Can't specify explicit initializer for arrays

问题:

Why does this compile in VS 2013

int main()
{

    int a[3] = { 1, 2, 3 };

    return 0;

}

but this gives the error

class TestClass
{

    int a[3] = { 1, 2, 3 };

};

How do I fix it?

回答1:

From Bjarne's C++11 FAQ page:

In C++98, only static const members of integral types can be initialized in-class, and the initializer has to be a constant expression. [...] The basic idea for C++11 is to allow a non-static data member to be initialized where it is declared (in its class).

The problem is, VS2013 doesn't implement all the features of C++11, and this is one of them. So what I suggest you use is std::array (take note of the extra set of braces):

#include <array>

class A
{
public:
    A() : a({ { 1, 2, 3 } }) {} // This is aggregate initialization, see main() for another example

private:
    std::array<int, 3> a; // This could also be std::vector<int> depending on what you need.
};

int main()
{
    std::array<int, 3> std_ar2 { {1,2,3} };
    A a;

    return 0;
}

cppreference link on aggregate initialization

If you're interested you can click on this link to see that what you did does compile when using a compiler that has implemented this feature (in this case g++, I've tried it on clang++ and it works too).

回答2:

Why: not yet implemented in that version of Visual C++.

Fix: use std::array and initialize in each constructor.

回答3:

In alternative to using std::array as the other answers suggest, you can use the approach described in this answer: mark the array as static in the class declaration (which would usually be in the header file), and initialize it in the source file. Like so:

test.h:

class TestClass
{
    static int a[3];
};

test.cpp:

int TestClass::a[3] = { 1, 2, 3 };

Eventually, this should become unecessary as MSVC catches up to C++11.