在c ++中创建一个常量数组

Question

Is there any reason why codeblocks is telling me that I can't make an array? I'm simply trying to do:

const unsigned int ARRAY[10] = {0,1,2,3,4,5,6,7,8,9};

and it's giving me

error: a brace-enclosed initializer is not allowed here before '{' token

I have changed other parts of the initializer, but the error is always saying the same thing. This doesn't seem to make sense, since this is one of the first things I learned in c++.

Answer 1

You say that you did this within a class, as a private variable.

Recall that (at the moment), member variables may not be initialised in the same place where you declare them (with a few exceptions).

struct T {
   std::string str = "lol";
};

is not ok. It has to be:

struct T {
   std::string str;
   T() : str("lol") {}
};

But, to add insult to injury, pre-C++0x you cannot initialise arrays in the ctor-initializer !:

struct T {
   const unsigned int array[10];
   T() : array({0,1,2,3,4,5,6,7,8,9}) {} // not possible :(
};

And, because your array's elements are const , you can't rely on assignment either:

struct T {
   const unsigned int array[10];
   T() {
       for (int i = 0; i < 10; i++)
          array[i] = i; // not possible :(
   }
};

However, as some other contributors have quite rightly pointed out, there seems little point in having a copy of the array for each instance of T if you can't modify its elements. Instead, you could use a static member.

So, the following will ultimately solve your problem in what's — probably — the best way:

struct T {
   static const unsigned int array[10];
};

const unsigned int T::array[10] = {0,1,2,3,4,5,6,7,8,9};

Hope this helps.

Answer 2

Since this is a private member variable in a class (according to the comment), this is indeed not allowed in C++03.

C++0x, partially supported by many modern compilers, allows the following to compile:

class C
{
    const unsigned int ARRAY[10];
 public:
    C() : ARRAY{0,1,2,3,4,5,6,7,8,9} {}
};
int main()
{
    C obj; // contains a non-static const member: non-assignable 
}

However, non-static const members only make sense if they contain different values in different instances of the class. If every instance is to contain the same {0,1,2,3,4,5,6,7,8,9} , then you should make it static , which also makes it possible to do this in C++98:

class C
{
    static const unsigned int ARRAY[10];
 public:
    C() {}
};
const unsigned int C::ARRAY[10] = {0,1,2,3,4,5,6,7,8,9};
int main()
{
    C obj;
}