C++ – When to use the brace-enclosed initializer

cc++11initializer-list

In C++11, we have that new syntax for initializing classes which gives us a big number of possibilities how to initialize variables.

{ // Example 1
  int b(1);
  int a{1};
  int c = 1;
  int d = {1};
}
{ // Example 2
  std::complex<double> b(3,4);
  std::complex<double> a{3,4};
  std::complex<double> c = {3,4};
  auto d = std::complex<double>(3,4);
  auto e = std::complex<double>{3,4};
}
{ // Example 3
  std::string a(3,'x');
  std::string b{3,'x'}; // oops
}
{ // Example 4
  std::function<int(int,int)> a(std::plus<int>());
  std::function<int(int,int)> b{std::plus<int>()};
}
{ // Example 5
  std::unique_ptr<int> a(new int(5));
  std::unique_ptr<int> b{new int(5)};
}
{ // Example 6
  std::locale::global(std::locale("")); // copied from 22.4.8.3
  std::locale::global(std::locale{""});
}
{ // Example 7
  std::default_random_engine a {}; // Stroustrup's FAQ
  std::default_random_engine b;
}
{ // Example 8
  duration<long> a = 5; // Stroustrup's FAQ too
  duration<long> b(5);
  duration<long> c {5};
}

For each variable I declare, I have to think which initializing syntax I should use and this slows my coding speed down. I'm sure that wasn't the intention of introducing the curly brackets.

When it comes to template code, changing the syntax can lead to different meanings, so going the right way is essential.

I wonder whether there is a universal guideline which syntax one should chose.

Best Answer

I think the following could be a good guideline:

  • If the (single) value you are initializing with is intended to be the exact value of the object, use copy (=) initialization (because then in case of error, you'll never accidentally invoke an explicit constructor, which generally interprets the provided value differently). In places where copy initialization is not available, see if brace initialization has the correct semantics, and if so, use that; otherwise use parenthesis initialization (if that is also not available, you're out of luck anyway).

  • If the values you are initializing with are a list of values to be stored in the object (like the elements of a vector/array, or real/imaginary part of a complex number), use curly braces initialization if available.

  • If the values you are initializing with are not values to be stored, but describe the intended value/state of the object, use parentheses. Examples are the size argument of a vector or the file name argument of an fstream.

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