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.
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int a{1}; // braces initialization
int b(1); // parentheses initialization
int c=1; // equal sign initialization
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“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:
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// Before
std::vector<int> v;
v.push_back(1);
v.push_back(2);
// After
std::vector<int>{1,2,3};
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* Avoid narrowing:
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double d=2.5;
int i{d}; // Compilation error
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* Avoid the most vexing parse:
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class A{};
A a(); // Compilation error declares a function named a that returns A.
A a{}; // Call A constructor
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That is why“Uniform initialization” should be preferred.
* 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:
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vector<int> v1(5, 10); // 5 copies of the value 10
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* When "=" is after "auto". This might be needed to enforce deduction to initializer_list:
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auto i1 {1}; // int with the value 1
auto i2 = {1}; // std::initializer_list<int> with an element equal to 1
