How to emulate C array initialization "int arr[] = { e1, e2, e3, ... }" behaviour with std::array?

Best I can think of is:

template<class T, class... Tail>
auto make_array(T head, Tail... tail) -> std::array<T, 1 + sizeof...(Tail)>
{
     std::array<T, 1 + sizeof...(Tail)> a = { head, tail ... };
     return a;
}

auto a = make_array(1, 2, 3);

However, this requires the compiler to do NRVO, and then also skip the copy of returned value (which is also legal but not required). In practice, I would expect any C++ compiler to be able to optimize that such that it's as fast as direct initialization.

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I'd expect a simple make_array.

template<typename ret, typename... T> std::array<ret, sizeof...(T)> make_array(T&&... refs) {
    // return std::array<ret, sizeof...(T)>{ { std::forward<T>(refs)... } };
    return { std::forward<T>(refs)... };
}

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Combining a few ideas from previous posts, here's a solution that works even for nested constructions (tested in GCC4.6):

template <typename T, typename ...Args>
std::array<T, sizeof...(Args) + 1> make_array(T && t, Args &&... args)
{
  static_assert(all_same<T, Args...>::value, "make_array() requires all arguments to be of the same type."); // edited in
  return std::array<T, sizeof...(Args) + 1>{ std::forward<T>(t), std::forward<Args>(args)...};
}

Strangely, can cannot make the return value an rvalue reference, that would not work for nested constructions. Anyway, here's a test:

auto q = make_array(make_array(make_array(std::string("Cat1"), std::string("Dog1")), make_array(std::string("Mouse1"), std::string("Rat1"))),
                    make_array(make_array(std::string("Cat2"), std::string("Dog2")), make_array(std::string("Mouse2"), std::string("Rat2"))),
                    make_array(make_array(std::string("Cat3"), std::string("Dog3")), make_array(std::string("Mouse3"), std::string("Rat3"))),
                    make_array(make_array(std::string("Cat4"), std::string("Dog4")), make_array(std::string("Mouse4"), std::string("Rat4")))
                    );

std::cout << q << std::endl;
// produces: [[[Cat1, Dog1], [Mouse1, Rat1]], [[Cat2, Dog2], [Mouse2, Rat2]], [[Cat3, Dog3], [Mouse3, Rat3]], [[Cat4, Dog4], [Mouse4, Rat4]]]

(For the last output I'm using my pretty-printer.)

Actually, let us improve the type safety of this construction. We definitely need all types to be the same. One way is to add a static assertion, which I've edited in above. The other way is to only enable make_array when the types are the same, like so:

template <typename T, typename ...Args>
typename std::enable_if<all_same<T, Args...>::value, std::array<T, sizeof...(Args) + 1>>::type
make_array(T && t, Args &&... args)
{
  return std::array<T, sizeof...(Args) + 1> { std::forward<T>(t), std::forward<Args>(args)...};
}

Either way, you will need the variadic all_same<Args...> type trait. Here it is, generalizing from std::is_same<S, T> (note that decaying is important to allow mixing of T, T&, T const & etc.):

template <typename ...Args> struct all_same { static const bool value = false; };
template <typename S, typename T, typename ...Args> struct all_same<S, T, Args...>
{
  static const bool value = std::is_same<typename std::decay<S>::type, typename std::decay<T>::type>::value && all_same<T, Args...>::value;
};
template <typename S, typename T> struct all_same<S, T>
{
  static const bool value = std::is_same<typename std::decay<S>::type, typename std::decay<T>::type>::value;
};
template <typename T> struct all_same<T> { static const bool value = true; };

Note that make_array() returns by copy-of-temporary, which the compiler (with sufficient optimisation flags!) is allowed to treat as an rvalue or otherwise optimize away, and std::array is an aggregate type, so the compiler is free to pick the best possible construction method.

Finally, note that you cannot avoid copy/move construction when make_array sets up the initializer. So std::array<Foo,2> x{Foo(1), Foo(2)}; has no copy/move, but auto x = make_array(Foo(1), Foo(2)); has two copy/moves as the arguments are forwarded to make_array. I don't think you can improve on that, because you can't pass a variadic initializer list lexically to the helper and deduce type and size -- if the preprocessor had a sizeof... function for variadic arguments, perhaps that could be done, but not within the core language.

