How to Initialize Member Arrays in C 0x Constructors: Why Does Using `std::initializer_list` Fail and How to Resolve It?

Initializer Lists and Member Arrays in C 0x

In the early stages of learning C 0x, it's common to encounter syntax errors when experimenting with new features. Specifically, this issue arises when attempting to initialize a member array within a constructor using an initializer list.

Consider the following code:

<code class="cpp">struct Foo
{
    int const data[2];

    Foo(std::initializer_list<int const>& ini)
    : data(ini)
    {}
};

Foo f = {1,3};</code>

Upon compilation, this code triggers the following error:

incompatible types in assignment of ‘std::initializer_list<const int>&’ to ‘const int [2]’

To resolve this error, the approach recommended in the provided answer involves employing a variadic template constructor instead of an initializer list constructor. Using this method ensures type compatibility and allows for flexible initialization of member arrays:

<code class="cpp">struct foo {
    int x[2];
    template <typename... T>
    foo(T... ts) : x{ts...} {} // curly braces syntax for initializer list
};

int main() {
    foo f1(1, 2);   // compiles successfully
    foo f2{1, 2};   // also compiles
    foo f3(42);    // x[1] is zero-initialized
}</code>

Alternatively, if maintaining constness is not crucial, you can opt for a method that involves populating the array within the constructor body:

<code class="cpp">struct foo {
    int x[2]; 
    foo(std::initializer_list<int> il) {
       std::copy(il.begin(), il.end(), x);
    }
};</code>

While this approach may be viable, it sacrifices the compile-time bounds checking provided by the variadic template constructor.

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