Related
I have a class defining an array of fixed length n, with some methods.
template<int n>
struct array_container{
/* some code here */
int array[n];
};
Let's say I want to add a constructor to array_container<3>, something along the lines of:
array_container<3>::array_container(int a0, int a1 ,int a2){
array[0] = a0;
array[1] = a1;
array[2] = a1;
}
I know of two ways to do this:
One is to copy the entire code of the generic class, replacing n with 3, and add my constructor:
template<>
struct array_container<3>{
/* some code here */
int array[3];
array_container(int a0, int a1 ,int a2){
array[0] = a0;
array[1] = a1;
array[2] = a1; }
};
This works correctly, but has the disadvantage of needing to copy all the code and methods from the generic base.
Another method is to add a constructor array_container(int a0, int a1, int a2); in the generic class, then define:
template<>
array_container<3>:: array_container(int a0, int a1 ,int a2){
array[0] = a0;
array[1] = a1;
array[2] = a1; }
This has the disadvantage of populating my generic base class with at best undefined or at worst incorrect constructors, such as
array_container<2>(int a0, int a1 ,int a2) (undefined or incorrect depending on whether or not I add the definition to the generic base or not).
Is there any approach that avoids both pitfalls? Ie. doesn't need to copy-paste the entire generic base code for the specialization, and doesn't add unnecessary constructors to the generic base?
Why not simply use
template <std::size_t size>
struct MyArrayWithFunctions : std::array<int, size> {
/* some functions */
};
std::array allows aggregate initialization and even deduces the size so you can simply write
MyArrayWithFunctions arr{1,2,3};
You can add a deduction guideline for your own class but why re-implement array?
Is there any approach that avoids both pitfalls? Ie. doesn't need to copy-paste the entire generic base code for the specialization, and doesn't add unnecessary constructors to the generic base?
Ignoring the fact that, as #Goswin von Brederlow mentions, you seem to be reinventing the wheel (std::array and aggregate initialization), C++20's requires-expressions allows you to define constructors in a primary template that are constrained to only certain specializations. E.g.:
#include <type_traits>
// Helper trait for constraining a ctor to a set
// of specializations.
template <int n, int... ns> struct is_one_of {
static constexpr bool value{false};
};
template <int n, int n0, int... ns> struct is_one_of<n, n0, ns...> {
static constexpr bool value{(n == n0) || is_one_of<n, ns...>::value};
};
template <int n, int... ns>
inline constexpr bool is_one_of_v{is_one_of<n, ns...>::value};
template <int n> struct array_container {
/* some code here */
int array[n];
// Constrained to n == 3 or n == 5.
template <typename... Args>
requires(std::is_same_v<Args, int> &&...) && (sizeof...(Args) == n) &&
is_one_of_v<n, 3, 5 /*, ... */> array_container(Args... args)
: array{args...} {}
};
// Demo.
array_container<3> arr3{1, 2, 3}; // OK
array_container<4> arr4{1, 2, 3, 4}; // Error (no matching ctor)
array_container<5> arr5{1, 2, 3, 4, 5}; // OK
The easier solution would be to construct it with an array(in place).
template<int n>
struct array_container{
int array[n];
array_container(std::array<int, n> arrayIn)
{
std::copy(arrayIn.begin(), arrayIn.end(), array);
}
};
Otherwise you can mess with variadic templates and parameter unpacking.
If happen to have a C++17 compiler you can use the following code:
#include <type_traits>
// class definition
template<int n>
struct array_container {
int array[n];
template<typename... Args, typename std::enable_if_t<(std::is_same_v<int, Args> && ...) && (sizeof...(Args) == n), bool> = true>
array_container(Args... args):
array{args...}
{}
};
// user defined template deduction guide
template<typename... Args>
array_container(Args...) -> array_container<sizeof...(Args)>;
and use it like
array_container<3> x {1,2,3};
or even let the size be deduced from the number of arguments like
// n deduced to 3 from the number of arguments
array_container x {1,2,3};
The constructor is a variadic template, taking any number of int arguments (the latter is enforced by the std::enable_if_t template parameter) and initializes the array member from them. A user defined deduction guide can be used to automatically deduce the n parameter from the number of arguments, that you pass to the constructor.