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Using trailing return syntax make_array can be further simplified

#include <array>
#include <type_traits>
#include <utility>

template <typename... T>
auto make_array(T&&... t)
  -> std::array<std::common_type_t<T...>, sizeof...(t)>
{
  return {std::forward<T>(t)...};
}

int main()
{
  auto arr = make_array(1, 2, 3, 4, 5);
  return 0;
}

Unfortunatelly for aggregate classes it requires explicit type specification

/*
struct Foo
{
  int a, b;
}; */

auto arr = make_array(Foo{1, 2}, Foo{3, 4}, Foo{5, 6});

In fact this make_array implementation is listed in sizeof... operator

c++17 version

Thanks to template argument deduction for class templates proposal we can use deduction guides to get rid of make_array helper

#include <array>

namespace std
{
template <typename... T> array(T... t)
  -> array<std::common_type_t<T...>, sizeof...(t)>;
}

int main()
{
  std::array a{1, 2, 3, 4};
  return 0; 
}

_{Compiled with -std=c++1z flag under x86-64 gcc 7.0}

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I know it's been quite some time since this question was asked, but I feel the existing answers still have some shortcomings, so I'd like to propose my slightly modified version. Following are the points that I think some existing answers are missing.

1. No need to rely on RVO

Some answers mention that we need to rely on RVO to return the constructed array. That is not true; we can make use of copy-list-initialization to guarantee there will never be temporaries created. So instead of:

return std::array<Type, …>{values};

we should do:

return {{values}};

2. Make make_array a constexpr function

This allow us to create compile-time constant arrays.

3. No need to check that all arguments are of the same type

First off, if they are not, the compiler will issue a warning or error anyway because list-initialization doesn't allow narrowing. Secondly, even if we really decide to do our own static_assert thing (perhaps to provide better error message), we should still probably compare the arguments' decayed types rather than raw types. For example,

volatile int a = 0;
const int& b = 1;
int&& c = 2;

auto arr = make_array<int>(a, b, c);  // Will this work?

If we are simply static_asserting that a, b, and c have the same type, then this check will fail, but that probably isn't what we'd expect. Instead, we should compare their std::decay_t<T> types (which are all ints)).

4. Deduce the array value type by decaying the forwarded arguments

This is similar to point 3. Using the same code snippet, but don't specify the value type explicitly this time:

volatile int a = 0;
const int& b = 1;
int&& c = 2;

auto arr = make_array(a, b, c);  // Will this work?

We probably want to make an array<int, 3>, but the implementations in the existing answers probably all fail to do that. What we can do is, instead of returning a std::array<T, …>, return a std::array<std::decay_t<T>, …>.

There is one disadvantage about this approach: we can't return an array of cv-qualified value type any more. But most of the time, instead of something like an array<const int, …>, we would use a const array<int, …> anyway. There is a trade-off, but I think a reasonable one. The C++17 std::make_optional also takes this approach:

template< class T > 
constexpr std::optional<std::decay_t<T>> make_optional( T&& value );

Taking the above points into account, a full working implementation of make_array in C++14 looks like this:

#include <array>
#include <type_traits>
#include <utility>

template<typename T, typename... Ts>
constexpr std::array<std::decay_t<T>, 1 + sizeof... (Ts)>
make_array(T&& t, Ts&&... ts)
    noexcept(noexcept(std::is_nothrow_constructible<
                std::array<std::decay_t<T>, 1 + sizeof... (Ts)>, T&&, Ts&&...
             >::value))

{
    return {{std::forward<T>(t), std::forward<Ts>(ts)...}};
}

template<typename T>
constexpr std::array<std::decay_t<T>, 0> make_array() noexcept
{
    return {};
}

Usage:

constexpr auto arr = make_array(make_array(1, 2),
                                make_array(3, 4));
static_assert(arr[1][1] == 4, "!");

---

C++11 will support this manner of initialization for (most?) std containers.

---

(Solution by @dyp)

Note: requires C++14 (std::index_sequence). Although one could implement std::index_sequence in C++11.

#include <iostream>

// ---

#include <array>
#include <utility>

template <typename T>
using c_array = T[];

template<typename T, size_t N, size_t... Indices>
constexpr auto make_array(T (&&src)[N], std::index_sequence<Indices...>) {
    return std::array<T, N>{{ std::move(src[Indices])... }};
}

template<typename T, size_t N>
constexpr auto make_array(T (&&src)[N]) {
    return make_array(std::move(src), std::make_index_sequence<N>{});
}

// ---

struct Point { int x, y; };

std::ostream& operator<< (std::ostream& os, const Point& p) {
    return os << "(" << p.x << "," << p.y << ")";
}

int main() {
    auto xs = make_array(c_array<Point>{{1,2}, {3,4}, {5,6}, {7,8}});

    for (auto&& x : xs) {
        std::cout << x << std::endl;
    }

    return 0;
}

---

С++17 compact implementation.

template <typename... T>
constexpr auto array_of(T&&... t) {
    return std::array{ static_cast<std::common_type_t<T...>>(t)... };
}

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While this answer is directed more towards this question, that question was marked as a duplicate of this question. Hence, this answer is posted here.

