Statically initializing constexpr std::array of objects containing function pointers

Question

I am trying to statically initialize a constexpr std::array of objects containing function pointers with the following code:

#include <array>

using TVoidVoid = void(*)(void);

class State{
public:
  constexpr State(TVoidVoid function) : function_{function}{}
private:
  TVoidVoid function_;
};

void OnEvent1(){}
void OnEvent2(){}
constexpr std::array<State, 10> states = {{OnEvent1}, {OnEvent2}};

int main(){}

I am compiling with:

g++ -Wall -Wextra -Wshadow -Weffc++ -Wstrict-aliasing -ansi -pedantic -Werror -std=c++14 main.cpp

I have trouble understanding the compiling error I'm getting:

main.cpp:14:69: error: too many initializers for ‘const std::array<State, 10>’
 constexpr std::array<State, 10> states = {{OnEvent1}, {OnEvent2}}

The compiler is g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0.

What could be the problem here? Many thanks in advance!

StoryTeller - Unslander Monica · Accepted Answer · 2019-03-08 18:13:01Z

2

The error message could be better. But what's tripping the initialization is in fact that you don't have enough braces. Recall that a std::array is an aggregate wrapping a raw array. So you need to initialize like this:

constexpr std::array<State, 10> states = {{ {OnEvent1}, {OnEvent2} }};

Otherwise, the somewhat inaccurate brace ellision detection algorithm assumes {OnEvent1} is to initialize the internal array, and the second clause is redundant.

Now you just need to provide a default c'tor for State, or adjust the array size.

answered Mar 8, 2019 at 18:13

StoryTeller - Unslander Monica

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4 Comments

Mihai Galos Over a year ago

thank you! I did not know I needed a default constructor for the the non-default initialized elements.

StoryTeller - Unslander Monica Over a year ago

@Mihai - Each c'tor is a different way to initialize objects of your class. The first two don't need it. It's the rest. You can also provide it in another way. TVoidVoid function = nullptr will make your existing c'tor usable as a default c'tor too.

Mirko Over a year ago

I'm a bit amazed that with extra {} (StoryTeller answer) and with less {} works too (my answer), but with the original quantity it did not.

StoryTeller - Unslander Monica Over a year ago

@Mirko - Initialization in C++ is crazy, sadly.

Mirko · Accepted Answer · 2019-03-08 18:21:12Z

1

You need a default constructor (for the last 8)

#include <array>

using TVoidVoid = void(*)(void);

class State{
public:
  // This static is equivalent to a TVoidVoid
  // used by the default constructor
  static void DefFunct() {}

  constexpr State(TVoidVoid function) : function_{function}{}

  // We create a default constructor for the 
  // empty elemnts of the array with our function
  constexpr State() : function_(DefFunct) {}

private:
  TVoidVoid function_;
};

void OnEvent1(){}
void OnEvent2(){}
constexpr std::array<State, 10> states = {OnEvent1, OnEvent2};

int main(){}

edited Mar 8, 2019 at 18:21

answered Mar 8, 2019 at 17:53

Mirko

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3 Comments

Mihai Galos Over a year ago

Thanks Mirko, I reformulated the question, but the code remains the same: I need an array of objects containing function pointers. I would like to construct these objects of type State in-place in the array.

Mirko Over a year ago

@MihaiGalos Edited the answer.

Mihai Galos Over a year ago

So the remaining items need to be default-constructed, which was producing the error. Of course. All elements are statically constructed. Awesome. Please remove the first answer and leave the one after the edit, perhaps add a few comments why this is needed. Then we can close this question! Many thanks!

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