See it live on godbolt.
The following code compiles in gcc 9.3, but not in gcc 10.2:
constexpr std::array<int, 2> opt = {1,2};
template <typename T>
constexpr auto f(const T& arr)
{
std::array<int, arr.size()> res{};
return res;
}
int main()
{
auto res = f(opt);
}
The code is in https://godbolt.org/z/8hb6M8.
The error given by gcc10.2 is that arr.size() is not a constant expression.
Which compiler is right? 9.3 or 10.2?
If 10.2 is right, how can I define a compile time array and pass its size (and the array) as an argument?
Not sure which one is right but for
how can I define a compile time array and pass its size (and the array) as an argument?
You can change the function to
template <typename T, std::size_t N>
constexpr auto f(const std::array<T, N>& arr)
{
std::array<int, N> res{};
return res;
}
and now the size gets uplifted into the template parameter.
Another alternative which works in both compilers and doesn't require changing the template declaration:
std::array<int, std::tuple_size_v<T>> res{};
The definition of constant expressions has been changed since C++20, you can find several changes on this.
The shorter answer is to change your function signature:
template <typename T>
constexpr auto f(const T& arr);
into:
template <typename T>
constexpr auto f(const T arr);
Then it works.
I'm looking for a way to populate an array with elements of values at compile time rather then run time. So what I'm looking for is something like this
#define numbar
Bar foo[] = {Bar(0),Bar(1)....Bar(numbar)}
Does c++ have any way to do this? Perhaps using macro's or something like that?
Assuming c++14:
constexpr auto num_bars = 100;
class Bar {
constexpr Bar(int i);
};
template <std::size_t ...I>
constexpr auto generate_bars_impl(std::index_sequence<I...>) {
return {Bar(I)...};
}
template <std::size_t N, typename Indices = std::make_index_sequence<N>>
constexpr auto generate_bars() {
return generate_bars_impl(Indices());
}
constexpr auto generated = generate_bars<num_bars>();
This will give you an std::initializer_list in C++14. C++11: You should implement index_sequence and I think that initializer_list has no constexpr constructor.
I am not sure if this is clever enough, but in the case you could use boost preprocessor to create the code as described in you example:
#include <boost/preprocessor/repetition/repeat.hpp>
#include <boost/preprocessor/seq/enum.hpp>
#define INIT(z, n, initializer)(initializer(n))
#define INIT_ARRAY(N, INITIALIZER) {BOOST_PP_SEQ_ENUM(BOOST_PP_REPEAT(5, INIT, Bar))}
struct Bar
{
constexpr Bar(int ii) : i(ii) {}
int i;
};
Bar data[] = INIT_ARRAY(5, Bar);
In general you can solve a lot of repetitive problems this way, but you often makes the code more complex and it less obvious what happens. So you should do this only if you have a lot of such think.
Of course you can also do some metaprogramming, which can lead to quite long compile times
(Note: This question is about not having to specify the number of elements and still allow nested types to be directly initialized.)
This question discusses the uses left for a C array like int arr[20];. On his answer, #James Kanze shows one of the last strongholds of C arrays, it's unique initialization characteristics:
int arr[] = { 1, 3, 3, 7, 0, 4, 2, 0, 3, 1, 4, 1, 5, 9 };
We don't have to specify the number of elements, hooray! Now iterate over it with the C++11 functions std::begin and std::end from <iterator> (or your own variants) and you never need to even think of its size.
Now, are there any (possibly TMP) ways to achieve the same with std::array? Use of macros allowed to make it look nicer. :)
??? std_array = { "here", "be", "elements" };
Edit: Intermediate version, compiled from various answers, looks like this:
#include <array>
#include <utility>
template<class T, class... Tail, class Elem = typename std::decay<T>::type>
std::array<Elem,1+sizeof...(Tail)> make_array(T&& head, Tail&&... values)
{
return { std::forward<T>(head), std::forward<Tail>(values)... };
}
// in code
auto std_array = make_array(1,2,3,4,5);
And employs all kind of cool C++11 stuff:
Variadic Templates
sizeof...
rvalue references
perfect forwarding
std::array, of course
uniform initialization
omitting the return type with uniform initialization
type inference (auto)
And an example can be found here.