A particular use that I feel hasn't been fully covered is a situation where you want to obtain a std::array of chars initialized with a rather long string literal but don't want to blow up the enclosing function. There are a couple of ways to go about this.

The following works but requires us to explicitly specify the size of the string literal. This is what we're trying to avoid:

auto const arr = std::array<char const, 12>{"some string"};

One might expect the following to produce the desired result:

auto const arr = std::array{"some string"};

No need to explicitly specify the size of the array during initialization due to template deduction. However, this wont work because arr is now of type std::array<const char*, 1>.

A neat way to go about this is to simply write a new deduction guide for std::array. But keep in mind that some other code could depend on the default behavior of the std::array deduction guide.

namespace std {
    template<typename T, auto N>
    array(T (&)[N]) -> array<T, N>;
}

With this deduction guide std::array{"some string"}; will be of type std::array<const char, 12>. It is now possible to initialize arr with a string literal that is defined somewhere else without having to specify its size:

namespace {
    constexpr auto some_string = std::array{"some string"};
}

auto func() {
    auto const arr = some_string;
    // ...
}

Alright, but what if we need a modifiable buffer and we want to initialize it with a string literal without specifying its size?

A hacky solution would be to simply apply the std::remove_cv type trait to our new deduction guide. This is not recommended because this will lead to rather surprising results. String literals are of type const char[], so it's expected that our deduction guide attempts to match that.

It seems that a helper function is necessary in this case. With the use of the constexpr specifier, the following function can be executed at compile time:

#include <array>
#include <type_traits>

template<typename T, auto N>
constexpr auto make_buffer(T (&src)[N]) noexcept {
    auto tmp = std::array<std::remove_cv_t<T>, N>{};

    for (auto idx = decltype(N){}; idx < N; ++idx) {
        tmp[idx] = src[idx];
    }
    return tmp;
}

Making it possible to initialize modifiable std::array-like buffers as such:

namespace {
    constexpr auto some_string = make_buffer("some string");
}

auto func() {
    auto buff = some_string;
    // ...
}

And with C++20, the helper function can even be simplified:

#include <algorithm>
#include <array>
#include <type_traits>

template<typename T, auto N>
constexpr auto make_buffer(T (&src)[N]) noexcept {
    std::array<std::remove_cv_t<T>, N> tmp;
    std::copy(std::begin(src), std::end(src), std::begin(tmp));
    return tmp;
}

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If std::array is not a constraint and if you have Boost, then take a look at list_of(). This is not exactly like C type array initialization that you want. But close.

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Create an array maker type.

It overloads operator, to generate an expression template chaining each element to the previous via references.

Add a finish free function that takes the array maker and generates an array directly from the chain of references.

The syntax should look something like this:

auto arr = finish( make_array<T>->* 1,2,3,4,5 );

It does not permit {} based construction, as only operator= does. If you are willing to use = we can get it to work:

auto arr = finish( make_array<T>= {1}={2}={3}={4}={5} );

or

auto arr = finish( make_array<T>[{1}][{2}[]{3}][{4}][{5}] );

None of these look like good solutions.

Using variardics limits you to your compiler-imposed limit on number of varargs and blocks recursive use of {} for substructures.

In the end, there really isn't a good solution.

What I do is I write my code so it consumes both T[] and std::array data agnostically -- it doesn't care which I feed it. Sometimes this means my forwarding code has to carefully turn [] arrays into std::arrays transparently.

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None of the template approaches worked properly for me for arrays of structs, so I crafted this macro solution:

#define make_array(T, ...) \
    (std::array<T,sizeof((T[]){ __VA_ARGS__ })/sizeof(T)> {{ __VA_ARGS__ }})

auto a = make_array(int, 1, 2, 3);

struct Foo { int x, y; };

auto b = make_array(Foo,
    { 1, 2 },
    { 3, 4 },
    { 5, 6 },
);

Note that although the macro expands its array arguments twice, the first time is inside sizeof, so any side effects in the expression will correctly happen only once.

Have fun!

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