2.5.6 Initializer list

What was the problem

Initializing a container like std::vector with a predefined sequence of values was quite verbose before C++11, as we had to insert the values one by one:

void transform 
(std::vector<int>& values, const std::vector<int>& multipliers); 
void up_down_transform(std::vector<int>& values) 
  // Pfff it is not even const. 
  std::vector<int> pattern(4); 
  pattern[0] = 0; 
  pattern[1] = 1; 
  pattern[2] = 0; 
  pattern[3] = -1; 
  // Can’t we have simple initialization, like we have for arrays? 
  // const int pattern[4] = {0, 1, 0, -1}; 
  transform(values, pattern); 
How the Problem is Solved

PIC <initializer_list>

However, thanks to the the introduction of std::initializer_list in C++11, we can now initialize our containers wich bracket enclosed values:

void transform 
(std::vector<int>& values, const std::vector<int>& multipliers); 
void up_down_transform(std::vector<int>& values) 
  const std::vector<int> pattern = {0, 1, 0, -1}; 
  transform(values, pattern); 
  // This one also works: 
  // transform(values, {0, 1, 0, -1}); 

It’s just like uniform initialization [2.1.5]! Or is it?

The Problem with std::initializer_list

Instances of std::initializer_list are created by the compiler when it encounters a list of values between brackets and if the target to which these values are assigned is, or can be constructed from, an std::initializer_list.

In the example above, we can create a vector from the values because std::vector defines a constructor taking an std::initializer_list as it sole argument. This constructor then copies the values from the list into the vector7.

I think it is important to emphasize that: the constructor copies the values from the list. There is no way it can move them, let alone wrap them.

So we have something that looks surreptitiously like the good old simple and efficient aggregate initialization, that is consequently identical in its syntax to uniform initialization [2.1.5], and that is actually a quite inefficient way to initialize something as soon as the element type is non trivial.

The worse part is that bracket initialization is pushed by the so-called “modern” C++ trend, propagating this inefficiency everywhere.


Avoid std::initializer_list.

If you really, really, want the target container to be const, then use an immediately invoked lambda to construct it:

    const std::vector<int> pattern = 
      []() -> std::vector<int> 
        std::vector<int> result(4); 
        result[0] = 0; 
        result[1] = 1; 
        result[2] = 0; 
        result[3] = -1; 
        return result; 
      }(); // Watch out for the parentheses here, 
           // it is a call to the lambda.

7Check [Bri20] for an illustration of the problem.