Initialize object containing C-style array as member variable (C++)

Since Color is an aggregate you can use aggregate initialization and put the array initializer directly in the braces like

Color c = {10, {2, 6, 9}};

If you have to initialize c with an array, since it is small, you can just unroll it like

Color c = {10, {myRGB[0], myRGB[1], myRGB[2]}};

Supposing that there is a need to use an intermediate array, here is how one can do it:

#include <utility>#include <cstddef>struct Color  //this struct can't be modified{    double grey;    double rgb[3];};template<size_t N, size_t... IX>auto make_c_impl(double grey, double (&rgb)[N], std::index_sequence<IX...>) {    static_assert(sizeof(rgb) == sizeof(Color::rgb), "Arrays sizes must match!");    return Color{grey, {rgb[IX]...}};}template<size_t N>auto make_c(double grey, double (&rgb)[N]) {    return make_c_impl(grey, rgb, std::make_index_sequence<N>{});}double myRGB[3] = {2, 6, 9};Color c = make_c(10, myRGB); //this line now works

Note, that this code will not actually produce any unnecessary copying with any level of optimization.

As complement to others answers, the error is because in c++ arrays are not copyable and trying to initialize an array from an lvalue has the semantic of invoking the copy-constructor, the same as:

double r1[3] = {0., 0., 0.};double r2[3] {r1} // doesn't compile

Your options are:

• do list-initialization as @NathanOliver did
• or expand the elements of the array to form a list-initialization as in the @SergeyA answer.