0,2.0},{3.0,4.0}};

    //method 1: declare and initialization in same line 
    //std::array<row, 2> a = { {1.0, 2.0}, {3.0, 4.0} };//error: Excess elements in struct initializer
    std::array<row, 2> a = {{ {1.0, 2.0}, {3.0, 4.0} }}; //double brace


    //method 2, declare, then initialize in different line
    std::array<row, 2> b;
    //b = { {1.0, 2.0}, {3.0, 4.0} };//error: No viable overloaded '='
    b = { { {1.0, 2.0}, {3.0, 4.0} } }; //double brace

    return 0;
}

Now I find double brace works from this post.

Just wondering why do we need extra pair of brace to initialize array of struct?

Does this answer your question? When can outer braces be omitted in an initializer list? — jignatius
– jignatius, Commented Dec 8, 2020 at 6:56

Adrian Mole · Accepted Answer · 2020-12-08 05:30:18Z

The literals (without the doubled braces) you are trying to use to initialize/assign your std::array variables do not match the type of those arrays. You need to explicitly make each of the 'top-level' elements a row object, like this, for example:

int main()
{
    struct row {
        double a0;
        double a1;
    };
    
    std::array<row, 2> a = { row{1.0, 2.0}, row{3.0, 4.0} };

    std::array<row, 2> b;
    b = { row{1.0, 2.0}, row{3.0, 4.0} };

    return 0;
}

This is because, without the double braces, your RHS literals are expected to be objects of the std::array<row,2> class (unambiguously). However, with the double-braces, you are using aggregate initialization rather than (copy) assignment (as mentioned in the post you link).

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