C++ 具有聚合初始化的常量匿名实例

Question

Basically Im wanting to fetch a pointer of a constant and anonymous object, such as an instance of a class, array or struct that is inialised with T {x, y, z...} . Sorry for my poor skills in wording.

The basic code that Im trying to write is as follows:

//Clunky, Im sure there is an inbuilt class that can replace this, any information would be a nice addition
template<class T> class TerminatedArray {
public:
  T* children;
  int length;

  TerminatedArray(const T* children) {
    this->children = children;
    length = 0;
    while ((unsigned long)&children[length] != 0)
      length++;
  }
  TerminatedArray() {
    length = 0;
    while ((unsigned long)&children[length] != 0)
      length++;
  }
  const T get(int i) {
    if (i < 0 || i >= length)
      return 0;
    return children[i];
  }
};

const TerminatedArray<const int> i = (const TerminatedArray<const int>){(const int[]){1,2,3,4,5,6,0}};

class Settings {
public:
  struct Option {
    const char* name;
  };
  struct Directory {
    const char* name;
    TerminatedArray<const int> const children;
  };

  const Directory* baseDir;
  const TerminatedArray<const Option>* options;

  Settings(const Directory* _baseDir, const TerminatedArray<const Option> *_options);
};


//in some init method's:
Settings s = Settings(
  &(const Settings::Directory){
    "Clock",
    (const TerminatedArray<const int>){(const int[]){1,2,0}}
  },
  &(const TerminatedArray<const Settings::Option>){(const Settings::Option[]){
    {"testFoo"},
    {"foofoo"},
    0
  }}
);

The code that I refer to is at the very bottom, the definition of s . I seem to be able to initialize a constant array of integers, but when applying the same technique to classes, it fails with:
error: taking address of temporary [-fpermissive]

I don't even know if C++ supports such things, I want to avoid having to have separate const definitions dirtying and splitting up the code, and instead have them clean and anonymous.

The reason for wanting all these definitions as constants is that Im working on an Arduino project that requires efficient balancing of SRAM to Flash. And I have a lot of Flash to my disposal.

---

My question is this. How can I declare a constant anonymous class/struct using aggregate initialization?

Answer 1

The direct (and better) equivalent to TerminatedArray is std::initializer_list :

class Settings {
public:
  struct Option {
    const char* name;
  };
  struct Directory {
    const char* name;
    std::initializer_list<const int> const children;
  };

  const Directory* baseDir;
  const std::initializer_list<const Option>* options;

  Settings(const Directory& _baseDir, const std::initializer_list<const Option>& _options);
};


//in some init method's:
Settings s = Settings(
  {
    "Clock",
    {1,2,0}
  },
  {
    {"testFoo"},
    {"foofoo"}
  }
);

https://godbolt.org/z/8t7j0f

However, this will almost certainly have lifetime issues (which the compiler tried to warn you about with "taking address of temporary"). If you want to store a (non-owning) pointer (or reference) then somebody else should have ownership of the object. But when initializing with temporary objects like this, nobody else does. The temporaries die at the end of the full expression, so your stored pointers now point to dead objects. Fixing this is a different matter (possibly making your requirements conflicting).

Somewhat relatedly, I'm not sure whether storing a std::initializer_list as class member is a good idea might. But it's certainly the thing you can use as function parameter to make aggregate initialization nicer.

Answer 2

&children[length] != 0 is still true or UB.

If you don't want to allocate memory, you might take reference to existing array:

class Settings {
public:
  struct Option {
    const char* name;
  };
  struct Directory {
    const char* name;
    std::span<const int> const children;
  };

  const Directory baseDir;
  const std::span<const Option> options;

  Settings(Directory baseDir, span<const Option> options);
};

//in some method:
const std::array<int, 3> ints{{1,2,0}};
const std::array<Settings::Option> options{{"testFoo"}, {"foofoo"}};
Settings s{"Clock", {ints}}, options};

Answer 3

First, you're not aggregate-initializing anything. This is uniform initialization and you're calling constructors instead of directly initializing members. This is because your classes have user-defined constructors, and classes with constructors can't be aggregate-initialized.

Second, you're not really able to "initialize a constant array of integers". It merely compiles. Trying to run it gives undefined behavior - in my case, trying to construct i goes into an infinite search for element value 0.

In C++, there's values on the stack, there's values on the heap and there's temporary values (I genuinely apologize to anyone who knows C++ for this statement).

• Values on the heap have permanent addresses which you can pass around freely.
• Values on the stack have temporary addresses which are valid until the end of the block.
• Temporary values either don't have addresses (as your compiler warns you) or have a valid address for the duration of the expression they're used for.

You're using such a temporary to initialize i , and trying to store and use the address of a temporary. This is an error and to fix it you can create your "temporary" array on the stack if you don't plan to use i outside of the block where your array will be.

Or you can create your array on the heap, use its address to initialize i , and remember to explicitly delete your array when you're done with it.

I recommend reading https://isocpp.org/faq and getting familiar with lifetime of variables and memory management before attempting to fix this code. It should give you a much better idea of what you need to do to make your code do what you want it to do.

Best of luck.