== Why is this an issue?

C+11 introduced “Uniform initialization”/"list initialization". It is a way to initialize an object from a braced-init-list. This adds a third way to initialize objects in {cpp} on top of parentheses and equal signs.

----
int a{1}; // braces initialization
int b(1); // parentheses initialization
int c=1; // equal sign initialization
----
“Uniform initialization” was introduced to address the confusion of the many initialization syntaxes in {cpp} and to give a syntax that, in concept, can be used in all initialization scenarios. It helps to:

* Initialize container in a way that wasn't possible before:

----
// Before
std::vector<int> v;
v.push_back(1);
v.push_back(2);
// After
std::vector<int>{1,2,3};
----

* Avoid narrowing:

----
double d=2.5;
int i{d}; // Compilation error
----

* Avoid the most vexing parse:

----
class A{};
A a();  // Compilation error declares a function named a that returns A.
A a{}; // Call A constructor
----

That is why “Uniform initialization” should be preferred.


=== Noncompliant code example

[source,cpp]
----
struct A {
  A(int i, int j, int z) {
    ...
  }
};

void f() {
  A a(1,2,3); // Noncompliant
}

struct B {
  A a = A(1, 2, 3); // Noncompliant
};
----


=== Compliant solution

[source,cpp]
----
struct A {
  A(int i, int j, int z) {
    ...
  }
};

void f() {
  A a{1,2,3};
}

struct B {
  A a{1, 2, 3};
};
----


=== Exceptions

* In some situations, “Uniform initialization” has surprising behavior. for example, in constructor overload resolution, “Uniform initialization” prefers the constructor with "std::initializer_list" as a parameter if possible, even if another constructor seems like a better match. Like in the "std::vector" case, you might need to go back to parentheses in order to call the non "intializer_list" constructor:

[source,cpp]
----
vector<int> v1(5, 10); //  5 copies of the value 10
----

* When "=" is after "auto". This might be needed to enforce deduction to initializer_list: 

[source,cpp]
----
auto i1 {1}; // int with the value 1
auto i2 = {1}; //  std::initializer_list<int> with an element equal to 1
----

* In single argument cases, "=" is okay:

[source,cpp]
----
int i = 39;
----

* When the braces are after the "=":

[source,cpp]
----
vector<int> v = { 1, 2, 4 };
----


== Resources

* {cpp} Core Guidelines - https://github.com/isocpp/CppCoreGuidelines/blob/e49158a/CppCoreGuidelines.md#es23-prefer-the\--initializer-syntax[ES.23: Prefer the `{}`-initializer syntax]


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== Comments And Links
(visible only on this page)

=== on 11 May 2020, 09:05:55 Geoffray Adde wrote:
* The description is crystal clear! I love the summary about initialization.
* In single argument cases, "=" is okay:  =>  maybe builtin single argument case ?
* In some situations, “Uniform initialization” has surprising behavior. => somehow to me, it makes sense because the curly braces syntax seems to indicate initializer list.
* Do we need more examples?

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