However, as #Johannes points out in the comment on #Xaade's answer, you can't initialize nested types with such a function. Example:
struct A{ int a; int b; };
// C syntax
A arr[] = { {1,2}, {3,4} };
// using std::array
??? std_array = { {1,2}, {3,4} };
Also, the number of initializers is limited to the number of function and template arguments supported by the implementation.
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.
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)... };
}
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.
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});
EDIT No longer relevant:
In fact this make_array implementation is listed in sizeof... operator
The code below introduces undefined behavior as per [namespace.std]/4.4
4.4 The behavior of a C++ program is undefined if it declares a deduction guide for any standard library class template.
# 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
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)... };
}
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;
}
C++20 UPDATE: Although there are some excellent answers that provide the desired functionality (such as Gabriel Garcia's answer that uses std::index_sequence), I am adding this answer because the simplest way to do this as of C++20 isn't mentioned: just use std::to_array(). Using the OP's last example of an array of structs:
struct A{ int a; int b; };
// C syntax
A arr[] = { {1,2}, {3,4} };
// using std::array
auto std_array = std::to_array<A>({ {1,2}, {3,4} });
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.
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.
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!
(Note: This question is about not having to specify the number of elements and still allow nested types to be directly initialized.)
This question discusses the uses left for a C array like int arr[20];. On his answer, #James Kanze shows one of the last strongholds of C arrays, it's unique initialization characteristics:
int arr[] = { 1, 3, 3, 7, 0, 4, 2, 0, 3, 1, 4, 1, 5, 9 };
We don't have to specify the number of elements, hooray! Now iterate over it with the C++11 functions std::begin and std::end from <iterator> (or your own variants) and you never need to even think of its size.
Now, are there any (possibly TMP) ways to achieve the same with std::array? Use of macros allowed to make it look nicer. :)
??? std_array = { "here", "be", "elements" };
Edit: Intermediate version, compiled from various answers, looks like this:
#include <array>
#include <utility>
template<class T, class... Tail, class Elem = typename std::decay<T>::type>
std::array<Elem,1+sizeof...(Tail)> make_array(T&& head, Tail&&... values)
{
return { std::forward<T>(head), std::forward<Tail>(values)... };
}
// in code
auto std_array = make_array(1,2,3,4,5);
And employs all kind of cool C++11 stuff:
Variadic Templates
sizeof...
rvalue references
perfect forwarding
std::array, of course
uniform initialization
omitting the return type with uniform initialization
type inference (auto)
And an example can be found here.
However, as #Johannes points out in the comment on #Xaade's answer, you can't initialize nested types with such a function. Example:
struct A{ int a; int b; };
// C syntax
A arr[] = { {1,2}, {3,4} };
// using std::array
??? std_array = { {1,2}, {3,4} };
Also, the number of initializers is limited to the number of function and template arguments supported by the implementation.
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.
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)... };
}
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.
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});
EDIT No longer relevant:
In fact this make_array implementation is listed in sizeof... operator
The code below introduces undefined behavior as per [namespace.std]/4.4
4.4 The behavior of a C++ program is undefined if it declares a deduction guide for any standard library class template.
# 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
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)... };
}
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;
}
C++20 UPDATE: Although there are some excellent answers that provide the desired functionality (such as Gabriel Garcia's answer that uses std::index_sequence), I am adding this answer because the simplest way to do this as of C++20 isn't mentioned: just use std::to_array(). Using the OP's last example of an array of structs:
struct A{ int a; int b; };
// C syntax
A arr[] = { {1,2}, {3,4} };
// using std::array
auto std_array = std::to_array<A>({ {1,2}, {3,4} });
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.
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.
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!