Initializing an array in C++

Most people know about member initialization lists in C++. Initialization lists allow you to initialize a variable when the class is instantiated, versus assigning a value within the constructor.

Have you ever tried to initialize an array? Many websites will tell you that this is not possible. In fact, if you try this you’ll get a compiler error:

class array {
public:
  bool vars[2];
  array() : vars[0](true), vars[1](true) {}
};

BUT, if all you want to do is zero-initialize the array then you’re in luck! Simply place parens after the array and the array will be zero-initialized. Try out the code below:

#include <iostream>

class arrayInit {
 public:
  bool vars[2];

  arrayInit() : vars() {}
};

class array {
 public:
  bool vars[2];
  array() {}
};

int main()
{
  arrayInit a;
  std::cout << "a.vars[0] = " << a.vars[0] << std::endl;
  std::cout << "a.vars[1] = " << a.vars[1] << std::endl;

  array b;
  std::cout << "b.vars[0] = " << b.vars[0] << std::endl;
  std::cout << "b.vars[1] = " << b.vars[1] << std::endl;
}

Using code like this in Visual Studio can generate warning C4351 because the behavior is different from previous versions of VC++. Here is the documentation from MSDN:

Compiler Warning (level 1) C4351

new behavior: elements of array ‘array’ will be default initialized

When an array is in a constructor’s member initialization list, the elements of the array will be default initialized. In previous versions of Visual C++, when an array was in a constructor’s member initialization list, the elements of the array may not have been default initialized in some cases.

If the array’s element type does not have a constructor, the elements of the array will be initialized with the corresponding zero representation for that type.

C4351 means that you should inspect your code. If you want the compiler’s previous behavior, remove the array from the constructor’s member initialization list.

If you want the new behavior, which is likely, because the array was explicitly added to the constructor’s member initialization list, use the warning pragma to disable the warning. The new behavior should be fine for most users.

One situation where the new behavior can result in unexpected behavior is when placement new is used to construct the object that has the array as a member, and the program depends on the contents that the memory (for the elements of the default initialized array) had before the call to placement new. In this case, the older compiler would have left the contents of memory unchanged, but the new behavior will cause default initialization of the array elements, overwriting the original contents in memory.