c++ Constructor overload ambiguity with initializer_list

Question

I have a class with two constructors:

myclass(std::initializer_list<int>);
myclass(std::initializer_list<std::initializer_list<int>);

The following declarations work

myclass obj1 = {{1,2},{3,4},{5,6}}; //a 3x2 matrix
myclass obj2 = {{1,2,3}}; // a 1x3 matrix

But the following declaration does not work, when I try to compile, it says that it is ambiguous

myclass obj3 = {{1},{2},{3}};  //a 3x1 matrix

The following solves the problem:

myclass obj3 = {std::initializer_list<int>({1}), 
                std::initializer_list<int>({2}), 
                std::initializer_list<int>({3})};

but I find this solution unconfortable and ugly. Is it possible to do something better?

Answer 1

The reason the compiler says {{1},{2},{3}} is ambiguous is that {1} can both be an int or an std::initializer_list<int> . So you either have a list of int s or a list of a list of int s and the compiler cannot decide what you actually want

When you do {{{1},{2},{3}}} or {{{1}},{{2}},{{3}}} the innermost braces represents an int , the next innermost represents a list of those int s and then the outermost braces represents a list of that list. You needs these extra braces to remove the ambiguity for the compiler.