C++ Designated Initializers (C++20) | `.field = value` for Aggregates

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C++ Designated Initializers (C++20) | `.field = value` for Aggregates

이 글의 핵심

Designated initializers use `.member = value` for aggregates in C++20. Improves readability for large option structs; order must match declarations.

What are designated initializers?

Initialize aggregate members by name (C++20), similar to C99 style but with C++ ordering rules.

struct Point {
    int x;
    int y;
    int z;
};

Point p1 = {1, 2, 3};

Point p2 = {.x = 1, .y = 2, .z = 3};
Point p3 = {.x = 1, .z = 3};  // y value-initialized
// Point p4 = {.z = 3, .x = 1};  // error: out of declaration order

Basic usage

struct Config {
    std::string host;
    int port;
    bool ssl;
    int timeout;
};

Config cfg = {
    .host = "localhost",
    .port = 8080,
    .ssl = true,
    .timeout = 30
};

Nested structs

struct Address {
    std::string street;
    std::string city;
    int zip;
};

struct Person {
    std::string name;
    int age;
    Address address;
};

Person p = {
    .name = "Alice",
    .age = 30,
    .address = {
        .street = "123 Main St",
        .city = "Seoul",
        .zip = 12345
    }
};

Compared to other styles

StyleNotes
Point p{1,2,3}Short; order-dependent
Point p{.x=1,...}Self-documenting; order must match declarations
Point p(1,2,3)Requires ctor (non-aggregate)
Default member initFills omitted members

Unlike C, out-of-order designation is not allowed in C++20.

Performance

Designated initialization is a source-level feature; generated code is typically the same as positional aggregate init.

Common mistakes

  1. Member order changed in refactor—update all designated lists.
  2. Type no longer aggregate (added ctor/virtual)—designators disappear.
  3. Mixing designated and positional in one initializer—follow your compiler’s rules (often disallowed).

FAQ

Q1: When to use?
A: Large config structs, tests, protocols where names beat position.

Q2: Only aggregates?
A: Yes—classes with private data or user ctors need different APIs.

Related: Aggregate initialization, List initialization.

  • C++ designated initializer (guide)
  • C++ Three-way comparison

Keywords

C++, designated initializers, C++20, aggregate initialization, struct.

  • C++ Aggregate initialization