C言語 共用体（union）の基本と実践的な活用法：構造体との違いを活かしたメモリ最適化

#include <stdio.h>

// 共用体の定義
union Data {
    int i;
    float f;
    char str[20];
};

int main() {
    // 共用体変数の宣言
    union Data data;

    // メンバへのアクセス
    data.i = 10;
    printf("data.i: %d\n", data.i);

    data.f = 220.5;
    printf("data.f: %f\n", data.f);

    // 注意：data.iの値はこの時点で壊れている
    printf("data.i (after data.f assign): %d\n", data.i);

    return 0;
}

data.i: 10
data.f: 220.500000
data.i (after data.f assign): 1130135552

#include <stdio.h>

struct MyStruct {
    char c;    // 1 byte
    int i;     // 4 bytes
    double d;  // 8 bytes
};

union MyUnion {
    char c;    // 1 byte
    int i;     // 4 bytes
    double d;  // 8 bytes
};

int main() {
    printf("Size of Struct: %zu bytes\n", sizeof(struct MyStruct));
    printf("Size of Union: %zu bytes\n", sizeof(union MyUnion));

    return 0;
}

Size of Struct: 16 bytes
Size of Union: 8 bytes

#include <stdio.h>

typedef enum {
    TYPE_INT,
    TYPE_FLOAT,
    TYPE_STR
} DataType;

typedef struct {
    DataType type;
    union {
        int i;
        float f;
        char *s;
    } value;
} Variant;

void printVariant(Variant v) {
    switch (v.type) {
        case TYPE_INT:
            printf("Integer: %d\n", v.value.i);
            break;
        case TYPE_FLOAT:
            printf("Float: %f\n", v.value.f);
            break;
        case TYPE_STR:
            printf("String: %s\n", v.value.s);
            break;
    }
}

int main() {
    Variant v1;
    v1.type = TYPE_INT;
    v1.value.i = 100;
    printVariant(v1);

    Variant v2;
    v2.type = TYPE_STR;
    v2.value.s = "Hello Union";
    printVariant(v2);

    return 0;
}

#include <stdio.h>
#include <stdint.h>

union IPAddress {
    uint32_t full_address;
    struct {
        uint8_t b1;
        uint8_t b2;
        uint8_t b3;
        uint8_t b4;
    } bytes;
};

int main() {
    union IPAddress ip;
    ip.full_address = 0xC0A80001; // 192.168.0.1 (Big Endian表現を想定)

    printf("Full: 0x%X\n", ip.full_address);
    printf("Bytes: %d.%d.%d.%d\n", ip.bytes.b1, ip.bytes.b2, ip.bytes.b3, ip.bytes.b4);

    return 0;
}

Full: 0xC0A80001
Bytes: 1.0.168.192

#include <stdio.h>
#include <stdint.h>

float fast_inverse_sqrt_raw(float number) {
    union {
        float f;
        uint32_t i;
    } conv;

    conv.f = number;
    // floatのビット表現を整数として扱い、ビット演算を行う
    conv.i = 0x5f3759df - (conv.i >> 1); 
    
    return conv.f;
}

int main() {
    float n = 4.0f;
    printf("Approx inverse sqrt of %f is %f\n", n, fast_inverse_sqrt_raw(n));
    return 0;
}

#include <stdio.h>

struct Vector3 {
    union {
        struct { float x, y, z; }; // 匿名構造体
        float v[3];                // 匿名共用体のメンバ
    };
};

int main() {
    struct Vector3 pos;
    
    // .value.x のような深い階層が不要
    pos.x = 1.0f;
    pos.y = 2.0f;
    pos.z = 3.0f;

    printf("Position: x=%f, y=%f, z=%f\n", pos.v[0], pos.v[1], pos.v[2]);

    return 0;
}