compile-time fizzbuzz that emits optimized code

fizzbuzz.cc

#include <cstdio>

struct String {
    char b[1000];
    size_t size;
    constexpr String() : b(), size() {}
};

constexpr String operator+(String x, char y) {
    x.b[x.size] = y;
    x.size = x.size + 1;
    return x;
}

constexpr String operator+(String x, const char *y) {
    while (*y != '\0')
        x = x + *y++;
    return x;
}

constexpr String operator+(String x, String y) {
    return x + y.b;
}

constexpr String numstr(int n) {
    String s;
    if (n >= 10)
        s = numstr(n / 10);
    return s + ('0' + n % 10);
}

constexpr String fizzbuzz(int n) {
    String s;
    for (int i = 1; i < n; ++i) {
        if (i % 15 == 0) {
            s = s + "FizzBuzz\n";
        } else if (i % 3 == 0) {
            s = s + "Fizz\n";
        } else if (i % 5 == 0) {
            s = s + "Buzz\n";
        } else {
            s = s + numstr(i) + '\n';
        }
    }
    return s;
}

int main() {
    constexpr String b = fizzbuzz(100);
    fwrite(b.b, 1, b.size, stdout);
    return 0;
}