C++ Initializer-List Constructors | `std::initializer_list` Ctors Explained

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C++ Initializer-List Constructors | `std::initializer_list` Ctors Explained

이 글의 핵심

An initializer_list constructor lets your type initialize from `{a,b,c}`. It takes precedence for brace syntax over many other ctors—know the priority rules and lifetime of the backing array.

What is an initializer-list constructor?

A constructor that takes std::initializer_list<T> lets users write:

#include <initializer_list>
#include <vector>

class MyVector {
    std::vector<int> data;
public:
    MyVector(std::initializer_list<int> list) {
        for (int x : list) data.push_back(x);
    }
};

int main() {
    MyVector vec = {1, 2, 3, 4, 5};
}

Priority over other ctors

class MyClass {
public:
    MyClass(int x) {
        std::cout << "int ctor: " << x << "\n";
    }
    MyClass(std::initializer_list<int> list) {
        std::cout << "initializer_list ctor\n";
    }
};

int main() {
    MyClass a(10);   // int ctor
    MyClass b{10};   // initializer_list ctor
    MyClass c = {10}; // initializer_list ctor
}

Empty braces vs empty list

class MyClass {
public:
    MyClass() { std::cout << "default\n"; }
    MyClass(std::initializer_list<int> list) { std::cout << "list\n"; }
};

int main() {
    MyClass x;      // default
    MyClass y{};    // default
    MyClass z{{}};  // initializer_list (empty)
}

How it works

The compiler typically allocates a const temporary array for {1,2,3} and passes pointer + length to initializer_list. Do not store an initializer_list past the full-expression unless you copy into a vector. Copies of initializer_list alias the same array.

Relation to std::vector

  • vector(n, val) vs vector{n, val} — different ctors (size/fill vs two-element list).
  • Custom types with both (int,int) and initializer_list need clear conventions.

Performance notes

  • Passing initializer_list is cheap; populating a vector from it may allocate.
  • Prefer range ctor or reserve when you know sizes upfront.

FAQ

Q: When to use?
A: APIs that take “list of values” with homogeneous type.

Q: Modifiable?
A: View is read-only—copy if you need to mutate.

Resources: Effective Modern C++, cppreference.

  • C++ initializer_list
  • C++ explicit keyword

Keywords

C++, initializer_list constructor, overload resolution, braced init, C++11.

  • C++ explicit keyword
  • C++ default and delete