Initializing std::tuple from initializer list

Question

Hi,

I'm wondering whether the tuple can be initialized by initializer list (to be more precise - by initializer_list of initializer_lists)? Considering the tuple definition:

typedef std::tuple< std::array<short, 3>,
                    std::array<float, 2>,
                    std::array<unsigned char, 4>,
                    std::array<unsigned char, 4> > vertex;

is there any way of doing the following:

static vertex const nullvertex = { {{0, 0, 0}},
                                   {{0.0, 0.0}},
                                   {{0, 0, 0, 0}},
                                   {{0, 0, 0, 0}} };

I just want to achieve same functionality I got using struct instead of tuple (thus only arrays are initialized by initializer_list):

static struct vertex {
    std::array<short, 3> m_vertex_coords;
    std::array<float, 2> m_texture_coords;
    std::array<unsigned char, 4> m_color_1;
    std::array<unsigned char, 4> m_color_2;
} const nullvertex = {
    {{0, 0, 0}},
    {{0.0, 0.0}},
    {{0, 0, 0, 0}},
    {{0, 0, 0, 0}}
};

There is no reason I must use tuples, just wondering. I'm asking, because I'm unable to go through g++ templates errors which are generated by my attempt of such tuple initialization.

@Motti: So I missed the proper syntax for uniform initialization -

static vertex const nullvertex = vertex{ {{0, 0, 0}},
                                         {{0.0, 0.0}},
                                         {{0, 0, 0, 0}},
                                         {{0, 0, 0, 0}} };

and

static vertex const nullvertex{ {{0, 0, 0}},
                                {{0.0, 0.0}},
                                {{0, 0, 0, 0}},
                                {{0, 0, 0, 0}} };

But it seems that all the trouble lies in arrays, which got no constructor for initializer_list and wrapping arrays with proper constructor seems not so easy task.

Answer 1

Initializer lists aren't relevant for tuples.

I think that you're confusing two different uses of curly braces in C++0x.

1. initializer_list<T> is a homogeneous collection (all members must be of the same type, so not relevant for std::tuple)
2. Uniform initialization is where curly brackets are used in order to construct all kinds of objects; arrays, PODs and classes with constructors. Which also has the benefit of solving the most vexing parse)

Here's a simplified version:

std::tuple<int, char> t = { 1, '1' }; 
// error: converting to 'std::tuple<int, char>' from initializer list would use
// explicit constructor 'std::tuple<_T1, _T2>::tuple(_U1&&, _U2&&) 
// [with _U1 = int, _U2 = char, _T1 = int, _T2 = char]'

std::tuple<int, char> t = std::tuple<int, char>{ 1, '1' }; 
// OK, but not an initializer list, uniform initialization

The error message says is that you're trying to implicitly call the constructor but it's an explicit constructor so you can't.

Basically what you're trying to do is something like this:

struct A { 
    explicit A(int) {}
};

A a0 = 3;
// Error: conversion from 'int' to non-scalar type 'A' requested

A a1 = {3}; 
// Error: converting to 'const A' from initializer list would use 
// explicit constructor 'A::A(int)'

A a2(3); // OK C++98 style
A a3{3}; // OK C++0x Uniform initialization